• 

This matters, since most existential risks (xrisk) we worry about today (like nuclear war, bioweapons, global ecological/societal crashes) only affect one planet. But if existential risk is the answer to the Fermi question, then the peril has to strike reliably. If it is one of the local ones it has to strike early: a multi-planet civilization is largely immune to the local risks. It will not just be distributed, but it will almost by necessity have fairly self-sufficient habitats that could act as seeds for a new civilization if they survive. Since it is entirely conceivable that we could have invented rockets and spaceflight long before discovering anything odd about uranium or how genetics work it seems unlikely that any of these local risks are “it”. That means that the risks have to be spatially bigger (or, of course, that xrisk is not the answer to the Fermi question).

Of the risks mentioned by George physics disasters are intriguing, since they might irradiate solar systems efficiently. But the reliability of them being triggered before interstellar spread seems problematic. Stellar engineering, stellification and orbit manipulation may be issues, but they hardly happen early — lots of time to escape. Warp drives and wormholes are also likely late activities, and do not seem to be reliable as extinctors. These are all still relatively localized: while able to irradiate a largish volume, they are not fine-tuned to cause damage and does not follow fleeing people. Dangers from self-replicating or self-improving machines seems to be a plausible, spatially unbound risk that could pursue (but also problematic for the Fermi question since now the machines are the aliens). Attracting malevolent aliens may actually be a relevant risk: assuming von Neumann probes one can set up global warning systems or “police probes” that maintain whatever rules the original programmers desire, and it is not too hard to imagine ruthless or uncaring systems that could enforce the great silence. Since early civilizations have the chance to spread to enormous volumes given a certain level of technology, this might matter more than one might a priori believe.

So, in the end, it seems that anything releasing a dangerous energy effect will only affect a fixed volume. If it has energy E and one can survive it below a deposited energy e, if it just radiates in all directions the safe range is — one needs to get into supernova ranges to sterilize interstellar volumes. If it is directional the range goes up, but smaller volumes are affected: if a fraction f of the sky is affected, the range increases as but the total volume affected scales as .

Self-sustaining effects are worse, but they need to cross space: if their space range is smaller than interplanetary distances they may destroy a planet but not anything more. For example, a black hole merely absorbs a planet or star (releasing a nasty energy blast) but does not continue sucking up stuff. Vacuum decay on the other hand has indefinite range in space and moves at lightspeed. Accidental self-replication is unlikely to be spaceworthy unless is starts among space-moving machinery; here deliberate design is a more serious problem.

The speed of threat spread also matters. If it is fast enough no escape is possible. However, many of the replicating threats will have sublight speed and could hence be escaped by sufficiently paranoid aliens. The issue here is if lightweight and hence faster replicators can always outrun larger aliens; given the accelerating expansion of the universe it might be possible to outrun them by being early enough, but our calculations do suggest that the margins look very slim.

The more information you have about a target, the better you can in general harm it. If you have no information, merely randomizing it with enough energy/entropy is the only option (and if you have no information of where it is, you need to radiate in all directions). As you learn more, you can focus resources to make more harm per unit expended, up to the extreme limits of solving the optimization problem of finding the informational/environmental inputs that cause desired harm (=hacking). This suggests that mindless threats will nearly always have shorter range and smaller harms than threats designed by (or constituted by) intelligent minds.

In the end, the most likely type of actual civilization-ending threat for an interplanetary civilization looks like it needs to be self-replicating/self-sustaining, able to spread through space, and have at least a tropism towards escaping entities. The smarter, the more effective it can be. This includes both nasty AI and replicators, but also predecessor civilizations that have infrastructure in place. Civilizations cannot be expected to reliably do foolish things with planetary orbits or risky physics.

The key issue here is the nature of the Great Filter, something we talk about when we discuss the Fermi Paradox.

The Fermi Paradox: loosely put, we live in a monstrously huge cosmos that is rather old. We only evolved relatively recently — our planet is ~4.6GYa old, in a galaxy containing stars up to 10GYa old in a universe around 13.7GYa old. Loosely stated, the Fermin Paradox asks, if life has evolved elsewhere, then where is it? We would expect someone to have come calling by now: a five billion year head start is certainly time enough to explore and/or colonize a galaxy only 100K light years across, even using sluggish chemical rockets.

We don't see evidence of extraterrestrial life, so, as economist Robin Hanson pointed out, there must be some sort of cosmic filter function (The Great Filter) which stops life, if it develops, from leaving its star system of origin and going walkabout. Hanson described two possibilities for the filter. One is that it lies in our past (pGF): in this scenario, intelligent tool-using life is vanishingly rare because the pGF almost invariably exterminates planetary biospheres before they can develop it. (One example: gamma ray bursts may repeatedly wipe out life. If this case is true, then we can expect to not find evidence of active biospheres on other planets. A few bacteria or archaea living below the Martian surface aren't a problem, but if our telescopes start showing us lots of earthlike planets with chlorophyll absorption lines in their reflected light spectrum (and oxygen-rich atmospheres) that would be bad news because it would imply that the GF lies in our future (an fGF).

The implication of an fGF is that it doesn't specifically work against life, it works against interplanetary colonization. The fGF in this context might be an emergent property of space exploration, or it might be an external threat — or some combination: something so trivial that it happens almost by default when the technology for interstellar travel emerges, and shuts it down for everyone thereafter, much as Kessler syndrome could effectively block all access to low Earth orbit as a side-effect of carelessly launching too much space junk. Here are some example scenarios: see below.

• 

Recently I noticed Voices in AI – Episode 16: A Conversation with Robert J. Sawyer, an interview by Byron Reese, which contained an interesting tidbit on existential risk.The interviewer Byron Reese, in the course of the conversation, attributed a connection between radio technology and existential risk to Carl Sagan: 

“[Sagan] said that his guess was civilizations had a hundred years after they got radio, to either destroy themselves, or overcome that tendency and go on to live on a timescale of billions of years.” 

Robert J. Sawyer picked up on this theme and elaborated:

“The window is very small to avoid the existential threats that come with radio. The line through the engineering and the physics from radio, and understanding how radio waves work, and so forth, leads directly to atomic power, leads directly to atomic weapons… and leads conceivably directly to the destruction of the planet.”

I wasn’t able to find a fully explicit statement of this thesis in Sagan’s writings (it may be there, of course, and I simply couldn’t find it), but I found something close to it: 

“Sagan has several possible explanations for why alien radio signals have proved so elusive. Maybe… ‘No civilization survives long enough to develop power levels adequate to make such communications. All civilizations destroy themselves shortly after achieving a technological level consonant with radio astronomy’.“ (Carl Sagan, Conversations with Carl Sagan, edited by Tom Head, p. 156)

My assumption is that the connection between radio technology and nuclear weapons obtains because radio technology means the use of electronics (the ability to build electronic devices, which are a much more sophisticated technology with more possibilities than, say, steam-power technology), as well as opening up the electromagnetic spectrum to scientific study. The study of the electromagnetic radiation may inevitably result in the development of a more general conception of radiation that leads to nuclear science and the ability to build nuclear weapons.

I have called radio (along with fusion and consciousness, inter alia) Technologies of Nature, that is to say, processes for which nature provides an existence proof, and which we can, in the fullness of technological development, attempt to reverse engineer. It should not surprise us that with the reverse engineering of the cosmos – that which nature has shown us that it is possible to do – we would eventually converge on the knowledge and the technology that would allow us to undo what nature has done.  

In other contexts Sagan often formulated the maturity of civilizations in terms of their mastery of radio technology, specifically, the ability to build radio telescopes. For example, Sagan wrote:

“Radio astronomy on Earth is a by-product of the Second World War, when there were strong military pressures for the development of radar. Serious radio astronomy emerged only in the 1950s, major radio telescopes only in the 1960s. If we define an advanced civilization as one able to engage in long-distance radio communication using large radio telescopes, there has been an advanced civilization on our planet for only about ten years. Therefore, any civilization ten years less advanced than we cannot talk to us at all.” (Carl Sagan, The Cosmic Connection: An Extraterrestrial Perspective, Chap. 31)

For Sagan, radio telescopes are a crucial technology because they enable SETI, the search for extraterrestrial intelligence. Possessing a sensitive radio telescope would make it possible for a civilization to detect a SETI signal, and only a little more advanced technology would allow for the transmission of a SETI signal to some other civilization that might detect it. But radio is also a crucial technology because of its above-noted connection with nuclear technology and therefore with anthropogenic existential risk. 

In human history, we built nuclear weapons before we built radio telescopes, but this appears to be a merely contingent development, and the two were only separated by about ten years of history. It could have easily happened that human beings might have built radio telescope before we built nuclear weapons, as well another civilization might have done (or may yet do).

In a counterfactual history (which would also require a counterfactual universe, that is to say, a universe different from the universe in which we in fact find ourselves), in which human beings built radio telescopes, and as soon as they turned on their radio telescopes they found that they lived in a densely populated universe, so that the sky was alive with signals from a proliferation of ETIs, and if all this had happened before we built nuclear weapons, the second half of the twentieth century would have been radically different than it was in fact.

This obviously didn’t happen, but we could push the counterfactual scenario harder, and we can imagine a civilization with more-or-less nineteenth century levels of technology (as when the telegraph and the telephone were invented) developing radio much earlier in its history than human beings did, relative to other technological developments. If we had had radio technology for a hundred years before we developed nuclear weapons, again history might have been radically different. And if we had nuclear weapons for a hundred years before we developed radio technology, again, this would have meant a radically different history.

All of these scenarios, and others as well, may be instantiated by other civilizations elsewhere in the cosmos. Sagan’s thesis as stated above by Reese and Sawyer posits that nuclear and radio technology are tightly-coupled. This may well be true, but it is not too difficult to imagine alternative scenarios in which nuclear and radio technology are only loosely-coupled, and I have elsewhere noted how in terrestrial history there are cases in which loosely-coupled science and technology prior to the industrial revolution could be separated by hundreds of years. If radio technology and nuclear technology were found separated by hundreds of years, a civilization might experience a longue durée in possession of the one without the other.

• 

      Survival Paradox

Survival during the pretechnological stages of evolution commonly depends on unrestrained reproduction and aggression. As technology advances, however, that becomes the way to extinction. Only through some sane resolution of the paradox can life survive into the post-technological stage.

     Lifeburst

The evolutionary leap into space from a planetary birthplace. The crucial event in the evolution of any race, comparable only to the earlier leap from water to dry land, it becomes possible only when primitive survival modes are superceded by intelligence, technological sophistication, and a holistic regard for the total space environment and its entire community of life and mind.

• 

(ed note: The world is on the eve of a nuclear war, but a small team of astronauts are visiting Pluto. They discover that the moon Charon is actually a huge alien computer with a weakly-godlike level artificial intelligence. It has been observing Terra for a few million years. It talks to the astronauts through a small floating silver sphere they call The Egg.)

      The Egg, settling now, came to rest a few centimeters above the table. “Of all the thousands of millions of your species, you four are the only ones to have wandered into deep space and survived—with the help of your doomed culture, of course, and with more than a bit of luck. You four are surely unique beasts.”
     “Cultural toolmakers? That's it? Animals that play with tools?" Jennifer caught that sense of powerlessness that afflicted the commanders. Curiosity forgotten, she felt the rage that powerlessness brings.
     “Cultural toolmakers indeed, Doctor. You’re your world’s only Lamarckian creatures. You do know of Jean-Baptiste Lamarck? His theory of evolution?”
     “I'm sure you know that I do.”
     “Yes. The Chevalier de Lamarck believed that life evolved by means of the inheritance of acquired characteristics. His model of evolution is often expressed among you by means of the giraffe; straining to reach the leaves of tall trees, the old knight might have said, the ancestors of giraffes repeatedly stretched their necks. Somehow, this stretching might be transmitted to their young, who were then born with yet longer necks, and again through the generations until there evolved that stately beast the giraffe, which so sadly disappeared recently from Earth.”
     “Lamarck was wrong, though.” Jennifer scratched her head, stared at the Egg’s swirling pastels.
     “Indeed he was, for most life-forms. For them, Darwin was correct; the offspring of giraffes, to simplify matters very considerably, were born with necks of differing lengths, and those with longer necks were able to obtain more food, and hence to produce more offspring which in turn inherited those longer necks, etcetera. In but one case is Lamarck’s model applicable to evolution, and that is the case of the cultural toolmaker.”
     "No, we know something about our own evolution. You keep saying it yourself. We’re animals—we’re Darwinian. This Lamarck business—it’s bumwad.” Jennifer felt a childish triumph; she had the thing on this one, visited Olduvai and other museums, seen the bones of her ancestors. “We’re no more Lamarckian than,—than giraffes.”
     “Mmmm?” The Egg rose a bit higher, colors swirling. “Consider. A human being, producing an invention, passes that invention on to the next generation via the route of what we call culture. This I would call the Lamarckian ‘genetic apparatus.’ As you can plainly see from your own history, cultural—Lamarckian—evolution is far faster than Darwinian evolution; physically and mentally you remain Pleistocene hunters, yet cultural evolution has permitted you to hunt down all your old prey. Yet still, when stressed by resource diminution, human beings go hunting large mammals—as Darwinian evolution constructed them to do. But they hunt with Lamarckian tools that grow momentarily in efficiency. And the only large mammal left in any numbers is—?”
     “People,” said Benai sadly.
     “Yes, my highly-trained hunter, people. The fiercest of prey. And the Lamarckian mode assures you that those people with the finest weapons spread the fastest at the expense of their less formidable fellows, until all available planetary resources are used up and everyone has these weapons. Then you die.” If a disembodied voice could have shrugged, Charon’s would have done so. “There’re many among you who explain this very situation with more than adequate lucidity, broadly transmitted among your billions; I’ve listened to such opinions from your thinkers since you realized the danger. Yet the weapons continue to evolve faster than you’re able to handle them. Quite soon Homo sapiens will become extinct—as surely as you have existed until now.”
     “Inexorably? How do you know? Are we bacteria in a Petri dish?” Jennifer’s involuntary species-loyalty set an edge to her voice.
     Smooth as silk, the Egg responded with its own question: “Does any of you understand the nature of a koan?”
     Pike knew, drawing on a long-standing interest in certain things Japanese. “In Zen Buddhism? A koan’s a little tale, or, riddle, a sort of a fable—usually with some internal inconsistency that stretches its meaning. Stretches the mind.” He shrugged, staring at the Egg.
     “Well said, Major Muir. Here, then, is the Toolmaker Koan, the puzzle that has tormented me since I first enjoyed awareness:
     “On any planet where life evolves, given time, certain adaptable organisms acquire Lamarckian evolutionary systems—culture—to enhance their own survival. Once Lamarckian evolution is initiated, though, its consequent extinction is assured. Or, to put it another way, animals with culture can see farther into the future than any other animals; but for cultural animals the future is brief. Why? Why must cultural toolmakers, the most gifted of the universe’s spontaneous expressions, so swiftly and inexorably beat themselves into extinction with their very giftedness? Ah, my dear humans, that’s a conundrum at the very heart of my own existence. The Toolmaker Kean: you, like me, are mad.”
     The Egg paused, seemingly for effect, then: “Don't mistake me, my unruly little animals. I speak from long, long experience. Experience, mind you, with thousands of millions of different species—including a number of different cultural-toolmaker species. I collect cultures, you see, and the occasional individual, in my memory.”
     “Sh*t,” growled Corson. “You’re trying to tell us that there’s all sorts of, uh, people, and you visit 'em and collect ’em like butterflies. What are you calling yourself, a god? And—”
     “No god, Colonel, no god at all. I’m a program in a machine, as I believe I’ve said before. My abilities— deus ex machina, god in a machine? No, I’m in no way magical, supernatural, what have you. I’m simply, as I’m so fond of saying, very good at what I do; the best, in fact, of whom—or which—I’m aware.”
     Corson pressed home. “Well, then, whatever you call yourself, you’re telling us that someone made you, right?”
     “Indeed. Your word ‘machine’ derives from a Greek word meaning ‘contrivance,’ and I was indeed contrived. By animal cultural toolmakers, in fact, although certainly not in my present form. No indeed!” The voice paused for a moment, then: ‘I've a story to was invited joyously to a place on the planetary surface that had been prepared in advance. When landed, I was nearly crippled by an explosion—a God-trap. The resurgence of religious wars, augmented by an embryonic understanding of nuclear physics, soon took that race also into extinction—but not before I had made my escape and my own decision.”
     “You came here?” Jennifer, now tracking a beast like a horned camel with her camera.
     “Not immediately. I’d learned a thing: that the Koan fulfills itself at the very instant when a toolmaker species acquires spacefaring capacity. Toolmakers are a disease of planetary surfaces, ill suited to the confines of their native worlds. But they become most dangerous when they harness the energies that enable them to leave the homeworld; here the Koan slaughters them, at the very threshold of Meta-stasis! Such a wonder, such a paradox; no wonder the heavens are silent.
     And it spoke again, low, its gloom almost palpable in the alien land.
     “I've erred once more; I’m doomed to err. My sensors Earthward suggest that your masters respond to your disappearance with the beginning spasms of their last war. A war my meddling had a part in initiating; the Koan unfolds even as we talk. ”
     “I’ve seen it before.” The voice of Charon was uncharacteristically subdued. “One might call it a Smuggler’s War; it occurs when a toolmakers defensive resources have outstripped its offensive weapons. It’s an ignominious extinction, I’d guess, not being an animal, but an extinction all too common, given the existence of the Toolmaker Koan. The warnings have been given, the smuggling demonstrated. Two cities have been destroyed. I'm reckoning within a week’s time, Earthwise, for the general burning to begin. An oddly formal inception to a species’ demise—but animals are often formal when confronting one another with death.”
     “What cities?” Jennifer thought of a dozen she mightn't mind, a hundred for which her heart would ache.
     “Small cities, as befits the initial steps of a Smuggler's War, cities well away from the central polities of human ideology. About thirty minutes ago two explosions occurred—one in the city of Curitiba in Brazil, the other in the city of Ogbomosho in the African People’s Republic of Nigeria.
"     “God, I had a clearance-six, and I never guessed! How it could have gone on for so long—” Corson’s hands fisted. “How many years, Charon?”
     "As nearly as I’m able to determine, Colonel, ever since the small spasm you call the One-Day War.” The Egg’s tones carried more than a trace of sadness and fatigue. “Your masters appear to have come to the realization at that time that they could never penetrate one another’s defensive nets above-board; and state apparats, I believe you might call them, were erected to oversee the wholesale smuggling of light nuclear weapons into one another's cities. I believe the functionaries of the ancient human drug trade played a large part in the operations. Whatever, because the weapons are concealed below-ground, their detonations are singularly, ah, dirty.”
     “You’re just letting it happen,” murmured Pike.
     “‘Letting’ it? How could I stop it?” The Egg dropped, coming virtually nose-on to the startled major.
     “But to allow six billion people to die, when maybe we could do something—anything—look at all the civilizations you’ve seen, try to think how we—we animals’ feel! We’re not just bacteria in a Petri dish, however much you like to think it.”
     “And it’s not just the people,” added Jennifer, “it’s knowledge, the cities, the universities, zoos, museums, ecosystems! It’s the f*cking world you’re talking about here!”
     “Ah, young Major, Doctor, people I don't know what to do with. Sterilize them, so they don’t continue reproducing? Feed them? Change them? None of these can I do. Museums and universities—why, I’ve paid a good deal of attention to them ever since you invented them. I can say with a fair degree of certainty, ”—that preening tone again—“that I have a good grip on all the information you’ve accumulated in institutions. No problem there. And zoos? Ecosystems? I’ve a complete inventory of genetic detailing from before what you call the Quaternary Extinctions— a Pleistocene fauna greater than anything in your zoos and a lot healthier, larger gene-pool, the works. But six billion people, all hell-bent on reproducing and eating and owning televisions and automatic weapons and washing machines … and room to keep it all in … for this disease I have no cure. Toolmaker Koan, you know. It’s only the spacebased presence that holds my interest, you and your cosmonautical friends between the Earth and her Moon. I can stay their hands, I believe, by sending you to them with Hwiliria. She is after all, a world in herself—such a world as you could build, given time and the inspiration. In her, they may find hope. But Earth—she must go her own way.”
     The Egg gave its rare and artificial chuckle. “As ambassadors of Koan and Metastasis, you still have a bit to learn. A good deal, in fact. You understand the Toolmaker Koan, but you don’t yet feel it.”
     “What do you mean, ‘feel’ the Koan? We all know what you mean—we feel it.” Jennifer fisted her hand, thumped the ground. “We ‘feel’ it more than you do!"
     “Not in its true horror, you don’t. Complacent little things, sitting at the apex of evolution—you think. There’s horror here, and even you, my intuitive doctor, even you’ve missed it so far. You must think; extend yourselves beyond animal lifespans—look ahead! I’ve watched worlds burn; I fear, machine that I am; you too must know that fear.”
     “Come on,” said Corson, “We’re—”
     “You’d like to return to Texas,” said the Egg. “Well, my friends, Texas is a momentary phenomenon, a fantasy. Do think, now—do you hope to wait a few years, then go home to Amarillo, or wherever? After your stupid war? Fifty years, say? Oh, little Colonel, think again! Try fifty thousand! Imagine this many years after your Smugglers War and its inevitable echoing spasms have erased your Earthbound kind. What would you find if you were a traveller, just dropping by?”
     “Something like Mars?” More than ever, Pike was uncomfortable with the direction of talk, the madness rippling through Charon’s words.
     “No, no, nothing like Mars! Your atmosphere’s too thick, you’re in the temperate zone, the region of liquid water. Think, think!”

     Nothing.

     “Well, then, I’ll project for you. Knowing something of the megatonnage your masters prepare to unleash, I propose an initial burning, yes, as you do. But your burning will not be so easily extinguished, my foolish beasts—I know. And it will be followed by a total covering of your Fair world in a dirty yellow cloud cover composed largely of nitrogen, oxygen, dust, water vapor and carbon dioxide, no? This being just the initial result. I've seen it, alas, and I know, and so I know space to be your eternal home. You’re not suited as planetary stewards—you’ll stew the world, and this is just a start.” The Egg emitted a frightful giggle. “Considering your likely expenditure of energy in this endeavor, I project the cloud cover’s lasting perhaps five decades in an imperfect process of self-enhancing feedback, during which your kindly Sun heats the upper atmosphere while the air at ground level remains cold and dark. Mountains, shorelines and other such irregularities produce stirring of these layers of temperature difference, and the stirring in turn produces convective storms of magnitudes unknown throughout your history—you evolved, after all, well adapted to a planet that is friendly to life … but your current direction is most unfriendly to life. Toolmaker Koan, my dears—the makers of Hwiliria did precisely this, grew storms on their homeworld. And these storms are such as tear trees from the ground and flatten anything of lesser structural integrity than—than geologic formations. Earth will for a time resemble her sister world, Venus—a cold, wet Venus, true, and one with oxygen and liquid water … but in all, a dark and howling Toolmaker Hell. ”
     Involuntarily, the four humans thought of their Earthly overlords, fingers on buttons, waiting.
     “Toolmaker Koan,” said the Egg again. “Consider further, though. During those five decades, a gradual cleansing is taking place: dust drops from the air, the perpetual mixing of water vapor and air pouring dirty rains across a scoured landscape, billions of tons of eroded soil and debris creeping off the continental surfaces into the rivers, thence to the seas, carrying with them trees, buildings, artifacts … are you with me? I speak here of eternal Man!”

Silence.

• 

     “Yes. And, after five or six decades, sunlight again begins to penetrate to the surface, the cold seas soaking up the warmth, more rains falling, the cleansing accelerating—after five, perhaps ten decades of a most unEarthly climate, the skies are again blue and landscapes begin to soak up the warmth. The poles are dusted, and the sun-heated dust warms them. They melt. The level of the seas begins to rise, and water creeps up above the continental shelves, broadening estuaries, slowing rivers, carrying away your lovely filthy cities that cluster along the coasts,"
     “And everything’s extinct?” Pike, staring at the Egg.
     “Certainly not! Do you imagine yourselves gods, that you could kill off all the life in that wonderful place? In no way could you. I could, if sufficiently piqued and deranged, by dropping a little sizzler into your nice stable star; but you couldn’t. Life is infinitely resilient. On land there would remain seeds, pockets of soil, tangled debris, underground ruins —the twisted wreck of a world, but teeming with life. Seeds; the seeds of land plants are extraordinarily resistant, some of them. Those favored, of course, would be the varieties that are adapted already to cold and inclement conditions, those that are accustomed to periods of inertia—conifers, many deciduous hardwoods, mosses, lichens, grasses, plenty of seeds. If We imagine a warming world, a world whose atmosphere contains perhaps as much as five percent carbon dioxide from your excesses, we’d see a paradise for the rebirth of the plant kingdom. But you—you are gypsies forever. Earth will not welcome you back; she’ll welcome, oh, insects, for example. As you know, there are more species of insects now than there are of all other animals together; and under the mutagenic influence of your waste, these would prosper and diversify as soon as the Sun's rays reached them again. And of course the microbes that run life’s machinery; these too will awaken to perform their essential tasks in Earth's metabolism. And even land vertebrates, small ones capable of subsisting for a time on microhabitats—guesses?”
     “Lizards an’ snakes, I’ll bet,” growled Corson. “You, you kind of hate us, don't you?”
     “No, no, or already you’d be extinct. But your world will; she'll prefer those insects, and perhaps other honorable cold-blooded beings—amphibia like frogs and salamanders, and perhaps even crocodilians and turtles. These are ectotherms, creatures of slow metabolism that fall into life-saving torpor when deprived of food. They’re uniquely well suited to survive bad conditions, for a time.”
     Jennifer, long silent, suddenly snapped, “Right. Ever so lovely. Not a warm-blooded thing, and—”
     “Oh, yes, the beings you call ‘warm-blooded,’ the endotherms, the physiologically-heated? The birds, and the mammals? The cute ones, the ones you love? Did you think you'd. leave them untouched?” The Egg chuckled, its lunacy coming in laughter-sounds.
     “Mice, and rats,” said Jennifer. “They’re small; they might live—”
     “Precisely,” said Charon; “Shrews and moles, too. The creatures you despise. Indeed, many small species of mammals might weather it out, particularly those that hibernate at times of stress. When active, tiny mammals are good at exploiting microhabitats, locating and consuming food that could not support larger warm-blood beings. Too, many small mammals move well in the dark, being nocturnal by nature; they would survive, in isolated pockets. Even the most primitive primates, the things you call tarsioids and tree-shrews, these are nocturnal beings; where the forest tangles persisted, they might live on. I would venture to say that birds, too, some of them, with their great mobility and small size, might wander from place to place, devouring insects and carrion—of which there would be plenty, I assure you, human beings being as numerous as they are.”
     The Egg’s tones brightened. “And the seas, the seas—well, certainly the sea mammals, those few seals and the like that remain now, they would become extinct—their high metabolic rates would not tolerate the dying of plankton, the falling of the ecologic pyramid. But fishes and other ectothermic beings, cold, stupid things like sharks, and crustacea and the like; some of these would persist, eating in their desultory way, surviving on a carrion-based foodchain until the sun came again, the plankton revived, and a measure of normal energy-fixing returned to the water. Would you wish to return to such a world?”
     “So—a planet of rats, a world of insects, a sea of sharks and crabs.” Benai shook his head sadly.
     “More or less. But grant your world her due. You’re aware of the concept of terraforming; Earth, Terra herself, invented it. She’d terraform herself, with no help at all. After a time your world would be well-nigh reforested, singing with birds and crawling with little mammals. Your seas would fill with fishes again. And—the intriguing part—all the little animals on land, as well as whatever lived in the seas, all of these would be mutating; in a newly roomy world, they would begin to adaptively radiate. In only fifty thousand years you would see mice such as you’ve never seen before; you’d see new birds, new rats, new insects. A world of small animals, but plenty of them. And perhaps the fastest rates of evolution would be among the ruins, the few and sorry remaining ruins of your, ah, ‘eternal cities.’ The Earth would be an alien world, with—most notably—alien microorganisms. Yours no longer. Alien, unthinkably alien, growing more so daily. Another few orders of magnitude in years, then—how alien could she become? Try this: You’re an interplanetary traveller, interested in the paleontology of worlds, the study of their evolutionary histories. You come to Earth, oh, say, some fifty million years hence. D’you see? Fifty thousand millennia—a wink in the eye of Time. I want you here, and your cohorts near Earth, to consider what then would be left of the great dominion of Man.”
     "Stainless steel,” suggested Pike tentatively.
     “Plastic,” said Oruna Benai.
     “Concrete pilings and footings,” offered Corson.
     “I don't know,” said Jennifer. “Have you ever seen old concrete flood-control works, how quickly they deteriorate after even ten years?”
     “Our physician has the right idea,” said Charon. “As for metal artifacts, their lives are measured in millennia, not thousands of millennia; and plastic, organically based, is a momentary phenomenon at best. Glass likewise; you will have seen ‘sea glass,’ bits of broken bottles washed by surf or swift streams. In a matter of days it becomes rounded and smoothed, eroded. And what about subduction?”
     “Subduction?”
     “Oh, dear, I expect there’s little use for geologists in space, unless they work with lunar materials. There's no subduction there. Plate tectonics? Continental drift?"
     “Oh, God, yes,” said Jennifer. “In tens of millions of years the continents themselves move all over the place!”
     “Precisely; and their edges are subducted into the mantle of the planet, remelted into the stuff that makes the world. Bemuse of your big moon, this is a never-ending process; your crust is tidal, it never stabilizes as did that of Mars. Your Eternal Cities, whatever was left of them, would cease to exist in only ten million years, never mind fifty; while anything you'd built in the uplands would be well on its way to the sea for its own subduction. Why, there’d be new mountains, new seas! And of course new animals and plants—I warrant all those rats and mice and things would by then have diversified, their descendants filling large animal econiches with all the profligacy of the mammals of the old Pleistocene. Different orders, different forms, but recognizable lifeways … a new world.” The voice changed, grew harsher. “A world that might kill you, were you to return, and justly so.”
     “You tell us, then,” said Benai, “to forget Earth, and that in fifty million years there will be no trace of humanity. What good is this, telling us about such a remote time?”
     “Here, now, I didn’t say there’d be no trace; there would very definitely be a trace, but it’d take a good eye, a trained paleontologist’s eye, to find it. Consider our hypothetical paleontologist—he or she, or it, comes to the Earth of this distant era, and begins to dig. Among the strata of fifty million years in his past, the past of this Earth, what does he find? We guess, no cities, no automobiles, not much of anything after the postnuclear storms and what have you. But he does find something. Doctor? I know you could guess. Let's say he digs his way to some fossil-bearing Pleistocene stratum, and considers the record from there on up.”
     “Uh, he’d have to, he’d find—he’d find the Quaternary Extinction!”
     “Citizen Doctor,” said Benai wryly, “we’re not all trained in paleontology.”
     “The Pleistocene, the epoch just before ours—the world was full of big animals, elephants, mammoths, rhinos, megatheria, giant cats and dogs, huge bears, all sorts of great herbivores—had been, for millions of years—and they died out when modern people evolved! There’d be a dieoff, a gap in the record!” Jennifer grew excited, waving her arms. “When we spread around the world, we hunted, we overhunted, killing the things we could catch, the strong slow things, and we burned, we hunted with fire—we destroyed their fodder … they used to think, the Victorians who discovered it, that the extinction was a natural event, but no explanation came to light until the seventies and eighties—better technique, you know, and computer modelling. The extinction was progressive, and it proceeded with the spread of people—we did it!”
     “Yes, Man has already marked his world with death,” chortled the Egg. “But I submit, Doctor, that there'd be a bit more to mark the ascendancy of Man. Not all large mammals died off"; some did very well indeed after that extinction. Guess?” The Egg was pleased, driving its point home.
     “The seas—the whales and all … but we’ve almost killed them off, too, and the war would take whatever’s left. On land? Well, cattle—we domesticated them, and of course they’re always falling into water, getting into quicksands, things like that … there’d be fossils of cattle after the Quaternary.”
     “Yes,” said the Egg, “and our paleontologist would be wondering, why only large herbivores? What sort of evolutionary event would favor them?”
     “But people,” said Corson. “People are everywhere— wouldn’t he find ’em?”
     “Oh, not necessarily, not at all,” said Jennifer. “Fossils—they’re almost impossibly rare, considering the sheer numbers of animals that die in a day. Our record of fossil man is tiny in volume, and the period of great human numbers, why, that’s only a few thousand years, a tiny slice of the fossil record. And you need the right conditions: still water, silts— people don’t get caught in such places. Cattle sometimes, but not people. And at the last, our own time, that’d be the time of the storms—a poor time for fossils, very poor … Charon’s right. We’ll be scoured away.
     “Very good,” said Charon. “Your world will hate you, and will scour away your works. Another mark, though, one more mark of the dominion of Man. Industry. You've had two centuries of industry, two centuries and a bit more, of systematic global metal-working and pollution. Pollution, ladies and gentlemen. Yours is a messy species, and you dump your metallic wastes into the rivers and seas, your planet’s circulatory system.
     “So—I recite the brief fossil mark of a species struggling for the ideological dominion of its world. From the rich animal diversity of the Pleistocene, we’d see first the gradual extinction of large animals with the ascent of Man, leaving only his domesticated satellites, cattle, horses, swine and perhaps dogs and the like. Second, a stratum of metal-rich clays … then nothing, a series of strata empty of large animals, a few blank pages recording a world denuded. Oh, then of course the rise of new forms from the little survivors, but my experience suggests that it would take many millions of years for the wound to heal. And by then, of course, your world will long have been most unfriendly to anything human.”

     The Egg paused. No one spoke.

     “Well, then! Are we not impressed with ourselves, little toolmakers? To think that a species, Homo sapiens sapiens, the self-named Man the Wise, that has existed for but forty thousand years—to think that such a species, in its moment of existence, could leave such a global mark, such a striking gap in the unfolding of its noble world! I do hope that you all understand me well; minus the Metastasis, your signature for eternity is a brief great dying, and a line of waste. Toolmakers, you see, are deadly to the planets on which they evolve. Your only hope is Meta-stasis; you will work with me, then and perhaps attain a measure of adulthood.

     “You will atone for my sins!”

---

     “Toolmaker Koan. All part of your science-fiction thing—yes, Charon told me. So? In the last century you Americans listened, listened, and found no ‘intelligent life,’ no cultural toolmakers; I've read about it. SETI, Search for Extraterrestrial Intelligence. But Charon says your search was flawed. You were looking for a beacon and never found one; yet several times during your search Charon says it detected the faint shell of an expanding electromagnetic bubble generated by a doomed toolmaker. Like rings on a pond, when you throw a stone. No beacon, just this expanding ring. And whenever such a ring’s edge passed, Charon knew that someone was doomed.”
     “Doomed?”
     “Doomed. Charon says that the use of electromagnetic transmission is a function of the military: the cutting edge, the spearhead, more appropriately, of communications science among cultural animals is always military, even though civilian transmission is often louder. To Charon, an electromagnetic bubble signifies not only the presence of a cultural toolmaker but the approach of its extinction. And by the time such a bubble's shell leaves its system of origin, it’s so thinned as to sound like random noise to simple radio ears on Earth. Inverse square? Is that what they call it? We didn't know what we were looking for! Like the pond’s ringwaves: fainter, more random, thinner with distance.”
     “Uh, ‘electromagnetic bubble.’ ” Pike looked at Jennifer. Jennifer looked back, waiting. “Yeah, I think ours started right around the beginning of the twentieth century, Marconi—” He knew he ought to know, but so much from school is lost …
     “Less than a century and a half,” said Jennifer, “a wink in the eye of time. ”
     “Yeah,” said Pike sadly. “You make it seem like a law of nature.”
     “Charon does; the Koan does, not I.”

• 

(ed note: Warning, spoilers for REVELATION SPACE by Alastair Reynolds

But there was more to this galaxy than astrophysics. As if a new layer of memories had been quietly laid over her previous ones. Khouri found herself knowing something more. That the galaxy was teeming with life; a million cultures dispersed pseudo-randomly across its great slowly rotating disk.

But this was the past—the deep, deep past.

“Actually.” Fazil said, “somewhere in the region of a billion years ago. Given that the Universe is only about fifteen times older than that, that's quite a hefty chunk of time, especially on the galactic timescale." He was leaning over the railinged walkway next to her, as if they were a couple pausing to stare at their reflections in a dark, bread-strewn duckpond. “To give you some perspective. humanity didn't exist a billion years ago. In fact, neither did the dinosaurs. They didn't get around to evolving until less than two hundred million years ago; a fifth of the time we're dealing with here. No: we're deep into the Precambrian here. There was life on Earth, but nothing multicellular—a few sponges if you were lucky." Fazil looked at the galaxy representation again. “But that wasn't the case everywhere."

The million or so cultures (although she could be infinitely precise about the number, it suddenly struck her as childishly pedantic to do so, like specifying one's age to the nearest month) had not all arisen at the same time. nor did they all hang around for the same length of time. According to Fazil (though she understood it on some basic level) it had taken until four billion years ago for the galaxy to reach the required state at which intelligent cultures could begin to arise. But once that point of minimal galactic maturity had been reached, the cultures had not all suddenly appeared in unison. It had been a progressive emergence of intelligence, some cultures having arisen on worlds where, for one reason or another, the pace of evolutionary change was slower than the norm, or life's ascendancy was subject to more than the usual quota of catastrophic setbacks.

But eventually—two or three billion years after life had first arisen on their homeworlds—some of these cultures had become spacefaring. When that point was reached, most cultures expanded rapidly into the galaxy, although there were always a few stay-at-homes who preferred to colonise only their own solar systems, or sometimes even just their own circum-planetary environments. But generally the pace of expansion was rapid, with a mean drift rate between one tenth and one hundredth of the speed of light. That sounded slow, but was in fact blindingly fast, given that the galaxy was billions of years old and only a hundred thousand light-years wide. Unrestricted, any of these spacefarers could have dominated the entire galaxy in the totally inconsequential time of a few tens of millions of years. And maybe if it had happened like that—a neatly imperialist domination by one power—things would have been very different.

But instead, the first culture had been at the slower end of the expansionist speed-range, and had impacted on the expansion wave of a second, younger upstart. And while younger, the second civilisation was not technologically inferior to the first, nor less capable of mustering aggression when it was required. There was what—for want of a better word—one might describe as a galactic war; a sudden sparking friction where these two swelling empires brushed against one another, grinding like vast flywheels. Soon. other ascendant cultures were embroiled in the conflict. Eventually—to one degree or another—several thousand spacefaring civilisations fell into the fray. They had many names for it, in the thousand primary languages of the combatants. Some of these names could not easily be translated into any meaningful human referent. But more than one culture called it something which might—with due allowance for the crudities of interspecies communication—be termed the Dawn War.

It was a war encompassing the entire galaxy (and the two smaller satellite galaxies which orbited the Milky Way)—one which consumed not just planets, but whole solar systems, whole star systems, whole clusters of stars, and whole spiral arms. She understood that evidence of this war was visible even now, if one knew where to look. There were anomalous concentrations of dead stars in some regions of the galaxy, and still-burning stars in odd alignments; husked components of weapons-systems light-years wide. There were voids where there ought to have been stars, and stars which—according to the accepted dynamics of solar-system formulation—ought to have had worlds, but which lacked them: only rubble, cold now. The Dawn War had lasted a long, long time—longer even than the evolutionary timescale of the hottest stars. But on the timescale of the galaxy, it had indeed been mercifully brief; a transforming spasm.

It was possible that no culture emerged intact; that none of the players who entered the Dawn War actually emerged, victorious or otherwise. The lengthscale of the war, while short by galactic time, was nonetheless hideously long by species-time. It was long enough for species to self-evolve, to fragment, to coalesce with other species or assimilate them; to remake themselves beyond recognition, or even to jump from organic to machine-life substrates. Some had even made the return trip, becoming machine, then returning to the organic when it suited their purposes. Some had sublimed, vanishing from the theatre of the war entirely. Some had converted their essences to data and found immortal storage in carefully concealed computer matrices. Others had self-immolated.

Yet in the aftermath, one culture emerged stronger than the others. Possibly they had been a fortunate small-time player in the main fray, now rising to supremacy amongst the ruins. Or possibly they were the result of a coalition, a merging of several battle-weary species. It hardly mattered, and they themselves probably had no hard data on their absolute origin. They were—at least then—a hybrid machinechimeric species. with some residual vertebrate traits. They did not bother giving themselves a name.

“Still.” Fazil said, “they acquired one, whether they liked it or not."

“What were they called?" she asked.

He waited before answering, and when he did, it was with almost theatrical gravity. “The Inhibitors. For a very good reason, which will shortly become apparent."

---

"I'll start with what I know," Volyova said, drawing in a generous inhalation of breath. “Once, the galaxy was a lot more populous than it is now. Millions of cultures, though only a handful of big players In fact. Just the way all the predictive models say the galaxy ought to be today, based on the occurrence rates of G-type stars and terrestrial planets in the right orbits for liquid water." She was digressing, but Pascale and Khouri decided not to fight it. "That's always been a major paradox, you know. On paper, life looks a lot commoner than we find it to be. Theories for the developmental timescales for tool-using intelligence are a lot harder to quantify, but they suffer from much the same problem. They predict too many cultures."

“Hence the Fermi paradox.” Pascale said.

“The what?” asked Khouri.

“The old dichotomy between the relative ease of interstellar flight, especially for robotic envoys—and the complete absence of any such envoys turning up from non-human cultures. The only logical conclusion was that no one else was around to send them, anywhere in the galaxy."

“But the galaxy's a big place." Khouri said. “Couldn't there be cultures elsewhere, except that we just don't know about them yet?"

“Doesn't work," Volyova said emphatically. Pascale nodding in agreement. “The galaxy's big, but not that big—and it's also very old. Once a single culture decided to send out probes, everyone else in the galaxy would know about it within a few million years. And the galaxy happens to be several thousand times older than that. Granted, several generations of stars had to live and die before there were enough heavy elements to sustain life, but even if machine-building cultures only arise once every million years or so, they've had thousands of opportunities to dominate the entire galaxy."

“To which there have always been two answers." Pascale said. “Firstly, that they are here, but we just haven't ever noticed them. Maybe that was conceivable a few hundred years ago, but no one takes it seriously now; not when every square inch of every asteroid belt in about a hundred systems has been mapped."

“Then maybe they never existed in the first place?"

Pascale nodded at Khouri. “Which was perfectly tenable until we knew more about the galaxy, which begins to look suspiciously accommodating of life, at least in the essentials; what Volyova just said—the right types of stars, and the right kind of planets in the right places. And the biological models were still arguing for a higher occurrence rate, right on up to intelligent cultures. "

“So the models were wrong," Khouri said.

“Except they probably weren't." Volyova was speaking now. “Once we got into space, once we left the First System, we began to find dead cultures all over the place. None had survived until much more recently than a million years ago, and some had gone out a lot earlier than that. But they all pointed to one thing. The galaxy had been a lot more fecund in the past. So why not now? Why was it suddenly so lonely?"

“The war, " Khouri said, and for a moment no one spoke. The silence was only interrupted when Volyova began speaking, softly and reverently, as if they were discussing something sacred.

“Yes,” she said, “The Dawn War—that was what they called it, wasn't it?”

“It was a billion years ago," Khouri said, and for a moment Volyova let her speak without interruption. “And it sucked up all those cultures and spat them out in shapes and forms a lot different to the ones they'd had when they went in. I don't think we can really understand what it was about, or who or what exactly survived it—except that they were more like machines than living creatures, although as far beyond anything we can envisage as our machines are beyond stone tools. But they had a name, or they were given it—I don't really remember the details. But I do remember the name."

"The Inhibitors." Volyova said.

Khouri nodded. “And they deserved it."

“Why?”

“It was what they did afterwards," Khouri said. “Not during the war, but in its aftermath. It was like they subscribed to a creed; a rule of discipline. Intelligent, organic life had given rise to the Dawn War. What they were now was something different; post-intelligent, I guess. Anyway, it made what they did a lot easier."

“Which was?"

“Inhibition. Literally: they inhibited the rise of intelligent cultures around the galaxy, so that nothing like the Dawn War could ever happen again."

Volyova took over now. “It wasn't just a case of annihilating any extant cultures which might have survived war. They also set about disturbing the conditions which could lead to intelligent life ever arising again. Not stellar engineering—I think that would have been too great an interference; too much an act which contradicted their own strictures—but inhibition on a lesser scale. They could have done it without tampering in the evolution of a single star, except in extreme cases—by altering cometary orbits, for instance, so that episodes of planetary bombardment lasted much longer than the norm. Life probably would have found niches in which to survive—deep underground, or around hydrothermal vents—but it would never have become very complex. Certainly nothing which would threaten the Inhibitors."

“You said this was a billion years ago." Pascale said. “And yet we've come all that way since then— from single-celled creatures right up to Homo sapiens. Are you saying we slipped through the net?"

“Exactly that," Volyova said. “Because the net was falling apart."

Khouri nodded. “The Inhibitors seeded the galaxy with machines, designed to detect the emergence of life and then suppress it. For a long time it looked like they worked as planned—that's why the galaxy isn't teeming today, although all the preconditions look favourable." She shook her head. “I sound like I actually know this stuff. "

“Maybe you do," Pascale said. “In any case, I want to hear what you have to say. All of it."

“All right, all right." Khouri fidgeted in her acceleration couch, doubtless trying to do what Volyova had been doing for the last hour: avoiding putting pressure on the bruises she had already gained. “Their machines worked fine for a few hundred million years," she said. “But then stuff started to go wrong. They started failing; not working as efficiently as intended. Intelligent cultures began to emerge which would have previously been suppressed at birth."

• 

3 Classification of existential risks

We shall use the following four categories to classify existential risks:

Bangs — Earth-originating intelligent life goes extinct in relatively sudden disaster resulting from either an accident or a deliberate act of destruction.

Crunches — The potential of humankind to develop into posthumanity is permanently thwarted although human life continues in some form.

Shrieks — Some form of posthumanity is attained but it is an extremely narrow band of what is possible and desirable.

Whimpers — A posthuman civilization arises but evolves in a direction that leads gradually but irrevocably to either the complete disappearance of the things we value or to a state where those things are realized to only a minuscule degree of what could have been achieved.

Armed with this taxonomy, we can begin to analyze the most likely scenarios in each category. The definitions will also be clarified as we proceed.

---

4 Bangs

This is the most obvious kind of existential risk. It is conceptually easy to understand. Below are some possible ways for the world to end in a bang. I have tried to rank them roughly in order of how probable they are, in my estimation, to cause the extinction of Earth-originating intelligent life; but my intention with the ordering is more to provide a basis for further discussion than to make any firm assertions.

• 

4.1 Deliberate misuse of nanotechnology

In a mature form, molecular nanotechnology will enable the construction of bacterium-scale self-replicating mechanical robots that can feed on dirt or other organic matter . Such replicators could eat up the biosphere or destroy it by other means such as by poisoning it, burning it, or blocking out sunlight. A person of malicious intent in possession of this technology might cause the extinction of intelligent life on Earth by releasing such nanobots into the environment.

The technology to produce a destructive nanobot seems considerably easier to develop than the technology to create an effective defense against such an attack (a global nanotech immune system, an “active shield” ). It is therefore likely that there will be a period of vulnerability during which this technology must be prevented from coming into the wrong hands. Yet the technology could prove hard to regulate, since it doesn’t require rare radioactive isotopes or large, easily identifiable manufacturing plants, as does production of nuclear weapons .

Even if effective defenses against a limited nanotech attack are developed before dangerous replicators are designed and acquired by suicidal regimes or terrorists, there will still be the danger of an arms race between states possessing nanotechnology. It has been argued that molecular manufacturing would lead to both arms race instability and crisis instability, to a higher degree than was the case with nuclear weapons. Arms race instability means that there would be dominant incentives for each competitor to escalate its armaments, leading to a runaway arms race. Crisis instability means that there would be dominant incentives for striking first. Two roughly balanced rivals acquiring nanotechnology would, on this view, begin a massive buildup of armaments and weapons development programs that would continue until a crisis occurs and war breaks out, potentially causing global terminal destruction. That the arms race could have been predicted is no guarantee that an international security system will be created ahead of time to prevent this disaster from happening. The nuclear arms race between the US and the USSR was predicted but occurred nevertheless.

• 

4.2 Nuclear holocaust

The US and Russia still have huge stockpiles of nuclear weapons. But would an all-out nuclear war really exterminate humankind? Note that: (i) For there to be an existential risk it suffices that we can’t be sure that it wouldn’t. (ii) The climatic effects of a large nuclear war are not well known (there is the possibility of a nuclear winter). (iii) Future arms races between other nations cannot be ruled out and these could lead to even greater arsenals than those present at the height of the Cold War. The world’s supply of plutonium has been increasing steadily to about two thousand tons, some ten times as much as remains tied up in warheads. (iv) Even if some humans survive the short-term effects of a nuclear war, it could lead to the collapse of civilization. A human race living under stone-age conditions may or may not be more resilient to extinction than other animal species.

• 

4.3 We’re living in a simulation and it gets shut down

A case can be made that the hypothesis that we are living in a computer simulation should be given a significant probability . The basic idea behind this so-called “Simulation argument” is that vast amounts of computing power may become available in the future (see e.g. ), and that it could be used, among other things, to run large numbers of fine-grained simulations of past human civilizations. Under some not-too-implausible assumptions, the result can be that almost all minds like ours are simulated minds, and that we should therefore assign a significant probability to being such computer-emulated minds rather than the (subjectively indistinguishable) minds of originally evolved creatures. And if we are, we suffer the risk that the simulation may be shut down at any time. A decision to terminate our simulation may be prompted by our actions or by exogenous factors.

While to some it may seem frivolous to list such a radical or “philosophical” hypothesis next the concrete threat of nuclear holocaust, we must seek to base these evaluations on reasons rather than untutored intuition. Until a refutation appears of the argument presented in [27], it would intellectually dishonest to neglect to mention simulation-shutdown as a potential extinction mode.

• 

4.4 Badly programmed superintelligence

When we create the first superintelligent entity , we might make a mistake and give it goals that lead it to annihilate humankind, assuming its enormous intellectual advantage gives it the power to do so. For example, we could mistakenly elevate a subgoal to the status of a supergoal. We tell it to solve a mathematical problem, and it complies by turning all the matter in the solar system into a giant calculating device, in the process killing the person who asked the question.

• 

4.5 Genetically engineered biological agent

With the fabulous advances in genetic technology currently taking place, it may become possible for a tyrant, terrorist, or lunatic to create a doomsday virus, an organism that combines long latency with high virulence and mortality .

Dangerous viruses can even be spawned unintentionally, as Australian researchers recently demonstrated when they created a modified mousepox virus with 100% mortality while trying to design a contraceptive virus for mice for use in pest control. While this particular virus doesn’t affect humans, it is suspected that an analogous alteration would increase the mortality of the human smallpox virus. What underscores the future hazard here is that the research was quickly published in the open scientific literature . It is hard to see how information generated in open biotech research programs could be contained no matter how grave the potential danger that it poses; and the same holds for research in nanotechnology.

Genetic medicine will also lead to better cures and vaccines, but there is no guarantee that defense will always keep pace with offense. (Even the accidentally created mousepox virus had a 50% mortality rate on vaccinated mice.) Eventually, worry about biological weapons may be put to rest through the development of nanomedicine, but while nanotechnology has enormous long-term potential for medicine it carries its own hazards.

• 

4.6 Accidental misuse of nanotechnology (“gray goo”)

The possibility of accidents can never be completely ruled out. However, there are many ways of making sure, through responsible engineering practices, that species-destroying accidents do not occur. One could avoid using self-replication; one could make nanobots dependent on some rare feedstock chemical that doesn’t exist in the wild; one could confine them to sealed environments; one could design them in such a way that any mutation was overwhelmingly likely to cause a nanobot to completely cease to function . Accidental misuse is therefore a smaller concern than malicious misuse.

However, the distinction between the accidental and the deliberate can become blurred. While “in principle” it seems possible to make terminal nanotechnological accidents extremely improbable, the actual circumstances may not permit this ideal level of security to be realized. Compare nanotechnology with nuclear technology. From an engineering perspective, it is of course perfectly possible to use nuclear technology only for peaceful purposes such as nuclear reactors, which have a zero chance of destroying the whole planet. Yet in practice it may be very hard to avoid nuclear technology also being used to build nuclear weapons, leading to an arms race. With large nuclear arsenals on hair-trigger alert, there is inevitably a significant risk of accidental war. The same can happen with nanotechnology: it may be pressed into serving military objectives in a way that carries unavoidable risks of serious accidents.

In some situations it can even be strategically advantageous to deliberately make one’s technology or control systems risky, for example in order to make a “threat that leaves something to chance”.

4.7 Something unforeseen

We need a catch-all category. It would be foolish to be confident that we have already imagined and anticipated all significant risks. Future technological or scientific developments may very well reveal novel ways of destroying the world.

Some foreseen hazards (hence not members of the current category) which have been excluded from the list of bangs on grounds that they seem too unlikely to cause a global terminal disaster are: solar flares, supernovae, black hole explosions or mergers, gamma-ray bursts, galactic center outbursts, supervolcanos, loss of biodiversity, buildup of air pollution, gradual loss of human fertility, and various religious doomsday scenarios. The hypothesis that we will one day become “illuminated” and commit collective suicide or stop reproducing, as supporters of VHEMT (The Voluntary Human Extinction Movement) hope, appears unlikely. If it really were better not to exist (as Silenus told King Midas in the Greek myth, and as Arthur Schopenhauer argued although for reasons specific to his philosophical system he didn’t advocate suicide), then we should not count this scenario as an existential disaster. The assumption that it is not worse to be alive should be regarded as an implicit assumption in the definition of Bangs. Erroneous collective suicide is an existential risk albeit one whose probability seems extremely slight.

• 

4.8 Physics disasters

The Manhattan Project bomb-builders’ concern about an A-bomb-derived atmospheric conflagration has contemporary analogues.

There have been speculations that future high-energy particle accelerator experiments may cause a breakdown of a metastable vacuum state that our part of the cosmos might be in, converting it into a “true” vacuum of lower energy density . This would result in an expanding bubble of total destruction that would sweep through the galaxy and beyond at the speed of light, tearing all matter apart as it proceeds.

Another conceivability is that accelerator experiments might produce negatively charged stable “strangelets” (a hypothetical form of nuclear matter) or create a mini black hole that would sink to the center of the Earth and start accreting the rest of the planet.

These outcomes seem to be impossible given our best current physical theories. But the reason we do the experiments is precisely that we don’t really know what will happen. A more reassuring argument is that the energy densities attained in present day accelerators are far lower than those that occur naturally in collisions between cosmic rays. It’s possible, however, that factors other than energy density are relevant for these hypothetical processes, and that those factors will be brought together in novel ways in future experiments.

The main reason for concern in the “physics disasters” category is the meta-level observation that discoveries of all sorts of weird physical phenomena are made all the time, so even if right now all the particular physics disasters we have conceived of were absurdly improbable or impossible, there could be other more realistic failure-modes waiting to be uncovered. The ones listed here are merely illustrations of the general case.

• 

4.9 Naturally occurring disease

What if AIDS was as contagious as the common cold?

There are several features of today’s world that may make a global pandemic more likely than ever before. Travel, food-trade, and urban dwelling have all increased dramatically in modern times, making it easier for a new disease to quickly infect a large fraction of the world’s population.

• 

4.10 Asteroid or comet impact

There is a real but very small risk that we will be wiped out by the impact of an asteroid or comet.

In order to cause the extinction of human life, the impacting body would probably have to be greater than 1 km in diameter (and probably 3 - 10 km). There have been at least five and maybe well over a dozen mass extinctions on Earth, and at least some of these were probably caused by impacts. In particular, the K/T extinction 65 million years ago, in which the dinosaurs went extinct, has been linked to the impact of an asteroid between 10 and 15 km in diameter on the Yucatan peninsula. It is estimated that a 1 km or greater body collides with Earth about once every 0.5 million years We have only catalogued a small fraction of the potentially hazardous bodies.

If we were to detect an approaching body in time, we would have a good chance of diverting it by intercepting it with a rocket loaded with a nuclear bomb.

• 

4.11 Runaway global warming

One scenario is that the release of greenhouse gases into the atmosphere turns out to be a strongly self-reinforcing feedback process. Maybe this is what happened on Venus, which now has an atmosphere dense with CO₂ and a temperature of about 450^O C. Hopefully, however, we will have technological means of counteracting such a trend by the time it would start getting truly dangerous.

---

5 Crunches

While some of the events described in the previous section would be certain to actually wipe out Homo sapiens (e.g. a breakdown of a meta-stable vacuum state) others could potentially be survived (such as an all-out nuclear war). If modern civilization were to collapse, however, it is not completely certain that it would arise again even if the human species survived. We may have used up too many of the easily available resources a primitive society would need to use to work itself up to our level of technology. A primitive human society may or may not be more likely to face extinction than any other animal species. But let’s not try that experiment.

If the primitive society lives on but fails to ever get back to current technological levels, let alone go beyond it, then we have an example of a crunch. Here are some potential causes of a crunch:

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5.1 Resource depletion or ecological destruction

The natural resources needed to sustain a high-tech civilization are being used up. If some other cataclysm destroys the technology we have, it may not be possible to climb back up to present levels if natural conditions are less favorable than they were for our ancestors, for example if the most easily exploitable coal, oil, and mineral resources have been depleted. (On the other hand, if plenty of information about our technological feats is preserved, that could make a rebirth of civilization easier.)

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5.2 Misguided world government or another static social equilibrium stops technological progress

One could imagine a fundamentalist religious or ecological movement one day coming to dominate the world. If by that time there are means of making such a world government stable against insurrections (by advanced surveillance or mind-control technologies), this might permanently put a lid on humanity’s potential to develop to a posthuman level. Aldous Huxley’s Brave New World is a well-known scenario of this type.

A world government may not be the only form of stable social equilibrium that could permanently thwart progress. Many regions of the world today have great difficulty building institutions that can support high growth. And historically, there are many places where progress stood still or retreated for significant periods of time. Economic and technological progress may not be as inevitable as is appears to us.

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5.3 “Dysgenic” pressures

It is possible that advanced civilized society is dependent on there being a sufficiently large fraction of intellectually talented individuals. Currently it seems that there is a negative correlation in some places between intellectual achievement and fertility. If such selection were to operate over a long period of time, we might evolve into a less brainy but more fertile species, homo philoprogenitus (“lover of many offspring”).

However, contrary to what such considerations might lead one to suspect, IQ scores have actually been increasing dramatically over the past century. This is known as the Flynn effect. It’s not yet settled whether this corresponds to real gains in important intellectual functions.

Moreover, genetic engineering is rapidly approaching the point where it will become possible to give parents the choice of endowing their offspring with genes that correlate with intellectual capacity, physical health, longevity, and other desirable traits.

In any case, the time-scale for human natural genetic evolution seems much too grand for such developments to have any significant effect before other developments will have made the issue moot.

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5.4 Technological arrest

The sheer technological difficulties in making the transition to the posthuman world might turn out to be so great that we never get there.

5.5 Something unforeseen

As before, a catch-all.

Overall, the probability of a crunch seems much smaller than that of a bang. We should keep the possibility in mind but not let it play a dominant role in our thinking at this point. If technological and economical development were to slow down substantially for some reason, then we would have to take a closer look at the crunch scenarios.

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6 Shrieks

Determining which scenarios are shrieks is made more difficult by the inclusion of the notion of desirability in the definition. Unless we know what is “desirable”, we cannot tell which scenarios are shrieks. However, there are some scenarios that would count as shrieks under most reasonable interpretations.

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6.1 Take-over by a transcending upload

Suppose uploads come before human-level artificial intelligence. An upload is a mind that has been transferred from a biological brain to a computer that emulates the computational processes that took place in the original biological neural network. A successful uploading process would preserve the original mind’s memories, skills, values, and consciousness. Uploading a mind will make it much easier to enhance its intelligence, by running it faster, adding additional computational resources, or streamlining its architecture. One could imagine that enhancing an upload beyond a certain point will result in a positive feedback loop, where the enhanced upload is able to figure out ways of making itself even smarter; and the smarter successor version is in turn even better at designing an improved version of itself, and so on. If this runaway process is sudden, it could result in one upload reaching superhuman levels of intelligence while everybody else remains at a roughly human level. Such enormous intellectual superiority may well give it correspondingly great power. It could rapidly invent new technologies or perfect nanotechnological designs, for example. If the transcending upload is bent on preventing others from getting the opportunity to upload, it might do so.

The posthuman world may then be a reflection of one particular egoistical upload’s preferences (which in a worst case scenario would be worse than worthless). Such a world may well be a realization of only a tiny part of what would have been possible and desirable. This end is a shriek.

6.2 Flawed superintelligence

Again, there is the possibility that a badly programmed superintelligence takes over and implements the faulty goals it has erroneously been given.

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6.3 Repressive totalitarian global regime

Similarly, one can imagine that an intolerant world government, based perhaps on mistaken religious or ethical convictions, is formed, is stable, and decides to realize only a very small part of all the good things a posthuman world could contain.

Such a world government could conceivably be formed by a small group of people if they were in control of the first superintelligence and could select its goals. If the superintelligence arises suddenly and becomes powerful enough to take over the world, the posthuman world may reflect only the idiosyncratic values of the owners or designers of this superintelligence. Depending on what those values are, this scenario would count as a shriek.

6.4 Something unforeseen.

The catch-all.

These shriek scenarios appear to have substantial probability and thus should be taken seriously in our strategic planning.

One could argue that one value that makes up a large portion of what we would consider desirable in a posthuman world is that it contains as many as possible of those persons who are currently alive. After all, many of us want very much not to die (at least not yet) and to have the chance of becoming posthumans. If we accept this, then any scenario in which the transition to the posthuman world is delayed for long enough that almost all current humans are dead before it happens (assuming they have not been successfully preserved via cryonics arrangements) would be a shriek. Failing a breakthrough in life-extension or widespread adoption of cryonics, then even a smooth transition to a fully developed posthuman eighty years from now would constitute a major existential risk, if we define “desirable” with special reference to the people who are currently alive. This “if”, however, is loaded with a profound axiological problem that we shall not try to resolve here.

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7 Whimpers

If things go well, we may one day run up against fundamental physical limits. Even though the universe appears to be infinite, the portion of the universe that we could potentially colonize is (given our admittedly very limited current understanding of the situation) finite , and we will therefore eventually exhaust all available resources or the resources will spontaneously decay through the gradual decrease of negentropy and the associated decay of matter into radiation. But here we are talking astronomical time-scales. An ending of this sort may indeed be the best we can hope for, so it would be misleading to count it as an existential risk. It does not qualify as a whimper because humanity could on this scenario have realized a good part of its potential.

Two whimpers (apart form the usual catch-all hypothesis) appear to have significant probability:

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7.1 Our potential or even our core values are eroded by evolutionary development

This scenario is conceptually more complicated than the other existential risks we have considered (together perhaps with the “We are living in a simulation that gets shut down” bang scenario).

A related scenario is described in [62], which argues that our “cosmic commons” could be burnt up in a colonization race. Selection would favor those replicators that spend all their resources on sending out further colonization probes.

Although the time it would take for a whimper of this kind to play itself out may be relatively long, it could still have important policy implications because near-term choices may determine whether we will go down a track that inevitably leads to this outcome. Once the evolutionary process is set in motion or a cosmic colonization race begun, it could prove difficult or impossible to halt it. It may well be that the only feasible way of avoiding a whimper is to prevent these chains of events from ever starting to unwind.

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7.2 Killed by an extraterrestrial civilization

The probability of running into aliens any time soon appears to be very small.

If things go well, however, and we develop into an intergalactic civilization, we may one day in the distant future encounter aliens. If they were hostile and if (for some unknown reason) they had significantly better technology than we will have by then, they may begin the process of conquering us. Alternatively, if they trigger a phase transition of the vacuum through their high-energy physics experiments (see the Bangs section) we may one day face the consequences. Because the spatial extent of our civilization at that stage would likely be very large, the conquest or destruction would take relatively long to complete, making this scenario a whimper rather than a bang.

7.3 Something unforeseen

The catch-all hypothesis.

The first of these whimper scenarios should be a weighty concern when formulating long-term strategy. Dealing with the second whimper is something we can safely delegate to future generations (since there’s nothing we can do about it now anyway).

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Simplistic warfare: As Larry Niven pointed out, any space drive that obeys the law of conservation of energy is a weapon of efficiency proportional to its efficiency as a propulsion system. Today's boringly old-hat chemical rockets, even in the absence of nuclear warheads, are formidably destructive weapons: if you can boost a payload up to relativistic speed, well, the kinetic energy of a 1Kg projectile traveling at just under 90% of c (τ of 0.5) is on the order of 20 megatons. Slowing down doesn't help much: even at 1% of c that 1 kilogram bullet packs the energy of a kiloton-range nuke. War, or other resource conflicts, within a polity capable of rapid interplanetary or even slow interstellar flight, is a horrible prospect.

Irreducible complexity: I take issue with one of Anders' assumptions, which is that a multi-planet civilization is largely immune to the local risks. It will not just be distributed, but it will almost by necessity have fairly self-sufficient habitats that could act as seeds for a new civilization if they survive. I've rabbited on about this in previous years: briefly, I doubt that we could make a self-sufficient habitat that was capable of maintaining its infrastructure and perpetuating and refreshing its human culture with a population any smaller than high-single-digit millions; lest we forget, our current high-tech infrastructure is the climax product of on the order of 1-2 billion developed world citizens, and even if we reduce that by an order of magnitude (because who really needs telephone sanitizer salesmen, per Douglas Adams?) we're still going to need a huge population to raise, train, look after, feed, educate, and house the various specialists. Worse: we don't have any real idea how many commensal microbial species we depend on living in our own guts to help digest our food and prime our immune systems, never mind how many organisms a self-sustaining human-supporting biosphere needs (not just sheep to eat, but grass for the sheep to graze on, fungi to break down the sheep droppings, gut bacteria in the sheep to break down the lignin and cellulose, and so on).

I don't rule out the possibility of building robust self-sufficient off-world habitats. The problem I see is that it's vastly more expensive than building an off-world outpost and shipping rations there, as we do with Antarctica — and our economic cost/benefit framework wouldn't show any obvious return on investment for self-sufficiency.

So our future-GF need not be a solar-system-wide disaster: it might simply be one that takes out our home world before the rest of the solar system is able to survive without it. For example, if the resource extraction and energy demands of establishing self-sufficient off-world habitats exceed some critical threshold that topples Earth's biosphere into a runaway Greenhouse effect or pancakes some low-level but essential chunk of the biosphere (a The Death of Grass scenario) that might explain the silence.

Griefers: suppose some first-mover in the interstellar travel stakes decides to take out the opposition before they become a threat. What is the cheapest, most cost-effective way to do this?

Both the IO9 think-piece and Anders' response get somewhat speculative, so I'm going to be speculative as well. I'm going to take as axiomatic the impossibility of FTL travel and the difficulty of transplanting sapient species to other worlds (the latter because terraforming is a lot harder than many SF fans seem to believe, and us squishy meatsacks simply aren't constructed with interplanetary travel in mind). I'm also going to tap-dance around the question of a singularity, or hostile AI. But suppose we can make self-replicating robots that can build a variety of sub-assemblies from a canned library of schematics, building them out of asteroidal debris? It's a tall order with a lot of path dependencies along the way, but suppose we can do that, and among the assemblies they can build are photovoltaic cells, lasers, photodetectors, mirrors, structural trusses, and their own brains.

What we have is a Von Neumann probe — a self-replicating spacecraft that can migrate slowly between star systems, repair bits of itself that break, and where resources permit, clone itself. Call this the mobile stage of the life-cycle. Now, when it arrives in a suitable star system, have it go into a different life-cycle stage: the sessile stage. Here it starts to spawn copies of itself, and they go to work building a Matrioshka Brain. However, contra the usual purpose of a Matrioshka Brain (which is to turn an entire star system's mass into computronium plus energy supply, the better to think with) the purpose of this Matrioshka Brain is rather less brainy: its free-flying nodes act as a very long baseline interferometer, mapping nearby star systems for planets, and scanning each exoplanet for signs of life.

Then, once it detects a promising candidate — within a couple of hundred light years, oxygen atmosphere, signs of complex molecules, begins shouting at radio wavelengths then falls suspiciously quiet — it says "hello" with a Nicoll-Dyson Beam.

(It's not expecting a reply: to echo Auric Goldfinger: "no Mr Bond, I expect you to die.")

A Dyson sphere or Matrioshka Brain collects most or all of the radiated energy of a star using photovoltaic collectors on the free-flying elements of the Dyson swarm. Assuming they're equipped with lasers for direct line-of-sight communication with one another isn't much of a reach. Building bigger lasers, able to re-radiate all the usable power they're taking in, isn't much more of one. A Nicoll-Dyson beam is what you get when the entire emitted energy of a star is used to pump a myriad of high powered lasers, all pointing in the same direction. You could use it to boost a light sail with a large payload up to a very significant fraction of light-speed in a short time ... and you could use it to vapourize an Earth-mass planet in under an hour, at a range of hundreds of light years.

Here's the point: all it takes is one civilization of alien ass-hat griefers who send out just one Von Neumann Probe programmed to replicate, build N-D lasers, and zap any planet showing signs of technological civilization, and the result is a galaxy sterile of interplanetary civilizations until the end of the stelliferous era (at which point, stars able to power an N-D laser will presumably become rare).

We have plenty of griefers who like destroying things, even things they've never seen and can't see the point of. I think the N-D laser/Von Neumann Probe option is a worryingly plausible solution to the identity of a near-future Great Filter: it only has to happen once, and it f**ks everybody.

(ed note: Robin Hanson is the briliant man who originated the entire concept of The Great Filter)

These are indeed scenarios of concern. But I find it hard to see how, by themselves, they could add up to a big future filter.

On griefers (aka “berserkers”), a griefer equilibrium seems to me unstable to their trying sometimes to switch to rapid growth within a sufficiently large volume that they seem to control. Sometimes that will fail, but once it succeeds enough then competing griefers have little chance to stop them. Yes there’s a chance the first civilization to make them didn’t think to encode that strategy, but that seems a pretty small filter factor.

On simple war, I find it hard to see how war has a substantial chance of killing everyone unless the minimum viable civilization size is large. And I agree that this min size gets bigger for humans in space, who are more fragile there. But it should get smaller for smart robots in space, or on Earth, especially if production becomes more local via nano-factories. The chance that the last big bomb used in a war happens to kill off the last viable group of survivors seems to me relatively small.

Of course none of these chances are low enough to justify complacency. We should explore such scenarios, and work to prevent them. But we should work even harder to find more worrisome scenarios.

So let me explain my nightmare scenario: general non-diminishing prey. Consider the classic post-apocalyptic scenario, such as described in The Road. Desperate starving people ignore the need to save and build for the future, and grab any food they can find, including each other. First all the non-human food is gone, then all the people.

Such situations have been modeled formally via “predator-prey dynamics”:

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These are differential equations giving the rates at which counts of predators and prey grow or decline as a function of each other. The standard formulation has a key term whereby prey count falls in proportional to the product of the predator count and the prey count. This formulation embodies an important feature of diminishing returns: the fewer prey are left, the harder it is for predators to find and eat them.

Without enough such diminishing returns, any excess of predators quickly leads to the extinction of prey, followed quickly by the extinction of predators. For example, when starving humans are given easy access to granaries, such granaries are emptied quickly. Not made low; emptied. Which is why granaries in famines are usually either well-protected, or empty.

In nature, there are usually many kinds of predators, and even more kinds of prey. So the real predator-prey dynamic is high-dimensional. The pairwise relations between most predators and preys do in fact usually involve strongly diminishing returns, both because predators must usually search for prey, and because some prey hiding places are much better than others.

If the relation between any one pair of predator and prey types happens to have no diminishing returns, then that particular type of prey will go extinct whenever there is a big enough excess of that particular type of predator. Since this selects against such prey, the prey we see in nature almost all have diminishing returns for all their practical predators.

Humans are general predators, able to eat a great many kinds of prey. And within human societies humans are also relatively general kinds of prey, since we mostly all use the same kinds of resources. So when humans prey on humans, the human prey can more easily go extinct.

For foragers, a key limit on human predation was simple distance. Foragers lived far apart, and were unpredictably located. Also, foragers had little physical property to grab, wives were not treated as property, and land was too plentiful to be worth grabbing. These limits mattered less for farmers, who did predate often via war.

The usual source of diminishing returns in farmer war predation has been the wide range of protection in places to hide; humans have often run to the mountains, jungle, or sea to escape human predators. Even so, humans and proto-humans have quite often driven one another to local relative extinction.

While the extinction of some kinds of humans relative to others has been common, the extinction of all humans in an area has been much less common. This is in part because, when there has been a local excess of humans, most have focused on non-human prey. Such prey are diverse, and most have strongly diminishing returns to human predation.

Even if humans expand into the solar system, and even if they create robot descendants, we expect our descendants to remain relatively general predators, at least for a long while. We also expect the physical resources that they collect to constitute relatively general prey, useful to a wide range of our descendants. Furthermore, we expect nature that isn’t domesticated or descended from humans to hold a decreasing quantity of useful resources.

Thus the future predator-prey dynamic should become lower dimensional than it has been in the past. To a perhaps useful approximation, there’d be only a few kinds of predators and prey. Which raises the key question: how strong are the diminishing returns to predation in that new world? That is, when some of our descendants hunt down others to grab resources, how fast does that task get harder as fewer prey remain?

One source of diminishing returns in predation is a diversity of approaches and interfaces. The more different are the methods that prey use to create and store value, the smaller the fraction of that value a predator can obtain via a simple hostile takeover. This increases the ratio of how hard prey and prey fight. As many have noted, in nature prey fight for their lives, while predators fight only for a meal. Even so, nature still has plenty of predation. Even if predators gain only part of the value contained in prey, they still predate if that costs them even less than this value.

As I said above, the main source of diminishing returns in predation among foragers was travel cost, and among farmers it was the diversity of physical places to run and hide. Such effects might still protect our descendants from predator-prey-dynamic extinction, even if they have only one kind of predator and prey. Alas, we have good reasons to fear that these factors may less protect our descendants.

The basic problem here is our improving techs for travel, communication, and surveillance. We are steadily able to move bits and people more cheaply, and to more cheaply and accurately watch spaces for activity. Yes moving out into the solar system would put more distance between things, and make them harder to see. But that one-time effect will be quickly overwhelmed by improving tech.

A colonized solar system is plausibly a place where predators can see most any civilized activities of any substantial magnitude, and get to them easily if not quickly. So if we ever reach a point where predators fight to grab civilized resources with little concern to save some for the future, they might be able to find and grab pretty much everything in the solar system. Much as easy-access granaries are quickly emptied in a famine.

Whether extinction results from such a scenario depends how small are minimum viable civilization seeds, how obscure and well protected are the nooks and crannies in which they might hide, and how many of them exist and try to hide. Yes, hidden viable seeds drifting at near light-speed to other stars could prevent extinction, but such a prey-collapse scenario could play out well before such seeds are feasible.

So, bottom line, the future great filter scenario that most concerns me is one where our solar-system-bound descendants have killed most of nature, can’t yet colonize other stars, are general predators and prey of each other, and have fallen into a short-term-predatory-focus equilibrium where predators can easily see and travel to most all prey. Yes there are about a hundred billion comets way out there circling the sun, but even that seems a small enough number for predators to careful map and track all of them.

Worry about prey-extinction scenarios like this is a reason I’ve focused on hidden refuges as protection from existential risk. Nick Beckstead has argued against refuges saying:

The most likely ways in which improved refuges could help humanity recover from a global catastrophe are scenarios in which well-stocked refuges with appropriately trained people help civilization to recover after a catastrophe that leaves a substantial portion of humanity alive but disrupts industrial and agricultural infrastructure, and scenarios in which only people in constantly-staffed refuges survive a pandemic purposely engineered to cause human extinction. I would guess that, in the former case, resources and people stocked in refuges would play a relatively small role in helping humanity to recover because they would represent a small share of relevant people and resources. The latter case strikes me as relatively far-fetched and I would guess it would be very challenging to do much better than the largely uncontacted peoples in terms of ensuring the survival of the species. (more)

Nick does at one point seem to point to the scenario that concerns me:

If a refuge is sufficiently isolated and/or secret, it would be easier to ensure that everyone in the refuge had an adequate food supply, even if that meant an inegalitarian food distribution.

But he doesn’t appear to think this relevant for his conclusions. In contrast, I fear that a predatory-collapse scenario is the most likely future great filter, where unequal survival key to preventing extinction.

Added 10a: Of course the concern isn’t just that some parties would have short term orientations, but that most would pursue short-term predation so vigorously that they force most everyone to put in similar effort levels, even if they take have long-term view. When enemies mass on the border, one might have to turn farmers into soldiers to resist them, even if it is harvest time.

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(ed note: The researchers under Dr. Coffin have discovered a cure for the common cold! They rush the tests, fearing that they will be scooped by another lab. Dr. Phillip's wife Ellie has a cold but she'd refuses the cure since she is cared of needles. So she is currently speaking funny due to a stuffed up nose. But two months after he got the treatement, Dr. Phillip wakes up to .... horror! )

      He awoke, suddenly, began to stretch, and sat bolt upright in bed, staring wildly about the room. Pale morning sunlight drifted in the window. Downstairs he heard Ellie stirring in the kitchen.
     For a moment he thought he was suffocating. He leaped out of bed, stared at the vanity table across the room. "Somebody's spilled the whole damned bottle—"
     The heavy sick-sweet miasma hung like a cloud around him, drenching the room. With every breath it grew thicker. He searched the table top frantically, but there were no empty bottles. His head began to spin from the sickening effluvium.
     He blinked in confusion, his hand trembling as he lit a cigarette. No need to panic, he thought. She probably knocked a bottle over when she was dressing. He took a deep puff, and burst into a paroxysm of coughing as acrid fumes burned down his throat to his lungs.
     "Ellie!" He rushed into the hall, still coughing. The match smell had given way to the harsh, caustic stench of burning weeds. He stared at his cigarette in horror and threw it into the sink. The smell grew worse. He threw open the hall closet, expecting smoke to come billowing out. "Ellie! Somebody's burning down the house!"
     "Whadtever are you talking about?" Ellie's voice came from the stair well. "It's just the toast I burned, silly."
     He rushed down the stairs two at a time—and nearly gagged as he reached the bottom. The smell of hot, rancid grease struck him like a solid wall. It was intermingled with an oily smell of boiled and parboiled coffee, overpowering in its intensity. By the time he reached the kitchen he was holding his nose, tears pouring from his eyes. "Ellie, what are you doing in here?"
     She stared at him. "I'b baking breakfast."
     "But don't you smell it?"
     "Sbell whadt?" said Ellie.
     On the stove the automatic percolator was making small, promising noises. In the frying pan four sunnyside eggs were sizzling; half a dozen strips of bacon drained on a paper towel on the sideboard. It couldn't have looked more innocent.
     Cautiously, Phillip released his nose, sniffed. The stench nearly choked him. "You mean you don't smell anything strange?"
     "I did't sbell eddythig, period," said Ellie defensively.
     "The coffee, the bacon—come here a minute."
     She reeked—of bacon, of coffee, of burned toast, but mostly of perfume. "Did you put on any fresh perfume this morning?"
     "Before breakfast? Dod't be ridiculous."
     "Not even a drop?" Phillip was turning very white.
     "Dot a drop."
     He shook his head. "Now, wait a minute. This must be all in my mind. I'm—just imagining things, that's all. Working too hard, hysterical reaction. In a minute it'll all go away." He poured a cup of coffee, added cream and sugar.
     But he couldn't get it close enough to taste it. It smelled as if it had been boiling three weeks in a rancid pot. It was the smell of coffee, all right, but a smell that was fiendishly distorted, overpoweringly, nauseatingly magnified. It pervaded the room and burned his throat and brought tears gushing to his eyes.
     Slowly, realization began to dawn. He spilled the coffee as he set the cup down. The perfume. The coffee. The cigarette....
     "My hat," he choked. "Get me my hat. I've got to get to the laboratory."

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     It got worse all the way downtown. He fought down waves of nausea as the smell of damp, rotting earth rose from his front yard in a gray cloud. The neighbor's dog dashed out to greet him, exuding the great-grandfather of all doggy odors. As Phillip waited for the bus, every passing car fouled the air with noxious fumes, gagging him, doubling him up with coughing as he dabbed at his streaming eyes.
     Nobody else seemed to notice anything wrong at all.
     The bus ride was a nightmare. It was a damp, rainy day; the inside of the bus smelled like the men's locker room after a big game. A bleary-eyed man with three-days' stubble on his chin flopped down in the seat next to him, and Phillip reeled back with a jolt to the job he had held in his student days, cleaning vats in the brewery.
     "It'sh a great morning," Bleary-eyes breathed at him, "huh, Doc?" Phillip blanched. To top it, the man had had a breakfast of salami. In the seat ahead, a fat man held a dead cigar clamped in his mouth like a rank growth. Phillip's stomach began rolling; he sank his face into his hand, trying unobtrusively to clamp his nostrils. With a groan of deliverance he lurched off the bus at the laboratory gate.

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     A gray-faced Dr. Coffin unlocked the door, backed quickly toward the wall. The room reeked of kitchen deodorant. "Stay right where you are," Coffin squeaked. "Don't come a step closer. I can't see you now. I'm—I'm busy, I've got work that has to be done—"
     "You're telling me," growled Phillip. He motioned Jake into the office and locked the door carefully. Then he turned to Coffin. "When did it start for you?"
     Coffin was trembling. "Right after supper last night. I thought I was going to suffocate. Got up and walked the streets all night. My God, what a stench!"
     "Jake?"
     Dr. Miles shook his head. "Sometime this morning, I don't know when. I woke up with it."
     "That's when it hit me," said Phillip.
     "But I don't understand," Coffin howled. "Nobody else seems to notice anything—"
     "Yet," said Phillip, "we were the first three to take the Coffin Cure, remember? You, and me and Jake. Two months ago."
     Coffin's forehead was beaded with sweat. He stared at the two men in growing horror. "But what about the others?" he whispered.
     "I think," said Phillip, "that we'd better find something spectacular to do in a mighty big hurry. That's what I think."

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     "But what's happened?" Coffin cried. "These foul smells, everywhere. You, Phillip, you had a cigarette this morning. I can smell it clear over here, and it's bringing tears to my eyes. And if I didn't know better I'd swear neither of you had had a bath in a week. Every odor in town has suddenly turned foul—"
     "Magnified, you mean," said Jake. "Perfume still smells sweet—there's just too much of it. The same with cinnamon; I tried it. Cried for half an hour, but it still smelled like cinnamon. No, I don't think the smells have changed any."
     "But what, then?"
     "Our noses have changed, obviously." Jake paced the floor in excitement. "Look at our dogs! They've never had colds—and they practically live by their noses. Other animals—all dependent on their senses of smell for survival—and none of them ever have anything even vaguely reminiscent of a common cold. The multicentric virus hits primates only—and it reaches its fullest parasitic powers in man alone!"
     Coffin shook his head miserably. "But why this horrible stench all of a sudden? I haven't had a cold in weeks—"
     "Of course not! That's just what I'm trying to say," Jake cried. "Look, why do we have any sense of smell at all? Because we have tiny olfactory nerve endings buried in the mucous membrane of our noses and throats. But we have always had the virus living there, too, colds or no colds, throughout our entire lifetime. It's always been there, anchored in the same cells, parasitizing the same sensitive tissues that carry our olfactory nerve endings, numbing them and crippling them, making them practically useless as sensory organs. No wonder we never smelled anything before! Those poor little nerve endings never had a chance!"
     "Until we came along in our shining armor and destroyed the virus," said Phillip.
     "Oh, we didn't destroy it. We merely stripped it of a very slippery protective mechanism against normal body defences." Jake perched on the edge of the desk, his dark face intense. "These two months since we had our shots have witnessed a battle to the death between our bodies and the virus. With the help of the vaccine, our bodies have won, that's all—stripped away the last vestiges of an invader that has been almost a part of our normal physiology since the beginning of time. And now for the first time those crippled little nerve endings are just beginning to function."
     "God help us," Coffin groaned. "You think it'll get worse?"
     "And worse. And still worse," said Jake.

---

     The tidal wave began to break two days later ... only a few people here, a dozen there, but enough to confirm the direst newspaper predictions. The boomerang was completing its circle.
     At the laboratory the doors were kept barred, the telephones disconnected. Within, there was a bustle of feverish—if odorous—activity. For the three researchers, the olfactory acuity had reached agonizing proportions. Even the small gas masks Phillip had devised could no longer shield them from the constant barrage of violent odors.

(ed note: Eventually they manage to discover a cure for the cure. With the side effect that they will have head-colds for the rest of their lives.)

• 

      Even here, even downtown, where the bones sprawled everywhere were those of strangers, I was wound up tight enough to burst, and ancient reflex fought with modern wisdom as I felt the irrational impulse to lift my head and cast about for an enemy’s scent.
     I saw again in my mind’s eye the familiar Carlson Poster, the first thing my father ran off when he got access to a mimeograph machine: a remarkably detailed sketch of thin, academic features surrounded by a mass of graying hair, with the legend, “WANTED: FOR THE MURDER OF HUMAN CIVILIZATION—WENDELL MORGAN CARLSON. An unlimited lifetime supply of hot-shot shells (magnesium incendiary bullets) will be given to anyone bringing the above head to the Council of Fresh Start.”

---

     Excerpts from I Worked With Carlson,
     by Jacob Stone, Ph.D., authorized version: Fresh Start Press 1986 (Mimeo)

     … The sense of smell is a curious phenomenon, oddly resistant to measurement or rigorous analysis. Each life form on Earth appears to have as much of it as they need to survive, plus a little. The natural human sense of smell, for instance, was always more efficient than most people realized, so much so that in the 1880s the delightfully eccentric Sir Francis Galton had actually succeeded, by associating numbers with certain scents, in training himself to add and subtract by smell, apparently just for the intellectual exercise.
     But through a sort of neurological suppressor circuit of which next to nothing is known, most people contrived to ignore all but the most pleasing or disturbing of the messages their noses brought them, perhaps by way of reaction to a changing world in which a finely-tuned olfactory apparatus became a nuisance rather than a survival aid. The level of sensitivity which a wolf requires to find food would be a hindrance to a civilized human packed into a city of his fellows.
     By 1982, Professor Wendell Carlson had raised olfactometry to the level of a precise science. In the course of testing the theories of Beck and Miles, Carlson almost absentmindedly perfected the classic “blast-injection” technique of measuring differential sensitivity in olfaction, without regard for the subjective impressions of the test subject. This not only refined his data, but also enabled him to work with life forms other than human, a singular advantage when one considers how much of the human brain is terra incognita.
     His first subsequent experiments indicated that the average wolf utilized his sense of smell on the order of a thousand times more efficiently than a human. Carlson perceived that wolves lived in a world of scents, as rich and intricate as our human worlds of sight and word. To his surprise, however, he discovered that the potential sensitivity of the human olfactory apparatus far outstripped that of any known species.

     This intrigued him …

     Genius he unquestionably was; it was not serendipity that brought him the Nobel Prize for isolating a cure for the entire spectrum of virus infections called “the common cold.”
     And so when I went to visit Carlson in Jacobi Hospital and the conversation turned to the self-defeating nature of uncontrollable rage, I attempted to distract the patient with a hasty change of subject.
     “You are probably the world’s greatest authority on olfactometry and the human olfactory apparatus—among other things—right?”
     “As far as there is one, I suppose so. What of it?” He shifted uneasily within his traction gear: wearing his radical persona, he was made uncomfortable by reference to his scientist-mode. He felt it had little to do with the Realities of Life—like nightsticks and grand juries.
     “Has it ever occurred to you,” I persisted to my everlasting regret, “that nearly all the undesirable by-products of twentieth-century living—Technological Man’s most unlovable aspects—quite literally stink? The whole world’s going rancid, Wendell, not just the Movement: automobiles, factory pollution, crowded cities—Wendell, why couldn’t you develop a selective suppressant for the sense of smell—controlled anosmia? Oh, I know a snort of formaldehyde will do the trick, and having your adenoids removed sometimes works. But a man oughtn’t to have to give up the smell of frying bacon just to survive in New York. And you know we’re reaching that pass—in the past few years it hasn’t been necessary to leave the city and then return to be aware of how evil it smells. The natural suppressor-mechanism in the brain—whatever it is—has gone about as far as it can go. Why don’t you devise a small-spectrum filter to aid it? It would be welcomed by sanitation workers, engineers—why, it would be a godsend to the man on the street!”
     Carlson was mildly interested. Such an anosmic filter would be both a mordant political statement and a genuine boon to mankind. He had been vaguely pleased by the success of his cold cure, and I believe he sincerely wished to make the world a happier place—however perverted his methods tended to be. We discussed the idea at some length, and I left.

     Had Carlson not been bored silly in the hospital, he would never have rented a television set. It was extremely unfortunate that the “Late Show” (ed. note: a television show of the period) that evening featured the film version of Alistair MacLean’s The Satan Bug. Watching this absurd production, Carlson was intellectually repelled by the notion that a virus could be isolated so hellishly virulent that “a teaspoon of it would sweep the earth of life in a few days.”
     But it gave him a wild idea—a fancy, a fantasy, and a tasty one.
     He checked with me by phone the next day, very casually, and I assured him from my experiences with advances in virus-vectoring that MacLean had not been whistling in the dark. In fact, I told him, modern so-called “bacterial warfare” made the Satan Bug look like child’s play. Carlson thanked me and changed the subject.
     On his release from the hospital, he came to my office and asked me to work with him for a full year, to the exclusion of all else, on a project whose nature he was reluctant to discuss. “Why do you need me?” I asked, puzzled.
     “Because,” he finally told me, “you know how to make a Satan Bug. I intend to make a God Bug. And you could help me.”
     “Huh?”
     “Listen, Jake,” he said with that delightful informality of his. “I’ve licked the common cold—and there are still herds of people with the sniffles. All I could think of to do with the cure was to turn it over to the pharmaceuticals people, and I did all I could to make sure they didn’t milk it, but there are still suffering folks who can’t afford the damned stuff. Well, there’s no need for that. Jake, a cold will kill someone sufficiently weakened by hunger—I can’t help the hunger, but I could eliminate colds from the planet in forty-eight hours … with your help.”
     “A benevolent virus-vector …” I was flabbergasted, as much by the notion of decommercializing medicine as by the specific nostrum involved.
     “It’d be a lot of work,” Carlson went on. “In its present form my stuff isn’t compatible with such a delivery system—I simply wasn’t thinking along those lines. But I’ll bet it could be made so, with your help. Those pharmaceuticals goniffs have made me rich enough to pay you twice what Columbia does, and we’re both due for sabbatical anyway. What do you say?”
     I thought it over, but not enough. The notion of collaborating with a Nobel Prize winner was simply too tempting. “All right, Wendell.”

---

     And near the end of the year there came a day when he called me on the telephone. I was spending the weekend, as always, with my wife and two sons in Harlem. Christmas was approaching, and Barbara and I were discussing the relative merits of plastic and natural trees when the phone rang. I was not at all surprised to hear Carlson’s reedy voice, so reminiscent of an oboe lately—the only wonder was that he had called during conventional waking hours.
     “Jake,” he began without preamble, “I haven’t the time or inclination to argue, so shut up and listen, right? Right. I advise and strongly urge you to take your family and leave New York at once—steal a car if you have to, or hijack a Greyhound (ed. note: a public transportation conveyance) for all of me, but be at least twenty miles away by midnight.”
     “But—”
     “… head north if you want my advice, and for God’s sake stay away from all cities, towns, and people in any number. If you possibly can, get upwind of all nearby industry, and bring along all the formaldehyde you can—a gun too, if you own one. Goodbye, my friend, and remember I do this for the greater good of mankind. I don’t know if you’ll understand that, but I hope so.”

---

     “Wendell, what on earth is this all about? My wife is scared half to …”
     “Remember that controlled-anosmia you told me about when I was in the hospital?” he went on conversationally. “You said the trouble with the world is that it stinks, right?”
     I stared at him, vaguely recalling my words.
     “Well,” he said, “I’ve got a solution.”

---

• 

     At precisely nine o’clock, Wendell Morgan Carlson stood on the roof of Columbia’s enormous Butler Library, held high in the air by fake Greek columns and centuries of human thought, gazing north across a quadrangle within which grass and trees had nearly given up trying to grow, toward the vast domed Low Library and beyond toward the ghetto in which my wife and children were waiting, oblivious. In his hands he held the flask I had failed to wrest from him, and within it were approximately two teaspoons of an infinitely refined and concentrated virus culture. It was the end result of our year’s work, and it duplicated what the military had spent years and billions to obtain: a strain of virus that could blanket the globe in about forty-eight hours. There was no antidote for it, no vaccine, no defense of any kind for virtually all of humanity. It was diabolical, immoral and quite efficient. On the other hand, it was not lethal.
     Not, that is, in and of itself. But Carlson had concluded, like so many before him, that a few million lives was an acceptable price for saving the world, and so at 9:00 P.M. on December 17, 1984, he leaned over the parapet of Butler Hall and dropped his flask six long stories to the concrete below. It shattered on impact and sprayed its contents into what dismal breeze still blew through the campus.

     Carlson had said one word to me that afternoon, and the word was “Hyperosmia.”

     Within forty-eight hours every man, woman and child left alive on earth possessed a sense of smell approximately a hundred times more efficient than that of any wolf that ever howled.
     During those forty-eight hours, a little less than a fifth of the planet’s population perished, by whatever means they could devise, and every city in the world spilled its remaining life into the surrounding countryside. The ancient smell-suppressing system of the human brain collapsed under unbearable demand, overloaded and burned out in an instant.
     The great complex behemoth called Modern Civilization ground to a halt in a little less than two days. In the last hours, those pitiably few city-dwellers on the far side of the globe who were rigorous enough of thought to heed and believe the brief bewildered death-cries of the great mass media strove valiantly—and hopelessly—to effect emergency measures. The wiser attempted, as I had, to deaden their senses of smell with things like formaldehyde, but there is a limit to the amount of formaldehyde that even desperate men can lay hands on in a day or less, and its effects are generally temporary. Others with less vision opted for airtight environments if they could get them, and there they soon died, either by asphyxiation when their air supply ran out or by suicide when, fervently hoping they had outlived the virus, they cracked their airlocks at last. It was discovered that human technology had produced no commonly available noseplug worth a damn, nor any air-purification system capable of filtering out Carlson’s virus. Mankind failed utterly to check the effects of the ghastly Hyperosmic Plague, and the Exodus began… .

     … I don’t believe Carlson rejoiced over the carnage that ensued, though a strict Malthusian might have considered it as a long-overdue pruning. But it is easy to understand why he thought it was necessary, to visualize the “better world” for which he spent so many lives: Cities fallen to ruin. Automobiles rotting where they stood. Heavy industry gone to join the dinosaurs. The synthetic-food industry utterly undone. Perfume what it had always been best—a memory—as well as tobacco. A wave of cleanliness sweeping the globe, and public flatulence at last a criminal offense, punishable by death. Secaucus, New Jersey abandoned to the buzzards. The back-to-nature communalists achieving their apotheosis, helping to feed and instruct bewildered urban survivors (projected catch-phrase: “If you don’t like hippies, next time you’re hungry, call a cop”). The impetus of desperation forcing new developments in production of power by sun, wind and water rather than inefficient combustion of more precious resources. The long-delayed perfection of plumbing. And a profoundly interesting and far-reaching change in human mating customs as feigned interest or disinterest became unviable pretenses. (As any wolf could have told us, the scent of desire can be neither faked nor masked).

     All in all an observer as impartial as Carlson imagined himself to be might have predicted that an ultimate cost of perhaps thirty to forty percent of its population (no great loss), the world ten or twenty years after Carlson would be a much nicer place to live in.
     Instead and in fact, there are four billion fewer people living in it, and this year Two A.C. we have achieved only a bare possibility of survival at a cost of eighty to ninety percent of our number.

     The first thing Carlson could not have expected claimed over a billion and a half lives within the first month of the Brave New World. His compartmentalized mind had not been monitoring current developments in the field of psychology, a discipline he found frustrating. And so he was not aware of the work of Lynch and others, conclusively demonstrating that autism was the result of sensory overload. Autistic children, Lynch had proved, were victims of a physiochemical imbalance which disabled their suppressor circuitry for sight, hearing, touch, smell, or any combination thereof, flooding their brains with an intolerable avalanche of useless data and shocking them into retreat. Lysergic acid diethylamide is said to produce a similar effect, on a smaller scale.
     The Hyperosmic Virus produced a similar effect, on a larger scale. Within weeks, millions of near-catatonic adults and children perished from malnutrition, exposure, or accidental injury. Why some survived the shock and adapted, while some did not, remains a mystery, although there exist scattered data suggesting that those whose sense of smell was already relatively acute suffered most.

• 

"They're like locusts. They're moving from planet to planet, their entire civilization. Once they consume every natural resource, they move on."

— President Thomas Whitmore, Independence Day

Planet looters are a race of aliens that have run out of something, and must steal it from others — frequently, from us here on Earth. This is peculiar in that their demonstrated level of technology makes one wonder why they'd target Earth when it would be far easier to find an uninhabited planet and strip-mine that.

To Handwave that problem, they often need some particular resource which is supposedly rare. Water is apparently one of them, and countless aliens have needlessly lost their lives in futile attempts to steal our oceans. For some reason they overlook comets, dwarf planets and moons in the outer solar system which are not only made of mostly water in convenient prepackaged frozen form but don't have anyone out there to stop them from simply flying away with it.

At other times, we are the resource, and they want to take us as slaves or tasty, tasty food. If they need food, they might as well go steal the cattle equivalents on several planets that should be much closer, with even less native hostility. As for slavery, it's hard to imagine any sufficiently advanced race that has figured out interplanetary space travel would need slaves for any reason other than to fulfill a cultural, religious or egotistical need to conquer the galaxy's "weak". Although maybe they'd make good pets.

Another way around the question is to make Earth's abundant ecosystems and temperate climates the resource. In this version, the aliens simply view primitive humans as unworthy pests infesting an ideal new home or vacation spot. This variation may be Invading Refugees.

It's fairly popular to make humans fill this role (to the horror of the Space Elves), as a Green Aesop about exploitation of natural resources.

Usually results in an Alien Invasion. Compare Space Pirates, Horde of Alien Locusts, Planet Eater, Mars Needs Water, and Mars Needs Women.

(ed note: see TV Trope page for list of examples)

• 

      They were closing in now. The leader was a gray bulk filling my sight scope, and every time I glanced over the wall a spanging sleet of bullets brought my head jerking down again. I had some shelter from behind which to shoot in a fragment of wall looming higher than the rest, like a single tooth left in a dead man's jaw, but I had to squeeze the trigger and then duck fast. Once in awhile one of their slugs would burst on my helmet and the gas would be sickly-sweet in my nostrils. I felt ill and dizzy with it.
     Kathryn was reloading her own rifle, I heard her swearing as the cartridge clip jammed in the rusty old weapon. I'd have given her my own, except that it wasn't much better. It's no fun fighting with arms that are likely to blow up in your face but it was all we had—all that poor devastated Terra had after the Baldics had sacked her twice in fifteen years.
     I fired a burst and saw the big gray barbarian spin on his heels, stagger and scream with all four hands clutching his belly, and sink slowly to his knees. The creatures behind him howled, but he only let out a deep-throated curse. He'd be a long time dying. I'd blown a hole clear through him, but those Gorzuni were tough.
     The slugs wailed around us as I got myself down under the wall, hugging the long grass which had grown up around the shattered fragments of the house. There was a fresh wind blowing, rustling the grass and the big war-scarred trees, sailing clouds across a sunny summer sky, so the gas concentration was never enough to put us out. But Jonsson and Hokusai were sprawled like corpses there against the broken wall. They'd taken direct hits and they'd sleep for hours.
     Kathryn knelt beside me, the ragged, dirty coverall like a queen's robe on her tall young form, a few dark curls falling from under her helmet for the wind to play with. "If we get them mad enough," she said, "they'll call for the artillery or send a boat overhead to blow us to the Black Planet."
     "Maybe," I grunted. "Though they're usually pretty eager for slaves."
     "John—" She crouched there a moment, the tiny frown I knew so well darkening her blue eyes. I watched the way leaf-shadows played across her thin brown face. There was a grease smudge on the snub nose, hiding the little freckles. But she still looked good, really good, she and green Terra and life and freedom and all that I'd never have again.
     "John," she said at last, "maybe we should save them the trouble. Maybe we should make our own exit."
     "It's a thought," I muttered, risking a glance above the wall.
     The Gorzuni were more cautious now, creeping through the trampled gardens toward the shattered outbuilding we defended. Behind them, the main estate, last knot of our unit's resistance, lay smashed and burning. Gorzuni were swarming around us, dragging out such humans as survived and looting whatever treasure was left. I was tempted to shoot at those big furry bodies but I had to save ammunition for the detail closing in on us.
     "I don't fancy life as the slave of a barbarian outworlder," I said. "Though humans with technical training are much in demand and usually fairly well treated."
     "I might trade on my own mechanical knowledge," she said. "And then again, I might not. Is it worth the risk, John, my dearest?"

---

     We were the conquered. They had scattered our armies. They had ravaged our cities. They had hunted us through the streets and the hills and the great deeps of space, and we could only snarl and snap at them and hope that the remnants of our navy might pull a miracle. But miracles are hard to come by.
     So far the Baldic League had actually occupied only the outer planets. The inner worlds were nominally under Commonwealth rule but the government was hiding or nonexistent. Only fragments of the navy fought on without authority or plan or hope, and Terra was the happy hunting ground of looters and slave raiders. Before long, I supposed bitterly, the outworlders would come in force, break the last resistance, and incorporate all the Solar System into their savage empire. Then the only free humans would be the extrasolar colonists, and a lot of them were barbaric themselves and had joined the Baldic League against the mother world.
     The captives were herded into cells aboard the tender, crammed together till there was barely room to stand. Kathryn wasn't in my cell either. I lapsed into dull apathy.
     When everyone was aboard, the deckplates quivered under our feet and acceleration jammed us cruelly against each other. Several humans died in that press. I had all I could do to keep the surging mass from crushing in my chest but of course the Gorzuni didn't care. There were plenty more where we came from.
     The boat was an antiquated and rust-eaten wreck, with half its archaic gadgetry broken and useless. They weren't technicians, those Baldics. They were barbarians who had learned too soon how to build and handle spaceships and firearms, and a score of their planets united by a military genius had gone forth to overrun the civilized Commonwealth.
     But their knowledge was usually by rote; I have known many a Baldic "engineer" who made sacrifices to his converter, many a general who depended on astrologers or haruspices for major decisions. So trained humans were in considerable demand as slaves. Having a degree in nuclear engineering myself, I could look for a halfway decent berth, though of course there was always the possibility of my being sold to someone who would flay me or blind me or let me break my heart in his mines.
     Untrained humans hadn't much chance. They were just flesh-and-blood machines doing work that the barbarians didn't have automatics for, rarely surviving ten years of slavery.
     We were taken up to a ship orbiting just above the atmosphere. Airlocks were joined, so I didn't get a look at her from outside, but as soon as we entered I saw that she was a big interstellar transport of the Thurnogan class, used primarily for carrying troops to Sol and slaves back, but armed for bear. A formidable fighting ship when properly handled.
     Guards were leaning on their rifles, all of Gorzuni race, their harness worn any way they pleased and no formality between officers and men. The barbarian armies' sloppy discipline had blinded our spit-and-polish command to their reckless courage and their savage gunnery. Now the fine-feathered Commonwealth navy was a ragged handful of hunted, desperate men and the despised outworlders were harrying them through the Galaxy.

---

       This ship was worse than usual, though. I saw rust and mold on the unpainted plates. The fluoros were dim and in places burned out. There was a faint pulse in the gravity generators. They had long ago been stripped and refurnished with skins, stolen hold goods, cooking pots, and weapons. The Gorzuni were all as dirty and unkempt as their ship. They lounged about gnawing chunks of meat, drinking, dicing, and looking up now and then to grin at us.

• 

      A dozen men clustered around the bartending robot—his cousin and family lawyer, Nikkolay Trask; Lothar Ffayle, the banker; Alex Gorram, the shipbuilder, and his son Basil; Baron Rathmore; more of the Wardshaven nobles whom he knew only distantly. And Otto Harkaman.
     Harkaman was a Space Viking. That would have set him apart, even if he hadn't topped the tallest of them by a head.
     "A pleasure, Lord Trask. I've met your lovely bride-to-be, and now that I meet you, let me congratulate both." Then, as they were having a drink together, he put his foot in it by asking: "You're not an investor in the Tanith Adventure, are you?"
     He said he wasn't, and would have let it go at that. Young Basil Gorram had to get his foot in, too.
     "Lord Trask does not approve of the Tanith Adventure," he said scornfully. "He thinks we should stay home and produce wealth, instead of exporting robbery and murder to the Old Federation for it."
     The smile remained on Otto Harkaman's face; only the friendliness was gone. He unobtrusively shifted his drink to his left hand.
     "Well, our operations are definable as robbery and murder," he agreed. "Space Vikings are professional robbers and murderers. And you object? Perhaps you find me personally objectionable?"
     "I wouldn't have shaken your hand or had a drink with you if I did. I don't care how many planets you raid or cities you sack, or how many innocents, if that's what they are, you massacre in the Old Federation. You couldn't possibly do anything worse than those people have been doing to one another for the past ten centuries."
     He turned to Basil Gorram. "You see, the gentleman isn't crazy, at all. That's what happened to the Terran Federation, by the way. The good men all left to colonize, and the stuffed shirts and yes-men and herd-followers and safety-firsters stayed on Terra and tried to govern the galaxy."
     "Well, maybe this is all new to you, captain," Rovard Grauffis said sourly, "but Lucas Trask's dirge for the Decline and Fall of the Sword-Worlds is an old song to the rest of us. I have too much to do to stay here and argue."
     Lothar Ffayle evidently did intend to stay and argue.
     "All you're saying, Lucas, is that we're expanding. You want us to sit here and build up population pressure like Terra in the First Century?"
     "With three and a half billion people spread out on twelve planets? They had that many on Terra alone. And it took us eight centuries to reach that."
     That had been since the Ninth Century, Atomic Era, at the end of the Big War. Ten thousand men and women on Abigor, refusing to surrender, had taken the remnant of the System States Alliance navy to space, seeking a world the Federation had never heard of and wouldn't find for a long time. That had been the world they had called Excalibur. From it, their grandchildren had colonized Joyeuse and Durendal and Flamberge; Haulteclere had been colonized in the next generation from Joyeuse, and Gram from Haulteclere.
     "We're not expanding, Lothar; we're contracting. We stopped expanding three hundred and fifty years ago, when that ship came back to Morglay from the Old Federation and reported what had been happening out there since the Big War. Before that, we were discovering new planets and colonizing them. Since then, we've been picking the bones of the dead Terran Federation."
     "Well, if we don't get a planetary monarchy to keep order, this planet will decivilize like anything in the Old Federation."
     "Oh, come, Lucas!" Alex Gorram protested. "That's pulling it out too far."
     "Yes, for one thing, we don't have the Neobarbarians," somebody said. "And if they ever came out here, we'd blow them to Em-See-Square in nothing flat. Might be a good thing if they did, too; it would stop us squabbling among ourselves."
     Harkaman looked at him in surprise. "Just who do you think the Neobarbarians are, anyhow?" he asked. "Some race of invading nomads; Attila's Huns in spaceships?"
     "Well, isn't that who they are?" Gorram asked.
     "Nifflheim, no! There aren't a dozen and a half planets in the Old Federation that still have hyperdrive, and they're all civilized. That's if 'civilized' is what Gilgamesh is," he added. "These are homemade barbarians. Workers and peasants who revolted to seize and divide the wealth and then found they'd smashed the means of production and killed off all the technical brains. Survivors on planets hit during the Interstellar Wars, from the Eleventh to the Thirteenth Centuries, who lost the machinery of civilization. Followers of political leaders on local-dictatorship planets. Companies of mercenaries thrown out of employment and living by pillage. Religious fanatics following self-anointed prophets."
     "You think we don't have plenty of Neobarbarian material here on Gram?" Trask demanded. "If you do, take a look around."

---

     "Well, have you any conception of the sort of thing you're setting yourself to?" The Space Viking snapped his lighter and puffed. "You know, of course, how big the Old Federation is. You know the figures, that is, but do they mean anything to you? I know they don't to a good many spacemen, even. We talk glibly about ten to the hundredth power, but emotionally we still count, 'One, Two, Three, Many.' A ship in hyperspace logs about a light-year an hour. You can go from here to Excalibur in thirty hours. But you could send a radio message announcing the birth of a son, and he'd be a father before it was received. The Old Federation, where you're going to hunt Dunnan, occupies a space-volume of two hundred billion cubic light-years. And you're hunting for one ship and one man in that. How are you going to do it, Lord Trask?"
     "I haven't started thinking about how; all I know is that I have to do it. There are planets in the Old Federation where Space Vikings come and go; raid-and-trade bases, like the one Duke Angus planned to establish on Tanith. At one or another of them, I'll pick up word of Dunnan, sooner or later."
     "We'll hear where he was a year ago, and by the time we get there, he'll be gone for a year and a half to two years. We've been raiding the Old Federation for over three hundred years, Lord Trask. At present, I'd say there are at least two hundred Space Viking ships in operation. Why haven't we raided it bare long ago? Well, that's the answer: distance and voyage-time. You know, Dunnan could die of old age—which is not a usual cause of death among Space Vikings—before you caught up with him. And your youngest ship's-boy could die of old age before he found out about it."

---

     Now he was studying his new profession of interstellar robbery and murder against which he had once inveighed. Otto Harkaman's handful of followers became his teachers. Vann Larch, guns-and-missiles, who was also a painter; Guatt Kirbey, sour and pessimistic, the hyperspatial astrogator who tried to express his science in music; Sharll Renner, the normal-space astrogator. Alvyn Karffard, the exec, who had been with Harkaman longest of all.

---

     "The humor escapes me," he admitted. "We came here on a fools' errand."
     "I'm sorry, Lucas." Harkaman was still shaking with mirth. "I know it's a letdown, but that pair of chiseling chicken thieves! I could almost pity them, if it weren't so funny." He laughed again. "You know what their idea was?"
     Trask shook his head. "Who are they?"
     "What I called them, a couple of chicken thieves. They raid planets like Set and Hertha and Melkarth, where the locals haven't anything to fight with—or anything worth fighting for. I didn't know they'd teamed up, but that figures. Nobody else would team up with either of them. What must have happened, this story of Duke Angus' Tanith adventure must have filtered out to them, and they thought that if they got here first, I'd think it was cheaper to take them in than run them out. I probably would have, too. They do have ships, of a sort, and they do raid, after a fashion. But now, there isn't going to be any Tanith base, and they have a no-good planet and they're stuck with it."
     "Can't they make anything out of it themselves?"
     "Like what? They have no equipment, and they have no men. Not for a job like that. The only thing they can do is space out and forget it."
     "Yes; Valkanhayn and Spasso don't own their ships," Harkaman explained. "They've gone in debt to their crews for supplies and maintenance till everybody owns everything in common. The ships look like it, too. They don't even command, really; they just preside over elected command-councils."

---

     "We aren't going to spend our time sitting here on Tanith," he told the two captains. "This planet is a raiding base, and 'raiding' is the operative word. And we are not going to raid easy planets. A planet that can be raided with impunity isn't worth the time it takes getting to it. We are going to have to fight on every planet we hit, and I am not going to jeopardize the lives of the men under me, which includes your crews as well as mine, because of under-powered and under-armed ships."
     Spasso tried to argue. "We've been getting along."
     Harkaman cursed. "Yes. I know how you've been getting along; chicken-stealing on planets like Set and Xipototec and Melkarth. Not making enough to cover maintenance expenses; that's why your ship's in the shape she is. Well, those days are over. Both ships ought to have a full overhaul, but we'll have to skip that till we have a shipyard of our own. But I will insist, at least, that your guns and launchers are in order. And your detection equipment; you didn't get a fix on the Nemesis till we were less than twenty thousand miles off-planet."

---

     As soon as the Lamia was on orbital watch, the Space Scourge was set down at the spaceport and work started on her. It was decided that Valkanhayn would take her to Gram; enough Nemesis people would go along to insure good faith on his part, and to talk to Duke Angus and the Tanith investors. Baron Rathmore, and Paytrik Morland, and several other Wardshaven gentlemen-adventurers for the latter function; Alvyn Karffard to act as Valkanhayn's exec, with private orders to supersede him in command if necessary, and Guatt Kirbey to do the astrogating.
     "We'll have to take the Nemesis and the Space Scourge out, first, and make a big raid," Harkaman said. "We can't send the Space Scourge back to Gram empty. When Baron Rathmore and Lord Valpry and the rest of them talk to Duke Angus and the Tanith investors, they'll have to have a lot more than some travel films of Tanith. They'll have to be able to show that Tanith is producing. We ought to have a little money of our own to invest, too."
     "Have you picked a place to raid?"
     "Three of them. First, Khepera. That's only thirty light-years from here. That won't amount to much; just chicken-stealing. It'll give our green hands some relatively safe combat-training, and it'll give us some idea of how Spasso's and Valkanhayn's people behave, and give them confidence for the next job."
     "And then?"
     "Amaterasu. My information about Amaterasu is about twenty years old. A lot of things can happen in twenty years. All I know of it—I was never there myself—is it's fairly civilized—about like Terra just before the beginning of the Atomic Era. No nuclear energy, they lost that, and of course nothing beyond it, but they have hydroelectric and solarelectric power, and nonnuclear jet aircraft, and some very good chemical-explosive weapons, which they use very freely on each other. It was last known to have been raided by a ship from Excalibur twenty years ago."
     "That sounds promising. And the third planet?"
     "Beowulf. We won't take enough damage on Amaterasu to make any difference there, but if we saved Amaterasu for last, we might be needing too many repairs."
     "It's like that?"
     "Yes. They have nuclear energy. I don't think it would be wise to mention Beowulf to Captains Spasso and Valkanhayn. Wait till we've hit Khepera and Amaterasu. They may be feeling like heroes, then."

---

     Khepera left a bad taste in Trask's mouth. He was still tasting it when the colored turbulence died out of the screen and left the gray nothingness of hyperspace. Garvan Spasso—they had had no trouble in inducing him to come along—was staring avidly at the screen as though he could still see the ravished planet they had left.
     "That was a good one; that was a good one!" he was crowing. He'd said that a dozen times since they had lifted out. "Three cities in five days, and all the stuff we gathered up around them. We took over two million stellars."
     And did ten times as much damage getting it, and there was no scale of values by which to compute the death and suffering.
     "Knock it off, Spasso. You said that before."
     There was a time when he wouldn't have spoken to the fellow, or anybody else, like that. Gresham's law, extended: Bad manners drive out good manners. Spasso turned on him indignantly.
     "Who do you think you are—?"
     "He thinks he's Lord Trask of Tanith," Harkaman said. "He's right, too; he is." He looked searchingly at Trask for a moment, then turned back to Spasso. "I'm just as tired as he is of hearing you pop your mouth about a lousy two million stellars. Nearer a million and a half, but two million's nothing to pop about. Maybe it would be for the Lamia, but we have a three-ship fleet and a planetary base to meet expenses on. Out of this raid, a ground-fighter or an able spaceman will get a hundred and fifty stellars. We'll get about a thousand, ourselves. How long do you think we can stay in business doing this kind of chicken-stealing."
     "You call this chicken-stealing?"
     "I call it chicken-stealing, and so'll you before we get back to Tanith. If you live that long."
     Khepera had been easy; the locals hadn't had anything to fight with. Small arms, and light cannon which hadn't been able to fire more than a few rounds. Wherever they had attempted resistance, the combat cars had swooped in, dropping bombs and firing machine guns and auto-cannon. Yet they had fought, bitterly and hopelessly—just as he would have, defending Traskon.
     Trask busied himself getting coffee and a cigarette from one of the robots. When he looked up, Spasso had gone away, and Harkaman was sitting on the edge of the desk, loading his short pipe.
     "Well, you saw the elephant, Lucas," Harkaman said. "You don't seem to have liked it."
     "Elephant?"
     "Old Terran expression I read somewhere. All I know is that an elephant was an animal about the size of one of your Gram megatheres. The expression means, experiencing something for the first time which makes a great impression. Elephants must have been something to see. This was your first Viking raid. You've seen it, now."
     He'd been in combat before; he'd led the fighting-men of Traskon during the boundary dispute with Baron Manniwel, and there were always bandits and cattle rustlers. He'd thought it would be like that. He remembered, five days, or was it five ages, ago, his excited anticipation as the city grew and spread in the screen and the Nemesis came dropping down toward it. The pinnaces, his four and the two from the Space Scourge, had gone spiraling out a hundred miles beyond the city; the Space Scourge had gone into a tighter circle twenty miles from its center; the Nemesis had continued her relentless descent until she was ten miles from the ground, before she began spewing out landing craft, and combat cars, and the little egg-shaped one-man air-cavalry mounts. It had been thrilling. Everything had gone perfectly; not even Valkanhayn's gang had goofed.
     Then the screenviews had begun coming in. The brief and hopeless fight in the city. He could still see that silly little field gun, it must have been around seventy or eighty millimeter, on a high-wheeled carriage, drawn by six shaggy, bandy-legged beasts. They had gotten it unlimbered and were trying to get it on a target when a rocket from an aircar landed directly under the muzzle. Gun, caisson, crew, even the draft team fifty yards behind, had simply vanished.
     "They don't have a chance," he'd said, half-sick. "But they keep on fighting."
     "Yes; stupid of them, isn't it?" Harkaman, beside him, had said.
     "What would you do in their place?"
     "Fight. Try to kill as many Space Vikings as I could before they got me. Terro-humans are all stupid like that. That's why we're human."

---

     The third city was at the forks of a river, like Tradetown on Tanith. Unlike it, this was a real metropolis. They should have gone there first of all. They spent two days systematically pillaging it. The Kheperans carried on considerable river-traffic, with stern-wheel steamboats, and the waterfront was lined with warehouses crammed with every sort of merchandise. Even better, the Kheperans had money, and for the most part it was gold specie, and the bank vaults were full of it.
     Unfortunately, the city had been built since the fall of the Federation and the climb up from the barbarism that had followed, and a great deal of it was of wood. Fires started almost at once, and it was almost completely on fire by the end of the second day. It had been visible in the telescopic screen even after they were out of atmosphere, a black smear until the turning planet carried it into darkness and then a lurid glow.

---

     "It was a filthy business."
     Harkaman nodded. "Robbery and murder always are. You don't have to ask me who said that Space Vikings are professional robbers and murderers, but who was it said that he didn't care how many planets were raided and how many innocents massacred in the Old Federation?"
     "A dead man. Lucas Trask of Traskon."

---

     They came straight down on Eglonsby, on Amaterasu, the Nemesis and the Space Scourge side by side. The radar had picked them up at point-five light-seconds; by this time the whole planet knew they were coming, and nobody was wondering why. Paul Koreff was monitoring at least twenty radio stations, assigning somebody to each one as it was identified. What was coming in was uniformly excited, some panicky, and all in fairly standard Lingua Terra.
     Garvan Spasso was perturbed. So, in the communication screen from the Space Scourge, was Boake Valkanhayn.
     "They got radio, and they got radar," he clamored.
     "Well, so what?" Harkaman asked. "They had radio and radar twenty years ago, when Rock Morgan was here in the Coalsack. But they don't have nuclear energy, do they?"
     "Well, no. I'm picking up a lot of industrial electrical discharge, but nothing nuclear."
     "All right. A man with a club can lick a man with his fists. A man with a gun can lick half a dozen with clubs. And two ships with nuclear weapons can lick a whole planet without them. Think it's time, Lucas?"
     He nodded. "Paul, can you cut in on that Eglonsby station yet?"
     "What are you going to do?" Valkanhayn wanted to know, against it in advance.
     "Summon them to surrender. If they don't, we will drop a hellburner, and then we will pick out another city and summon it to surrender. I don't think the second one will refuse. If we are going to be murderers, we'll do it right, this time."
     "Space Vikings Nemesis and Space Scourge, calling the city of Eglonsby. Space Vikings...."
     He repeated it for over a minute; there was no reply.
     "Vann," he called Guns-and-Missiles. "A subcrit display job, about four miles over the city."
     He laid the phone down and looked to the underside viewscreen. A little later, a silvery shape dropped away from the ship's south pole. The telescopic screen went off, and the unmagnified screen darkened as the filters went on. Valkanhayn, aboard the other ship, was shouting a warning about his own screens. The only unfiltered screen aboard the Nemesis was the one tuned to the falling missile. The city of Eglonsby rushed upward in it, and then it went suddenly dark. There was an orange-yellow blaze in the other screens. After a while, the filters went off and the telescopic screen went on again. He picked up the phone.
     "Space Vikings calling Eglonsby; this is your last warning. Communicate at once."
     Less than a minute later, a voice came out of one of the speakers:
     "Eglonsby calling Space Vikings. Your bomb has done great damage. Will you hold your fire until somebody in authority can communicate with you? This is the chief operator at the central State telecast station; I have no authority to say anything to you, or discuss anything."
     "Oh, good, that sounds like a dictatorship," Harkaman was saying. "Grab the dictator and shove a pistol in his face and you have everything."
     "There is nothing to discuss. Get somebody who has authority to surrender the city to us. If this is not done within the hour, the city and everybody in it will be obliterated."
     Only minutes later, a new voice said:
     "This is Gunsalis Jan, secretary to Pedrosan Pedro, President of the Council of Syndics. We will switch President Pedrosan over as soon as he can speak directly to the personage in supreme command of your ships."
     "That is myself; switch him to me at once."
     After a delay of less than fifteen seconds they had President Pedrosan Pedro.
     "We are prepared to resist, but we realize what this would cost in lives and destruction of property," he began.
     "You don't begin to. Do you know anything about nuclear weapons?"
     "From history; we have no nuclear power of any sort. We can find no fissionables on this planet."
     "The cost, as you put it, would be everything and everybody in Eglonsby and for a radius of almost a hundred miles. Are you still prepared to resist?"
     The President of the Council of Syndics wasn't and said so.

---

     The "great damage" caused by the bomb was about equal to the effect of a medium hurricane; he had seen worse from high winds at Traskon. Mostly it had been moral, which had been the kind intended.
     He spoke to the President, ignoring the others. "We want it understood that we control this city, and we expect complete submission. As long as you remain submissive to us, we will do no damage beyond removal of the things we wish to take from it, and there will be no violence to any of your people, or any indiscriminate vandalism. This visit we are paying you will cost you heavily, make no mistake about that, but whatever the cost, it will be a cheap price for avoiding what we might otherwise do."
     The President and the Syndics exchanged relieved glances. Let the taxpayers worry about the cost; they'd come out of it with whole skins.
     "You understand, we want maximum value and minimum bulk," he continued. "Jewels, objects of art, furs, the better grades of luxury goods of all kinds. Rare-element metals. And monetary metals, gold and platinum. You have a metallic-based currency, I suppose?"

---

     Before sunset, they were back over Eglonsby. The loot might run as high as a half-billion Excalibur stellars. Boake Valkanhayn and Garvan Spasso were simply beyond astonishment and beyond words.
     They gathered up machinery, and stocks of steel and light-metal alloys. The city was full of warehouses, and the warehouses were crammed with valuables. In spite of the socialistic and egalitarian verbiage behind which the government operated, there seemed to be a numerous elite class and if gold were not a monetary metal it was not despised for purposes of ostentation. There were several large art museums. Vann Larch, their nearest approach to an art specialist, took charge of culling the best from them.

---

     Beowulf was bad.
     Valkanhayn and Spasso had both been opposed to the raid. Nobody raided Beowulf; Beowulf was too tough. Beowulf had nuclear energy and nuclear weapons and contragravity and normal-space craft, they even had colonies on a couple of other planets of their system. They had everything but hyperdrive. Beowulf was a civilized planet, and you didn't raid civilized planets, not and get away with it.
     And beside, hadn't they gotten enough loot on Amaterasu?
     "No, we did not," Trask told them. "If we're going to make anything out of Tanith, we're going to need power, and I don't mean windmills and waterwheels. As you've remarked, Beowulf has nuclear energy. That's where we get our plutonium and our power units."
     "There are, or were, three chief sources of fissionable ores," Harkaman said. "The last ship to raid here and get away was Stefan Kintour's Princess of Lyonesse, sixty years ago. He hit one on the Antarctic continent; according to his account, everything there was fairly new. He didn't mess things up too badly, and it ought to be still operating. We'll go in from the south pole, and we'll have to go in fast."
     They shifted personnel and equipment. They would go in bunched, the pinnaces ahead; they and the Space Scourge would go down to the ground, while the better-armed Nemesis would hover above to fight off local contragravity, shoot down missiles, and generally provide overhead cover. Trask transferred to the Space Scourge, taking with him Morland and two hundred of the Nemesis ground-fighters. Most of the single-mounts, landing craft and manipulators and heavy-duty lifters went with him, jamming the decks around the vehicle ports of Valkanhayn's ship.
     They jumped in to six light-minutes, and while Valkanhayn's astrogator was still fiddling with his controls they began sensing radar and microray detection. When they came out again, they were two light-seconds off the south pole, and half a dozen ships were either in orbit or coming up from the planet. All normal-space craft, of course, but some were almost as big as the Nemesis.
     From there on, it was a nightmare.
     Ships pounded at them with guns, and they pounded back. Missiles went out, and counter-missiles stopped them in rapidly expanding and quickly vanishing globes of light. Red lights flashed on the damage board, and sirens howled and klaxons squawked. In the outside-view screens, they saw the Nemesis vanish in a blaze of radiance, and then, while their hearts were still in their throats, come out of it again. Red lights went off on the board as damage-control crews and their robots sealed the breaches in the hull and pumped air back into evacuated areas, and then more red lights came on.
     Missiles were coming up from the planet, now. Valkanhayn's detection officer was trying to locate the source. While he was trying, a big melon-shaped thing fell away from the Nemesis, and in the jiggling, radiation-distorted intership screen Harkaman's image was laughing.
     "Hellburner just went off; target about 50° south, 25° east of the sunrise line. That's where those missiles are coming from."
     Counter-missiles sped toward the big metal melon; defense missiles, robot-launched, met them. The hellburner's track was marked first by expanding red and orange globes in airless space and then by fire-puffs after it entered atmosphere. It vanished into the darkness beyond the sunset, and then made sunlight of its own. It was sunlight; a Bethe solar-phoenix reaction, and it would sustain itself for hours. He hoped it hadn't landed within a thousand miles of their objective.
     The ground operation was a nightmare of a different sort. He went down in a command car, with Paytrik Morland and a couple of others. There were missiles and gun batteries. There were darting patterns of flights of combat vehicles, blazing gunfire, and single vehicles that shot past or blew up in front of them. Robots on contragravity—military robots, with missiles to launch, and working robots with only their own mass to hurl, flung themselves mindlessly at them. Screens that went crazy from radiation; speakers that jabbered contradictory orders. Finally, the battle, which had raged in the air over two thousand square miles of mines and refineries and reaction plants, became two distinct and concentrated battles, one at the packing plant and storage vaults and one at the power-unit cartridge factory.
     Three pinnaces came down to form a triangle over each; the Space Scourge hung midway between, poured out a swarm of vehicles and big claw-armed manipulators; armored lighters and landing craft shuttled back and forth. The command car looped and dodged from one target to the other; at one, keg-like canisters of plutonium, collapsium-plated and weighing tons apiece, were coming out of the vaults, and at the other lifters were bringing out loads of nuclear-electric power-unit cartridges, some as big as a ten liter jar, to power a spaceship engine, and some small as a round of pistol ammunition, for things like flashlights.
     Every hour or so, he looked at his watch, and it would be three or four minutes later.
     At last, when he was completely convinced that he had really been killed, and was damned and would spend all eternity in this fire-riven chaos, the Nemesis began firing red flares and the speakers in all the vehicles were signaling recall. He got aboard the Space Scourge somehow, after assuring himself that nobody who was alive was left behind.
     There were twenty-odd who weren't, and the sick bay was full of wounded who had gone up with cargo, and more were being helped off the vehicles as they were berthed. The car in which he had been riding had been hit several times, and one of the gunners was bleeding under his helmet and didn't seem aware of it. When he got to the command room, he found Boake Valkanhayn, his face drawn and weary, getting coffee from a robot and lacing it with brandy.

• 

• 

• 

      To do justice to another side of the xenophobia argument, it might be safe to say that the universe awaiting the Vulcans was not one they had ever imagined. It was ironic that the sudden beacon in their sky, the da'Nikhirch, or Eye of Fire, which stirred many Vulcans to even more intense interest in neighboring interstellar space, and which (some said) heralded the birth of Surak, was also to be the cause of such terrible anguish for the planet. No one has ever proved that it was a sunkiller bomb that made sigma-1014 Orionis go nova, but the destruction of the hearthworld of the Inshai Compact planets certainly suited the expansionist aims of their old enemies in trade, the "nonaligned" planets of the southern Orion Congeries.

     With the great power and restraining influence of Inshai suddenly gone, a reign of terror began in those spaces. Wars, and economic and societal collapse, decimated planetary populations in waves of starvation and plague while the decentralized interstellar corporations, their fleets armed with planetcracker weapons, fought over trade routes and sources of raw materials—blackmailing worlds into submission, destroying those that would not submit. In the power vacuum the surviving Compact worlds could not maintain their influence, or their technology, much of which derived from Inshai. They, too, turned to extortion and conquest to survive. Formerly peaceful worlds like Etosha and depopulated ones like Duthul became the home bases of the companies and guilds who degenerated over centuries into the Orion pirates. And Vulcan looked out into the darkness, where these dangerous next-door neighbors were stirring, not realizing that they had already turned on the houselights for them. It was around the time of Surak's birth, when the FireEye's light reached Vulcan, that the first electromagnetic signals from Vulcan reached Etosha in their turn, and notice was taken.

     Their first contacts with the Orion pirates, forty-five years later, would have been enough to disillusion even a Terran steeped in all old Earth's legends of bug-eyed monsters intent on stealing their women and subjugating their planet. The Vulcans had no such legends: they expected to deal with strangers hospitably, and courteously, though always from strength. They had no idea that their strengths were in areas that would mean nothing to the Duthulhiv pirates who first reached them.

     The subterfuge used by the raiders had worked on many another world. They surveyed Vulcan for months, monitoring communications, learning the languages, and assessing the world's resources for marketability. Then initial contacts began, properly stumbling ones made by conventional radio from pirate scout craft transmitting from several lightweeks outside the system. The pirates used a simple series of trinary signal-pulses expressing atomic ratios and so forth. Their own records, preserved on Last Etosha, make it plain that no one ever cracked this code as swiftly as the Vulcans did. "It was almost as if they had been expecting it," one pirate scientist was reported to have said. Messages began to flow back and forth immediately.

     When communication was established, the pirates offered peaceful trade and cultural opportunities; the first messages were debated for several standard months in councils around the planet. Several wars or declarations of wars were in fact put on hold, or postponed, while the officials conducting them were recalled to their capitals to assist in the discussions. Finally the Vulcans decided to receive the strangers as a united front. On this at least they were agreed, that their own position of strength would be stronger yet if they acted all together. If a more pressing reason for this lay in the minds of various parties—the idea that this way, no one faction would be allowed to get a jump on the others—then no one voiced it out loud. The date for the first physical meeting was set: nine Irhheen of the Vulcan old-date 139954, equivalent in Terran dating to January 18-19, 22 B.C. There at the agreed landing place at Shi'Kahr—then a tiny village of a ritually neutral tribe—five hundred twenty-three of the great ones of Vulcan gathered: clan and tribal chieftains, priestesses and clerics, merchants, scientists and philosophers, who went out in all their splendor to meet the strangers in courtesy and bring them home in honor. What met them in turn, when the strangers' landing craft settled, were phasers that stunned those who were to be held for ransom or sold into slavery, and particle-beam weapons that blew to bloody rags those who tried to fight or escape.

     By the merest accident, Surak was not there: an aircar malfunction had detained him at the port facility at ta'Valsh. When the news reached him, he immediately offered to go to the aliens and to "deal peace" with them. No government would support him in this, most specifically since half the nations on the planet that day were mourning their leaders: the other half were staring in rage at ransom demands radioed to them from the slaver ships in orbit.

     Thus war broke out—'Ahkh, "the" War, Vulcans called it, thereby demoting all other wars before it to the rank of mere tribal feuds. No ransoms were paid—and indeed if they had been, they would have beggared the planet. But the Vulcans knew from their own bitter experience with one another that once one paid Danegeld, one never got rid of the Dane. The space fleets of the planet were then no more than unarmed trading ships: in one of those lacunae that puzzle historians, the Vulcans had never even thought of carrying their warfare into space. But the ships did not stay unarmed long, and some of the armaments were of the kind that would not show on any enemy's sensors. The chief psi-talents of the planet, great builders and architects, and technicians who had long mastered the subtleties of the undermind, went out in the ships and taught the Duthulhiv pirates that weapons weren't everything. Metal came unraveled in ships' hulls; pilots calmly locked their ships into suicidal courses, unheeding of the screams of the crews: and the Vulcans beamed images of the destruction back to Etosha, lest there should be any confusion about the cause. The message was meant to be plain: kill us, and die.

     That meeting was to prove the rock on which Vulcan pacifism first and most violently ran aground. S'task's handling of it differed terribly from his master's, as they differed at that part of their lives on everything else. S'task was at the meeting at Shi'Kahr, one of those who was taken hostage. He it was who organized the in-ship rebellion that cost so many of the slavers their lives: he was the one who broke the back of the torturer left alone with him, broke into and sabotaged the ship's databanks, and then—after releasing the other hostages safely on Vulcan—crashed the luckless vessel into the pirate mothership at the cost of thousands of pirates' lives, and almost his own. Only his astonishing talent for calculation saved him, so that weeks later, after much anguished searching, he was picked up drifting in a lifepod in an L5 orbit, half starved, half dead of dehydration, but clinging to life through sheer rage. They brought him home, and Surak hurried to his couchside—to rebuke him. The words "I have lost my best student to madness" are the beginning of the breaking of the Vulcan species.

• 

     We come out of hyperspace at the right time and Dimo, my navigator, says it’s the right place. Right on the Rift at the edge of civilized space. Should be nothing there but a moon that once had a planet and a star and an atmosphere but now for whatever reason is all alone. Should be cold and dead and nothing on it but a billion-year-old Aaru station worth a fortune. Dimo says it’s there, it’s there. So it’s a bit of a surprise to find no moon and sixteen thousand (Dimo counts them) fab brain fighters.
     The Hegemony got here first. Nothing to do now but run before they see us.

     They see us.

---

3.9. Hegemony

     Technology sometimes becomes an end in itself, and when it does the galaxy is threatened by Hegemony. No culture seeks such a state, but sometimes one falls into an unstoppable spiral of development, a positive feedback loop that eventually sustains itself as its own purpose. Once autonomous species become enslaved by the hegemonic mind, melded with their technologies, with the inexorable purpose of expansion and envelopment and assimilation.

     Regions of the galaxy afflicted by hegemony are swiftly quarantined and left to burn out on their own unless more direct action is required and war on an enormous scale is necessary to destroy or suppress the expansion. The problem is that the seeds of hegemonic technology do not require life—they are essentially data and they can lie dormant for millions of years until allowed free by well-meaning xeno-archaeologists with inadequate network defenses.

     So a Hegemony, or a hegemonic event, happens when some kind of automation (natural, psychic, or mystical) achieves at once a very high degree of autonomy (not intelligence, necessarily), technology, and prioritizes reproduction. This automation then attempts to convert everything that isn’t it into it.

     We might have a Von Neumann plague: reproducing factory robots lose the directive to mine for ore and now simply reproduce as the end goal, converting solar systems into hordes of robots seeking more mass. (natural physics based hegemony)

     Or perhaps a virulent software package that infests, subverts, and re-purposes software-bearing equipment (including sophonts) to the purpose of spreading the signal. A computer virus that can infect people as well as machines. (psychic physics based hegemony. Arguably a better example can be found in Arthur C. Clarke's Childhood's End, since that one uses telepathy)

     A mystic hegemony might be such a powerful memetic idea that people that think it can do nothing but proselytize it, perpetuating it, and converting whole populations into unproductive, unthinking broadcasters of The Word. On high tech worlds this might be fine as the selfless fabs continue to feed and maintain ten billion useless sophonts until they go extinct from failure to breed. (mystical physics based hegemony)

     The key questions for a hegemony are:

• what physics does it use?
• how does it reproduce?
• how does it travel?
• what is the impact of being infected?

     And of course since they reproduce so rapidly, hegemonies typically mutate. Eventually they mutate into non-virulent forms but they may go through different phases first, changing one or more of their key properties.

---

6.1. Facts

     We need to talk. We need to come to an understanding about the ways in which this universe (of this space-opera scifi role-playing game) is not your universe. Now, it’s like your universe in a lot of ways, and most of those ways you will understand intuitively: when you drop things they fall, when you accelerate you go faster, and so on. However, there are details of real world physics that are not the same in this universe. The upside of this is that Elysium Flare uses a kind of “folk physics”—that is, if you’re untrained in physics you probably won’t even suspect a difference. Here are the differences anyway.

     There are three different branches of physics here, and they overlap somewhat.

6.1.1. Natural physics

     Natural physics is like ours, mostly, and is manipulated by technology. That is, natural physics is used to understand the universe enough to invent gadgets that manipulate the physics to produce useful effects. This gives us vehicles, weapons, sensors, and so on. Technology is something anyone can use, so throughout the universe it is the most common physical manipulation around. But it’s not the only way to manipulate the universe!

     These physics are represented and manipulated with mathematics and modelling and algorithms, and these things are easily stored and transmitted and interpreted. The use of natural physics is therefore more widespread than the others. You can teach it and you can explain it on a mass scale.

     The gaps and origins in natural physics, the places it cannot describe, are generally comprehensible by another branch of physics: the mystical.

6.1.2. Mystical physics

     These are the set of universal laws that are manipulable by faith and arcana. This requires dedication and wisdom and experience and morality. Anyone can access this by making the right sacrifices and staying on the path laid down by mystics before you. It allows and explains certain limited exceptions to natural physics and its gaps and exceptions are partially comprehensible by psychic physics.

     The representation and reasoning of mystical physics uses obscure symbologies and the details of the presentation are important—a particular paper or ink, the right frame of mind, written in the right place at the right time. Aesthetics have concrete powers. Consequently transmitting mystical calculations is useless—the medium here really is the message. This limits the rate at which mystical understanding can be propagated and it is commonly only taught from master to pupil.

     The trappings of mystical physics are special things and special places and special times and special people. None of these can be readily fabricated but rather they are found. If they were once fabricated, that art is long lost.

6.1.3. Psychic physics

     There is another set of laws that is even more personal, that resides inside the individual mind. Not everyone can manipulate this kind of physics, but in principle any kind of mind might be capable. Psychic physics allow certain limited exceptions to natural physics and is partially comprehensible by natural physics— it is understood that certain patterns of brain activity are associated with psychic activity, for example, and it can in principle be damped or amplified by natural technology, but the why of it all is permanently and provably elusive.

     Psychic physics is internalized—it is not calculated in real space but rather is a unique and internal calculation that takes place in the instinctive part of receptive minds. It can be reproduced but it is mostly about effort and desire and talent and so psychics can only very loosely define how they do what they do. They certainly cannot communicate it even to other psychics but can only nod knowlingly at the description of how it feels when it works

     Psychic physics doesn’t make things external to the mind but there are things that can be linked to or powered by the brain.

---

3.1. Manichae

     Several of the great Hegemonies in the previous cycle were born on human worlds and so humans have a reputation for irresponsible scientific and psychic research and development. It’s said that anything humans discover they can turn into a weapon or, as the Hegemonies proved, use to turn themselves into weapons. This is to some extent true and it’s certainly the case that many if not most of the major arms manufacturers are of human origin.

3.3. Orpheani

     The Orpheani ("Dreamers" to many) originated in the complex organic storms of the upper atmosphere of a gas planet. Their evolution was slow and while they came to consciousness and language relatively early, they did not become tool users until discovered by another species. This species (the Dreamers call them the Rescue) provided them with mechanisms to contain the Dreamers as individuals and to allow manipulation. (the Dreamers bodies are gaseous, not solid like humans. They wear mechanical suits so they can manipulate solid objects)

     Only one of the Hegemonies on record was Orpheanic and it was nearly the end of the species as the hegemonic infection did not infiltrate the Dreamers themselves but rather their suits and vehicles. Enslaved inside suddenly hegemonic shells that once were their interface with their industry, many Dreamers went insane and others simply died. Today the Dreamers are also considered the masters of security design in hardware, software, arcana, and even in mental training.

6.1.5. Chaos and the Void

     The worst fear is that a Hegemony riding the Horrors (weakly-godlike chaos creatures) will be so successful that the collapse will span the galaxy and then everything will end. There is evidence in the night sky that this has happened before.

---

• 

6.2. The Hub

6.2.6. Hegemony

     The Hegemony has made no inroads into the Hub. This is certainly a good thing — a Hegemony loose in the Hub would be devastating, placing trillions upon trillions of lives at risk.

6.3. The Rim

6.3.6. Hegemony

     The Hegemony does not seem to be interested in the Rim— several worlds have been dismantled but they are all on the fringes of expansion that is directed into an arm and not outwards into the Rim.

6.4. The Gulfs

6.4.6. Hegemony

     Hegemonies often originate near the Gulfs as physics and causality are weak there, allowing for technologies impossibly advanced and dangerously self-willed. An abandoned but functional research outpost should be terrifying: fortunes might be made but the risk of unleashing a Hegemonic algorithm is substantial.

6.5. The Manichaean Arm

6.5.1. Species

     …In fact the Manichaean Arm has the highest diversity of any arm, being home to at least ninety known species. This diversity may explain the arm’s resistance to hegemonic infiltrations—a hegemony that specializes will eventually find its targets exhausted. That’s not to say that a more adapative hegemony is impossible....

• 

WARNING! WARNING! WARNING!

With the voice and under the authority of the Galactic Volumetric Registry of the Conclave of Galactic Polities, this buoy issues the following warning:

The designated volume, including the englobed moon and its satellites, is a SECURED AREA by order of the Presidium of the Conclave. This volume MAY NOT BE APPROACHED for any reason.

This moon contains the remnant of a Class Three Perversion, including autonomous defensive technologies and other operational mechanisms, nanoviruses, infectious memes, certainty-level persuasive communicators, puppet ecologies, archives which must be presumed to contain resurrection seeds, and unknown other existential risks.  These dangers have not been disarmed, suppressed, or fully contained.

ACCEPT NO COMMUNICATION REQUESTS originating from within the englobed volume.  Memetic and information warfare systems are not known to be entirely inactive.

If any communication requests are received from the englobed volume, or other activity is noted within it, you must depart IMMEDIATELY, and report this activity to the Conclave Commission on Latent Threats WITHOUT DELAY.  A renascence of a perversion of this class poses a most serious and imminent threat to all local space and extranet-local systems.

Further, the englobement systems surrounding this volume are equipped for containment of remaining unidentified threats and the prevention of access. Any vessel approaching within 250,000 miles of the englobement grid, or attempting to communicate through the englobement grid, or attempting to actively scan through the englobement grid, will be fired upon without further warning.  Your presence has already been reported to higher authority, and escaping after transgressing the englobement grid will therefore not preserve you.

Naval vessels should note that this area has been deemed a black-level existential threat zone by the Presidium of the Conclave. This englobement grid does not respond to standard Accord command or diagnostic sequences. Interaction should not be attempted without explicit authorization and clearance from the Commission on Latent Threats.

DO NOT APPROACH, COMMUNICATE WITH, OR EXAMINE THIS VOLUME FURTHER!

No further warnings will be given.

• 

...Blights are regions which are interdicted due to the presence of either active hostile or runaway seed AIs, or their remnants – “operational mechanisms, nanoviruses, infectious memes, certainty-level persuasive communicators, puppet ecologies, archives which must be presumed to contain resurrection seeds”, and so forth, which pose potential existential threats.

They include not just large and active perversions such as the Leviathan Consciousness, but also areas formerly occupied by such and not yet known to be cleansed, such as the Charnel Cluster, and areas as small as a single moon or asteroid known to be the site of a failed experiment...

• 

(ed note: the post-apocalyptic novel takes place two thousand years after the disaster removed all oxygen from Terra's atmosphere, leaving only nitrogen. Humans survive using genetically engineered organisms that produce oxygen, more than conventional plants at any rate. The rest of the technology base is pretty crude, since nitric acid rain has dissolved anything made of metal. A few decades after the disaster a bunch of aliens called "Observers" appear. They travel cryogenically suspended in mobile comets and visit terrestrial planets with nitrogen atmospheres. An observer nicknamed "Bones" has befriended the Nomad couple Kahvi and Errih, and helps by dragging their raft/habitat around the ocean.)

      There had been no explosions, but this was only mildly encouraging. Many plants contained both reducing tissue and nitrates, arranged with varying degrees of intimacy. They burned with varying rates when something did light them; the nitrogen real-life mutated so frequently and grew so rapidly that one could never be sure just what an apparently familiar type would do. Pseudolife was far more reliable, but there was little of that in sight.
     Finally, however, they decided that there had been no witnesses to the presence of child or native, and went to work on the cargo.
     The copper was in sacks similar to the anchors, each containing twenty of the two-kilogram nuggets brought ashore by the pseudoliving metal-collecting robots which still bred and operated in the oceans. The change in Earth's air had been much harder on natural life than on the artificial varieties. Fifteen of the sacks were on the raft. Kahvi dropped each in turn into the meterdeep acid, and her husband carried them ashore, not lifting them above the surface until it was unavoidable. Bones moved some of them as close to shore as possible without appearing above the surface.
     The glass, similarly wrapped, consisted of whole window panes salvaged from the harbor bottom by the native. Vast numbers of these still lay where they had settled into the mud as the houses disintegrated around them. There had only been enough oxygen in Earth's atmosphere to make the oceans about one-hundredth-normal in nitric acid as it combined with the nitrogen, but rain, rivers, and even estuaries had often been much more concentrated during the fix. Metals in general were now completely dissolved, except for the nuggets which the pseudoliving collectors were still reducing in their mindless way. Glass and ceramics, of course, could still be found.
     By the time the last of the cargo was above high water mark the sun was low in the west. The jailbird had not reappeared, and if there had been any Hillers at the fire site they had shown no interest in the jail side of the ridge. There had been no more smoke, and no sounds, from that direction.

---

     "I think I see some of what's going on," she said slowly. Tell me, did any of these people use—well —dirty language."
     "Such as what?"
     "I—I can't give you any examples. About—you know—the things which were done to ruin the world." Fyn thought he saw what she meant.
     "You mean things like expressions from the Science Myths." Even the word science caused the woman to cringe a little, but she managed to answer.
     "Yes—that sort of thing."
     "I wouldn't say so—not the words, anyway. They did say they were trying to capture the animals they called Invaders to find out how they could be killed. They dodged the words, but what they were planning was certainly—that word you just now didn't like. Calling it something else doesn't change it. They didn't believe the usual story about the way the air changed—"
     "It's not a story! That's what happened. People tried to use—that method—to grow more food. It was a way to get more nitrogen, that their food plants needed, into the ground. The nitrogen combined with the oxygen, and there was a lot more nitrogen even then, so—"
     "I've heard the details. Many times. The point is that these people don't believe it; they think the Invaders did something to get the oxygen out of the air. They want to change it back, and think they'll have to get rid of the animals first."
     "Most of us, of course, believe that sinners experimenting with pseudolife made the change—they wanted nitrates to make food with, though I can't see what way that would work, and produced an organism which could take nitrogen from the air and release nitrates into the ground. It needed oxygen from the air, too, and worked too well. The Hemenway youngsters insist that the animals, which they call Invaders, did the same thing to get rid of the so they could live here.

(ed note: One of the three major components of fertilizer for crop growth is nitrogen. Some microorganisms and plants perform nitrogen fixation but they do a poor job of it. The disaster was caused by some idiot scientist genetically engineering a microorganism to do a better job. For some reason {see below} the germs got out of control and removed all the oxygen from the atmosphere.

The Delinquents are teenagers who don't believe that's how the disaster happened. They are convinced that the Observer aliens are the guilty party, which is not true. The Delinquents call the Observers "Invaders")

     "I wonder if Bones knows what really happened, though; have you ever asked?"
     "No. Neither of us ever thought of it," Earrin admitted.
     "Will you?" "Sure. I've never really believed there was enough oxygen loose in the air to make it breath able, but it would be nice to find out." He ignored Genda's gasp of outrage and turned to the Observer. The children listened and watched in fascination. Since the man's gestures served mainly to supplement his words, helping Bones distinguish phonemes which Observer auditory and nervous systems could not distinguish, some of the more observant youngsters began to catch on to the system very quickly. The return signals, however, were another matter; they were made not only with the two main handling tentacles but with the dozen much finer tendrils which formed a fore-and-aft fringe across the top of the head on each side of the great mouth. Since the translation came at intervals rather than continuously, none of the children made any progress with this aspect of the system.

     The translation itself was not always clear; neither Fyn nor the human listeners had an adequate information background. Human beings in general still knew some physics and astronomy, since many books still existed. They knew much more chemistry, since the technology which kept the remnants of humanity alive was based on biochemical products of earlier times.
     Mankind was in more or less the position of a motorcycle gang whose members could not have built their machines from metal ores or refined their own fuel, but were kept supplied with spare parts and gasoline. The continuation of the supplies was due to the fact that the resources of the biochemical culture were self-renewing, pseudoliving organisms and tissues. No person now living could have produced the pseudolife from raw chemicals, but many of them could still manipulate it with the aid of such agents as the Evolution plant's enzymes.

     It was clear that Bones had arrived on Earth after the air change, and was not responsible for it— He/they had travelled frozen in a body which an astronomer of earlier times would unhesitatingly have called a comet nucleus. These, traveling slowly between the stars, were constantly carrying vast numbers of Bones' species through the Galaxy in obedience to their basic psychological drive—the need to know. Automatic controls would place the comet into an appropriately close orbit and awaken the crew when and only when sensing apparatus identified an atmosphere as being primarily nitrogen, with enough traces of oxides of that element to indicate the presence of nitro-life. Bones' vehicle was the comet which now rode sixty degrees behind the moon in the latter's orbit; his/their landing craft was on the sea bottom thousands of kilometers from the Boston area.

     There was no doubt, however, that Bones was denying categorically that his kind had had anything to do with the change in the world's air.

     Zhamia, who had the best conception of the times and distances which must be involved, was rather suspicious of the account. She admitted that it was hard for her to believe in a coincidence which had apparently brought Bones' comet to the Solar system within a few years, or at most a few centuries, of the time the change had taken place. She put this to the Observer.
     Bones made it clear that no coincidence was involved. Many comets had undoubtedly visited the System; until the Earth conditions were correct, they would merely have gone into shortperiod orbits around the sun and waited, boiling off some of their ice at each perihelion passage until they were consumed. Their personnel would have died without ever being aroused (the Observers are hive entities, so dying does not bother them). When one crew was awakened, the other comets would, if they possessed mass enough, leave automatically in search of another habitable system.

• 

     "It may be the action of your kind which changed your world; I have been trying to decide. However, I must also decide why it always seems to happen, on every world I have seen."
     "Every one?" Mort took the question up instantly.
     "I certainly have not visited them all; how many there may be is knowledge not yet acquired. I have landed only on those with proper living conditions, and presumably only a small fraction of those. On every one of them there was much chemical evidence that some time earlier—always until now very much earlier—the world had had an oxygen-rich atmosphere. I have been trying to learn why this always changed, and it was a delight to find a world where the change seems to have been so recent. Perhaps I can really know. Perhaps your companion is right."

     "Have any oxygen-breathing people ever been living on the worlds you have seen?" asked Zhamia.
     "No, this is the first. This is why I am hopeful. Perhaps some of your people can supply knowledge —I have never before learned things indirectly, from other beings. This itself is a fascinating new field in which to learn. In any case, while I do know that one of your theories is wrong—I did not do anything to your air, and did not come here until it was as it is now—I do not know what is right, and wish very much to find out. It always seems to happen, on world after world. The air is always like this when we arrive—otherwise we wouldn't arrive—but close investigation always indicates that at some earlier time there was indeed much free oxygen. There always seems to have been life adapted to such a condition inevitably; oxygen is far too active to be in an atmosphere for long without life forms to maintain the supply." Kahvi nodded; basic biochemistry was part of even Nomad education. "It seems reasonable to infer that the ordinary course of evolution eventually produced an organism, probably microscopic in size and rapidly reproducing, which oxidized nitrogen; but inference is of course not knowledge by itself. However that may be, the same change seems to have occurred on all the worlds I have seen. An organism develops which oxidizes nitrogen with free oxygen, and a new equilibrium is reached between the two—the one this world has now, with only a trace of free oxygen."

     "The story is that men made the organism. We know they made many—most of the ones which keep us alive now are artificial," Kahvi agreed.
     "I can't see, though, why they'd make one which destroyed the air, except by mistake; and how could anyone make such a mistake? They would have had to think, first, surely. That's why most of the Nomads we know doubt that the world ever did have oxygen in its air."
     "You must know," she said at last, "what sort of life form does oxidize the nitrogen. Is it a pseudolife of the sort people made, or nitro-life like yours which is most of what grows now?"
     "It is nitro-life," the Observer replied. "There are a large number of species. However, much of the pseudolife you use is of the same variety. It would seem possible that artificial life made by your people was indeed responsible for the change."
     "But pseudolife is so stable, and nitro-life mutates so easily."
     "Which is why a single mistake on the part of your life technicians could have sufficed."

     "Could you make any of the kinds which oxidize nitrogen?"
     "Yes."
     "Would it be more difficult than making other kinds, or might it happen by accident?"
     "It could hardly happen by accident. All the organisms able to do this, on every world I remember, use one or another of four enzymes—you know what they are?"
     "Of course. The symbol is plain enough; we've talked about such things in our own life systems."

     "Those enzymes use a very surprising metal."
     "What's surprising about it?"
     "We have not been able to learn why it is so widespread in every planet's crust. It is one of the standing mysteries, which presents itself on world after world. It is a highly unreactive metal, which I would expect to find uncombined and highly localized. It should not be so thoroughly spread through a planet's soil and crust that a microbe can count on finding enough of its atoms whenever it needs them for its personal chemistry. One hypothesis is that a scientific race used and scattered it, but there has been no way to tell; I have never found a use for it myself, except in the most limited quantities in the laboratory. That is one reason I want to talk to these people of yours who seem to be somewhat scientific in nature. They might have knowledge of their own, even if memories are gone."
     Kahvi had her doubts about this; she felt that she knew pretty accurately the scientific status of the Delinquents. Essentially, they had probably been playing with cultures and Evolution Plant enzymes on a hit-or-miss basis. However, that was Bones problem.
     "What is this metal?" she asked. "Some obscure heavy element they never mentioned either in Surplus School or Citizen's Training, I suppose."
     "You know about it," Bones replied. A handling tentacle reached out and touched the gold bracelet on her left wrist.

     A slow grin spread over Kahvi's face. Her lip trembled, and she suddenly burst into laughter; peal after peal of uncontrollable laughter. Bones was familiar with this manifestation of the human nervous system's reaction to incongruity, but was mystified by the lack of any obvious incongruity this time.
     "I'll tell them that you know just what happened to the world's air—"
     "But I don't!" objected the Observer, startled.
     "But you certainly do. A hundred million—a billion—ten billion—whatever it was—people like me wearing gold ornaments. Jewelry. That's what spread the metal over the world so well. It's obvious. It wasn't your doing, it wasn't science's doing."

(ed note: So the original genetically-engineered nitrogen-fixing organism would not have destroyed Terra's oxygen, as long as it didn't encounter any large quantities of gold. It is just Terra's bad luck that humans love gold, so they accumulated large quantities of the stuff and spread it across the globe in the form of jewelry.)

• 

(ed note: our heroes have discovered the planet Magyan, which was accidentally settled by humans from the continent of Mu about twelve thousand years ago. Unfortunately they have been at war with the aliens of the planet Teff-el for almost as long. During our heroe's visit, the Tefflan unleash their ultimate weapon)

      Anto Rayl's feet sounded in the ball outside, his breath coming in gasps. "Aarn—Aarn—they've started! Their weapon—it's fire. It's started on the night side—weird fire—it burns blue—come—"
     The three Solarians were on their feet already, racing for the ports of the Sunbeam . In seconds Canning joined them and Martin.
     Out of the city in an instant—behind a thousand little fleeting scout and spy ships.
     "The fleet—Tefflan fleet—sneaked out somehow—got out into space, and came within about ten million miles of Magya before they were detected. Hundreds of oversize scout ships. They dropped thousands and thousands of bombs. They were going slowly then of course.
     "The bombs dropped to the planet Painted black—on the night side—they have been discovered only now. The fleet chased the Tefflan fleet away, and they thought that no damage had been done till the bombs began to land," Anto Rayl explained rapidly.
     Around the world from the afternoon side where they were to the night side they raced. It was a weird sight that met them. Enormous tongues of flame that stretched shimmering and pale-blue a hundred miles into the stratosphere, pale, blue, wavering light. They marched and countermarched, they rose and fell, and always sank.
     They started here and there. And they ended always on the ground. But flame, burning hot flames that were still mingled with the blue flame, and tinged, under powerful lights, with brown smoke. A dozen, darting, black silhouettes shot through the flames.
     "Spy ships."
     Aarn tuned in the radio. A Magyan's voice sounded sharp— "The temperature of the flames is so low they would not burn human flesh. But the chemical activity is strange. The flames on the ground are exceedingly hot, the ground itself is becoming incandescent. The vapors given off are foul and poisonous. They are red-brown in color—"
     "Great God in heaven! The catalyst!" Carlisle almost shrieked it. "Magya's doomed as sure as Teff-el if we can't stop that! It's the catalyst— the catalyst— don't you fools see— the nitrogen catalyst— the atmosphere here is sixty per cent nitrogen— twenty-one per cent oxygen— they've found the catalyst and the whole atmosphere of the planet is burning to foul, poisonous, burning nitro-compounds! The whole atmosphere is going—the catalyst is never used, and the atmosphere is burning itself away!"
          Anto Rayl bad gone pale; Aarn was looking pale, too. "Certain?" he asked sharply.
"What else?" snapped Carlisle. "Tell them to get samples —even if it costs lives. There's only one hope we can have. Only one way we can stop it, if—"
     "Attention—attention all Magyan scouts and spies—" Anto Rayl had snatched the microphone from Aarn's hand. "Anto Rayl, C-8-N32 speaking—commander In chief of spies and scouts speaking. Listen, and all government forces listen: Carlisle, chemist of the Other Space, says this is a catalyst which causes oxygen and nitrogen to combine, burning up oxygen of the atmosphere. Deadly to us if not stopped. Get a sample. He needs it. Get enough for all chemists to work on. That is an order—C-8-N32 speaking."

---

     The weird, blue flames leaped high, while on the ground they continued to glow, mingled with the brown fumes and the red flare of normal burning.
     Carlisle spoke rapidly: "The impossible catalyst. We've been looking for it for a century and a half. You see, nitrogen has one exothermic compound with oxygen—N₂O₅. That gives off heat in forming. Very little, comparatively, but some. That's why the flames were cold. N₂O₅ is the basis of nitric acid. it is a terrible oxidizing agent. All the organic matter in the soil is burning.
     "We want it because, controlled, it would generate free power from the air, and, more important, manufacture fixed nitrates for, literally, less than nothing at all. Uncontrolled, it is burning the precious oxygen and the nitrogen to form the deadly, corrosive nitric compounds.
     "Every particle of organic matter will be attacked. It will dissolve in the oceans and poison every species of creature. It will burn the air till there is no air, and sucks it out of your cities and leaves the people poisoned, and gasping. The oxygen will go, and with the nitrogen, to such a little volume the air pressure will fall, and then, no amount of locks will save your cities forever—"
     "Any luck?" asked Aarn, looking at Carlisle.
     The air in this room was very good, a little rich in oxygen. They were trying to help Carlisle all they could. In an air-tight retort, Carlisle had a sample of the deadly stuff. He looked up at Aarn's entrance. His face was pale, and haggard, his eyes tired.
     "Better take a rest, Carlisle," said Aarn.
     "No luck. Can't analyze it. Hasn't a chemical formula. Can't rest—the air pressure has fallen so now they can't keep up the pressure inside. That's only twenty days of it. And we can't stop it. We've got to stop it"
     "Know it. But you won't stop it being tired. Knock off for a while."
     "Say, Aarn, why don't they fix the locks here? They say they can't keep up the air pressure because the locks leak so. But they don't fix them. I thought they were proof against gases. Designed to keep poisonous gases out, and guaranteed gas-tight."
     "Uh—in the usual way," snorted Aarn. "They kept the poisonous gas out by pushing the good air out faster then it could come in. Result—they never planned on this. Then, too, it wouldn't do a bit of good to fix the locks, and rocks leak."
     "Rocks leak?"
     "Certainly! Full of cracks that they never worried about. Porous, so that it can seep through. With a pressure difference of nearly ten pounds to the square inch, that means leakage. And they can't suck it in fast enough to keep it up. Men not on active duty are being kept asleep as much as possible. Use less air."
     "Why can't they suck enough air in—build larger filters."
     "Time. They can't build them in time. They've started, of course. But, you see, they have to be proof against those ultra-microscopic particles. The worst of it is that it isn't the solid stuff that's doing the damage. Remember how terrible the gases were when they started the things—and the filters didn't get out the fine stuff.
     "It killed thousands—we missed the worst of it in the Sunbeam . In the civilian cities it was worse because they didn't have ships to retire to while adjustments were made. The filters would easily handle this—if they didn't have to be nearly choked off to stop the fine stuff."
     'I can't analyze this. I haven't found a poison for the catalyst. And I can't see any other way out. The rest of the chemists here are as helpless as I am."
     Carlisle turned wearily back to his work, and Aarn went out. He got into a little scout ship and was let out through a very small lock. In the twenty days that had passed, Magya had changed. The whole world was blanketed under a pall of white snow. But the snow was slushy and about two feet deep.
     It glowed blue, and the air glowed with a dim blue haze, and a constant rain of white crystals that fluttered gently down added to the slush. There were no more red flames of burning organic material. That was all burned away long since. The air was full of the floating particles of the catalyst dust. The ground was covered with it. The water was full of it.
     Thick, oily brooklets of thick nitric acid boiled and fumed brown as they crept down to the sea, bearing their load of undissolved, solid nitric pentoxide. The streams hissed and boiled water and brown vapor as they met the sea and dissolved. The air outside, could it have been smelled by any living creature, was a burning, terrible poison. Billions of tons of atmosphere had already been burned away.
     The burning was slower now, for each pound of nitrogen had carried down with it three pounds of oxygen, and the atmosphere was almost nothing but nitrogen and carbon dioxide. There was no water in the air, for the nitric oxide absorbed it, drank it greedily. The carbon dioxide was formed from every scrap of organic matter that had been on the planet.
     There were no fish in the sea, no plant nor any animal on the land, and no bird in the air. The humus in the soil was burned; the very rocks were being eaten by the corrosive, oily stuff.
     The nights were cold now, and the thick rivulets froze. The days were hot, and the snow melted under Anrel's rays and ran into the sea, liquid itself. The sea was an ocean of strong nitric acid. The very spy ship Aarn rode was being eaten slowly by the corrosive gas, and a trail of light brown fumes floated out behind.
     Out on Magya's four moons there was a deadly activity. Men were working with the grim determination that Teff-el should die, even as Magya perished, for there was little hope. Even if the catalyst stopped now, they feared the end would be inevitable.
     The air glowed blue, night and day. The great, clear stars of this space were invisible, for the light in the air hid them.
     Aarn settled to the planet and opened a little trap. It closed, and as the ship rose, there was a mass of nitric oxide in it. A moment later he had taken a similar sample from the water of the nitric-acid sea. Then he rose, and at an altitude of ten thousand feet took a sample of the thin air.

---

Aarn went down to the control room, and sat thinking. Aarn was not a chemist; he was a physicist, but like any scientist, he knew something of almost all others.      He tried to recall what he knew of nitrogen, of oxygen, of their mutual behavior.
     With a start he sat up, and finally got Carlisle's notes. "He writes a misbegotten sort of a hand," he said disgustedly, looking at the sloppy notes.
     The book was spotted with everything imaginable, and in many places there were irregular holes that grew when looked at, the paper was so rotted by acids and bases.
     Aarn sat down and at last came to what he wanted. A spectral analysis of the catalyst. Simple—thus far. But how were those elements combined?
     He looked at the list of them. And one he had rather hoped to see was there— titanium. With a sigh, he settled deeper and considered. Titanium, he remembered, was one of the few elements which burned readily in nitrogen; also oxygen. Perhaps that was the active principle.
     If he could just find some substance that would combine with the titanium permanently—he'd suggest that to Carlisle when he woke up, anyway.
     Spencer appeared presently, looking tired. "Been working out on Ma-ran. They're pushing the work. They have it almost all finished. Final plans called for an engine as powerful as the driving mechanism of Ma-kanee, but they've made it bigger. The air here's getting pretty bad. I hear they can't get oxygen. Why can't they take that damnable residue out side and break it down?"
     "The catalyst," said Aarn with a gentle sigh. "That, like the famous French phrase of the 1915's 'c'est la guerre' ("That's war!"), is the answer to all questions here, at present. They can't get the catalyst out of the snow or the liquid, and if they try to break it up, it just goes right back together, and the catalyst tends to escape into the air of the room—and that's why they don't do it."

---

     "THAT IDEA is interesting—you have a notion it acts some thing like the haemoglobin in the blood, then—carries combined nitrogen to oxygen, is freed of the nitrogen by the oxygen, and immediately goes back for more nitrogen in some way?" Carlisle seemed to turn the idea over in his mind. "Interesting thought—but the thing is, titanium wouldn't do that—it's just against all chemistry."
     "Uhmmmm—so's this blinkin' catalyst. You haven't made any better suggestions, I take it. Now what would stop it, if that's the case? You know—something like carbon monoxide in the blood. Combines with the haemoglobin and stays that way."
     "The chloride is a liquid. And peculiarly stable. The flouride is a solid—I've tried all those things—all the gases I could think of."
     "Well, try some more. Try something like ClO₂—a compound of chlorine for instance, with oxygen, so while the oxygen of the compound grabs the nitrogen, the chlorine can slip in."
     "This isn't a game of puss in the corner," objected Carlisle. "But I get your idea—I think I'll try something on that idea. But it will have to be an oxy-compound, because nothing else can exist—out there!"

---

     It was the twenty-third day when Carlisle came out. The air in the city was unbreathably thin. The oxygen content was so low that people were gasping for breath under the slightest exertion, while many of the old, those with weak heart or lungs, had already gone into eternal sleep.
     But Aarn recognized Carlisle's joy as he came out. "I've got something—it worked a little—" It was a sealed glass tube, a peculiar greenish-yellow liquid that had the characteristic color of substances green by transmitted and yellow by reflected light. It was volatile, thin liquid, and Carlisle was volatile today.
     "Come—look—" Aarn was already beside him. The laboratory door shut, and the air inside was noticeably richer, the oxygen content higher.
     In the glass dome, there was a little pile of the deadly white nitrogen snow, and a steady blue glow, where oxygen and nitrogen fed into it. Carlisle broke the stem of his sealed tube, and a pungent, biting odor spread through the room. The liquid boiled instantly, and frost collected on the tube. Carlisle held it in a cloth now, as the gas was sucked into the bell jar.
     Instantly the blue flame died, and went out. Carlisle resealed his tube in a flame, and no amount of shaking revived the catalyst action.
     "I've heated it; I've separated the N₂O₅, and I've done everything else. The catalyst is dead—and stays that way!"
     "Can we make that?" demanded Aarn.
     "We've got to," answered Carlisle. "And we can. It takes— I don't know how little. It's the poison. It's a chlorine compound—you were right. My Lord, the thing—it's done! We—we can live here!"
     "Shut up!" snapped Aarn. The man was nearly hysterical from the release of strain. "Get to work and teach the other chemists. We need tons of it—we've got to open those filters first—let the poisonous air come in, and take that catalyst out. Is the anticatalyst poisonous?"
     "No—see." Carlisle took a deep breath of the fumes. "It's biting—acrid—a heavy dose might be, but only as hydrochloric acid is. It burns a bit, but in the low concentration. Send all the men up—I've got the poison!"

---

     Hours later flasks of the stuff were coming out. A thin, volatile liquid. Chemists were regulating the flow into the great air filters of the city. In every city similar scenes were being witnessed. The catalyst was no longer deadly. The air could be sucked in more rapidly, the poisons eliminated.
     Hours later still—great retorts set up—glass chambers with their loads of deadly snow were being cooked, and plentiful, rich air came out— Three days—the gas was boiling out of great tanks now, being dropped from low-flying machines, and high-flying machines, and everywhere the catalyst sucked it in—and died.
     The flame of deadly blue died, and the nitric oxide at last began to cease its constant fall. Little light flakes drifted down for days, as slow release of the catalyst poison found the last active trace of catalyst.
     Already, huge beam stations had been set up, and great sunbeams reached out to tap the main beam, and hour after hour, great fan arcs reached out across the ground, and the burning arcs caught the snow, converted it in an instant to vapor at thousands of degrees—and smashed the snow back to pure oxygen and nitrogen, the lesser oxides of nitrogen breaking down instantly in the arc. Other stations were being built by the sides of the lakes, by the oceans, at the mouths of rivers. The thick, oily rivers, loaded with the terrible, burning stuff flashed into clean, burning electric flame, and the nitric gases returned to the air as nitrogen and oxygen.
     In the cities, life was easy once more, air plentiful, and the outside air pressure rapidly rising. It would be years, perhaps, before the last of the terrible nitric oxide was gone, before men could venture forth into—. Rains were falling again, now. After a brief three days the terrific arcs that boiled away seas and lakes and rivers were throwing rain clouds into the air, and rain began to wash the stuff in thick torrents to the sea, clearing the land rapidly.
     The water was loaded with thousands of tons of soluble stuff. Landslides, even minor quakes, changed the topography as the terrific erosive actions became evident. In mere days, the erosion of centuries had taken place!
     But it would be years before the soil was fit to bear again, centuries before the seas had been freed of their deadly load of acid. For, active as nitric acid is, and unstable as its compounds are, there were billions on billions of tons of it.
     "It will be a generation," said Carlisle, "before the land will be fertile again. Then it will be superfertile, for the place is soaked in nitrates as no other land ever was. The Tefflans have solved the problem of nitrates for all time to come."

• 

“The meteorite itself was so massive that it didn’t notice any atmosphere whatsoever,” said Rebolledo. “It was traveling 20 to 40 kilometers per second, 10 kilometers—probably 14 kilometers—wide, pushing the atmosphere and building such incredible pressure that the ocean in front of it just went away.”

These numbers are precise without usefully conveying the scale of the calamity. What they mean is that a rock larger than Mount Everest hit planet Earth traveling twenty times faster than a bullet. This is so fast that it would have traversed the distance from the cruising altitude of a 747 to the ground in 0.3 seconds. The asteroid itself was so large that, even at the moment of impact, the top of it might have still towered more than a mile above the cruising altitude of a 747. In its nearly instantaneous descent, it compressed the air below it so violently that it briefly became several times hotter than the surface of the sun.

“The pressure of the atmosphere in front of the asteroid started excavating the crater before it even got there,” Rebolledo said. “Then, when the meteorite touched ground zero, it was totally intact. It was so massive that the atmosphere didn’t even make a scratch on it.”

Unlike the typical Hollywood CGI depictions of asteroid impacts, where an extraterrestrial charcoal briquette gently smolders across the sky, in the Yucatan it would have been a pleasant day one second and the world was already over by the next. As the asteroid collided with the earth, in the sky above it where there should have been air, the rock had punched a hole of outer space vacuum in the atmosphere. As the heavens rushed in to close this hole, enormous volumes of earth were expelled into orbit and beyond—all within a second or two of impact.

“So there’s probably little bits of dinosaur bone up on the moon?” I asked.

“Yeah, probably.”

• 

(ed note: A wealthy soap company heir and amateur astronomer co-discover a new comet. It is going to pass really close to Terra. A documentary producer persuades Hamner to have his family's company sponsor a television documentary series on the subject. The first episode features a pair of scientist: Dr. Sharps and Dr. Forrest. They figure it would be better if they made an analogy for the audience…)

      "Oh." Harvey nodded. "So what have you got on Hamner-Brown? How far away at its closest point?"
     "Zero," Forrester said. He didn't crack a smile.
     "Uh—you mean it's coming down our throats?"
     "I doubt it." Now he smiled. "Zero within the limits of prediction. Which is a good half-million miles error."

     Harvey relaxed. So, he noticed, did everyone in the room, including Charlene. They took Forrester seriously here. He turned to Sharps. "Tell us, what would happen if the comet did hit us? Suppose we got unlucky."
     "You mean the head? The nucleus? Because it looks as if we might actually pass through the outer coma. Which is nothing more than gas."
     "No, I mean the head. What happens? The end of the world?"
     "Oh, no. Nothing like that. Probably the end of civilization."

     There was silence in the room for a moment. Then for another. "But," Harvey said, his voice puzzled, "Dr. Sharps, you told me that a comet, even the head, is largely foamy ice with rocks in it. And even the ice is frozen gases. That doesn't sound dangerous." In fact, Harvey thought, I asked to get it on the record.
     "Several heads," Dan Forrester said. "At least it looks that way. I think it's beginning to calve already. And if it does it now, it will do it later. Probably. Maybe."
     "So it's even less dangerous," Harvey said.
     Sharps wasn't listening to Harvey. He rolled his eyes toward the ceiling. "Calving already?"
     Forrester's grin widened. "Ook ook."

     Then he noticed Harvey Randall again. "You asked about danger," he said. "Let's look at it. We have several masses, largely the same material that boils off to form the coma and the tail: fine dust, foamy frozen gases, with pockets where the really volatile stuff has been long gone, and maybe a few rocks embedded in there. Hey—" Randall looked up at Forrester.
     Forrester was grinning his cherubic smile. "That's probably why it's so bright already. Some of the gases are interacting. Think what we'll see when they really get to boiling near the Sun! Ook ook."
     Sharps was getting that thoughtful, lost look again. Harvey said quickly, "Dr. Sharps—"
     "Oh. Yes, certainly. What happens if it hits? Which it won't.
     Well, what makes the nucleus dangerous is that it's big, and it's coming fast. Enormous energies."
     "Because of the rocks?" Harvey asked. Rocks he could understand. "How big are those rocks?"
     "Not very," Forrester said. "But that's theory—"
     "Right." Sharps was aware of the camera again. "That's why we need the probe. We don't know. But I'd guess the rocks are small, from the size of a baseball to the size of a small hill."
     Harvey felt relief. That couldn't be dangerous. A small hill?
     "But of course that doesn't matter," Sharps said. "They'll be embedded in the frozen gases and water ice. It would all hit as several solid masses. Not as a lot of little chunks."
     Harvey paused to think that over. This film would take careful editing. "It still doesn't sound dangerous. Even nickel-iron meteors usually burn out long before they hit the ground. In fact, in all history there's only been one recorded case of anyone being harmed by a meteor."
     "Sure, that lady in Alabama," Forrester said. "It got her picture in Life. Wow, that was the biggest bruise I ever saw. Wasn't there a lawsuit? Her landlady said it was her meteor because it ended up in her basement."
     Harvey said, "Look. Hamner-Brown will hit atmosphere a lot harder than any normal meteorite, and it's mostly ice. The masses will burn faster, won't they?"
     He saw two shaking heads: a thin face wearing insectile glasses, and a thick bushy beard above thick glasses. And over against the wall Mark was shaking his head too. Sharps said, "They'd bore through quicker. When the mass is above a certain size, it stops being important whether Earth has an atmosphere or not."
     "Except to us," Forrester said, deadpan.

     Sharps paused a second, then laughed. Politely, Harvey thought, but it was done carefully. Sharps took pains to avoid offending Forrester. "What we need is a good analogy. Um . . ." Sharps's brow furrowed.

     "Hot fudge sundae," said Forrester.

     "Hah?"
     Forrester's grin was wide through his beard. "A cubic mile of hot fudge sundae. Cometary speeds."
     Sharps's eyes lit up. "I like it! Let's hit Earth with a cubic mile of hot fudge sundae."
     Lord God, they've gone bonkers, Harvey thought. The two men raced each other to the blackboard. Sharps began to draw. "Okay. Hot fudge sundae. Let's see: We'll put the vanilla ice cream in the center with a layer of fudge over it . . ."
     He ignored the strangled sound behind him. Tim Hamner hadn't said a word during the whole interview. Now he was doubled over, holding himself, trying to hold in the laughter. He looked up, choked, got his face straight, said, "I can't stand it!" and brayed like a jackass. "My comet! A cubic mile of hot … fudge … sun … dae …"
     "With the fudge as the outer shell," Forrester amplified, "so the fudge will heat up when the Hammer rounds the Sun."
     "That's Hamner-Brown," Tim said, straight-faced.
     "No, my child, that's a cubic mile of hot fudge sundae. And the ice cream will still be frozen inside the shell," said Sharps.
     Harvey said, "But you forgot the—"
     "We put the cherry at one pole and say that pole was in shadow at perihelion." Sharps sketched to show that when the comet rounded the Sun, the cherry at the oblate spheroid's axis would be on the side away from Sol. "We don't want it scorched. And we'll put crushed nuts all through it, to represent rocks. Say a two-hundred-foot cherry?"
     "Carried by the Royal Canadian Air Force," Mark said.
     "Stan Freberg! Right!" Forrester whooped. "Shhhh … plop! Let's see you do that on television!"
     "And now, as the comet rounds the Sun, trailing a luminous froth of fake whipped cream, and aims itself down our throats … Dan, what's the density of vanilla ice cream?"
     Forrester shrugged. "It floats. Say two-thirds."
     "Right. Point six six six it is." Sharps seized a pocket calculator from the desk and punched frantically. "I love these things. Used to use slide rules. Never could figure out where the decimal point went.
     "A cubic mile to play with. Five thousand two hundred and eighty feet, times twelve for inches, times two point five four for centimeters, cube that … We have two point seven seven six times ten to the fifteenth cubic centimeters of vanilla ice cream. It would take a while to eat it all. Times the density, and lo, we have about two times ten to the fifteenth grams. Couple of billion tons. Now for the fudge …" Sharps punched away.
     Happy as a clam, Harvey thought. A very voluble clam equipped with Texas Instruments' latest pocket marvel.
     "What do you like for the density of hot fudge?" Sharps asked.
     "Call it point nine," Forrester said.
     "Haven't any of you made fudge?" Charlene demanded. "It doesn't float. You test it by dripping it into a cup of cold water. Or at least my mother did."
     "Say one point two, then," Forrester said.
     "Another billion and a half tons of hot fudge," Sharps said. Behind him Hamner made more strangled noises.
     "I think we can ignore the rocks," Sharps said. "Do you see why, now?"
     "Lord God, yes," Harvey said. He looked at the camera with a start. "Uh, yes, Dr. Sharps, it certainly makes sense to ignore the rocks."
     "You're not going to show this, are you?" Tim Hamner sounded indignant.
     "You're saying no?" Harvey asked.
     "No … no …" Hamner doubled over and giggled.
     "Now, she's coming at cometary speeds. Fast. Let's see, parabolic speed at Earth orbit is what, Dan?"
     "Twenty-nine point seven kilometers per second. Times square root of two."
     "Forty-two kilometers a second," Sharps announced. "And we've got Earth's orbital velocity to add. Depends on the geometry of the strike. Shall we say fifty kilometers a second as a reasonable closing velocity?"
     "Sounds good," Forrester said. "Meteors go from twenty to maybe seventy. It's reasonable."
     "Right. Call it fifty. Square that, times a half. Times mass in grams. Bit over two times ten to the twenty-eight ergs. That's for the vanilla ice cream. Now we can figure that most of the hot fudge boiled away, but understand, Harvey, at those speeds we're just not in the atmosphere very long. If we come in straight it's two seconds flat! Anyway, whatever mass you burn up, a lot of the energy just gets transferred to the earth's heat balance. That's a spectacular explosion all by itself. We'll figure twenty percent of the hot-fudge energy transfers to Earth, and"—more buttons pressed, and dramatic rise in voice—"our grand total is two point seven times ten to the twenty-eighth ergs. Okay, that's your strike."
     "Doesn't mean much to me," Harvey said. "It sounds like a big number …"
     "One followed by twenty-eight zeros," Mark muttered.
     "Six hundred and forty thousand megatons, near enough," Dan Forrester said gently. "It is a big number."
     "Good God, pasteurized planet," Mark said.
     "Not quite." Forrester had his own calculator out of the belt case. "About three thousand Krakatoas. Or three hundred Thera explosions, if they're right about Thera."
     "Thera?" Harvey asked.
     "Volcano in the Mediterranean," Mark said. "Bronze Age. Where the Atlantis legend comes from."
     "Your friend's right," Sharps said. "I'm not sure about the energy, though. Look at it this way. All of mankind uses about ten to the twenty-ninth ergs in a year. That's everything: electric power, coal, nuclear energy, burning buffalo chips, cars—you name it. So our hot fudge sundae pops in with about thirty percent of the world's annual energy budget."
     "Um. Not so bad, then," Harvey said.
     "Not so bad. Not so bad as what? A year's energy in one minute," Sharps said. "It probably hits water. If it hits land, it's tough for anyone under it, but most of the energy radiates back out to space fairly quickly. But if it hits water, it vaporizes it. Let's see, ergs to calories … damn. I don't have that on my gadget."
     "I do," Forrester said. "The strike would vaporize about sixty million cubic kilometers of water. Or fifty billion acrefeet, if you like that. Enough to cover the entire U.S.A. with two hundred and twelve feet of water."
     "All right," Sharps said. "So sixty million cubic kilometers of water go into the atmosphere. Harvey, it's going to rain. A lot of that water is moving across polar areas. It freezes, falls as snow. Glaciers form fast … slide south … yeah. Harvey, the historians believe the Thera explosion changed the world's climate. We know that Tamboura, about as powerful as Krakatoa, caused what historians of the last century called 'the year without a summer'. Famine. Crop failure. Our hot fudge sundae will probably trigger an ice age. All those clouds. Clouds reflect heat. Less sunlight gets to Earth. Snow reflects heat too. Still less sunlight. It gets colder. More snow falls. Glaciers move south because they don't melt as fast. Positive feedback."
     It had all turned dead serious. Harvey asked, "But what stops ice ages?"
     Forrester and Sharps shrugged in unison.
     "So," Hamner said, "my comet's going to bring about an ice age?" Now you could see the long lugubrious face of his grandfather, who could look bereaved at a $60,000 funeral.
     Forrester said, "No, that was hot fudge sundae we were talking about. Um—the Hammer is bigger."
     "Hamner-Brown. How much bigger?"
     Forrester made an uncertain gesture. "Ten times?"
     "Yes," said Harvey. There were pictures in his mind. Glaciers marched south across fields and forests, across vegetation already killed by snow. Down across North America into California, across Europe to the Alps and Pyrenees. Winter after winter, each colder, each colder than the Great Freeze of '76-'77. And hell, they hadn't even mentioned the tidal waves. "But a comet won't be as dense as a cubic mile of h-h-h—"
     It was just one of those things. Harvey leaned back in his chair and belly-laughed, because there was just no way he could say it.

• 

Mature people who were children in the 1950s grew up knowing that the entire world could die screaming in any given five-minute interval from atomic attack.

But we were carefully taught what to do when the enemy nuclear ICBMs started exploding cities into radioactive mushroom clouds. You duck and cover. Then stick your head between your knees so you can kiss your little heinie good-bye, before the blast of atomic radiation converts your body into a macabre silhouette on the wall.

In other words: all that "duck and cover" crap is just some busy-work to take the children's mind off their impending demise and to keep them quiet as they wait for death.

• 

• 

• 

A nuclear holocaust, nuclear apocalypse or atomic holocaust is a theoretical scenario involving widespread destruction and radioactive fallout causing the collapse of civilization, through the use of nuclear weapons. Under such a scenario, some or all of the Earth is made uninhabitable by nuclear warfare in future world wars.

Besides the immediate destruction of cities by nuclear blasts, the potential aftermath of a nuclear war could involve firestorms, a nuclear winter, widespread radiation sickness from fallout, and/or the temporary loss of much modern technology due to electromagnetic pulses. Some scientists, such as Alan Robock, have speculated that a thermonuclear war could result in the end of modern civilization on Earth, in part due to a long-lasting nuclear winter. In one model, the average temperature of Earth following a full thermonuclear war falls for several years by 7 to 8 degrees Celsius on average.

Early Cold War-era studies suggested that billions of humans would nonetheless survive the immediate effects of nuclear blasts and radiation following a global thermonuclear war. Some scholars argue that nuclear war could indirectly contribute to human extinction via secondary effects, including environmental consequences, societal breakdown, and economic collapse. Additionally, it has been argued that even a relatively small-scale nuclear exchange between India and Pakistan involving 100 Hiroshima yield (15 kilotons) weapons, could cause a nuclear winter and kill more than a billion people.

Since 1947, the Doomsday Clock of the Bulletin of the Atomic Scientists has visualized how close the world is to a nuclear war.

The threat of a nuclear holocaust plays an important role in the popular perception of nuclear weapons. It features in the security concept of mutually assured destruction (MAD) and is a common scenario in survivalism. Nuclear holocaust is a common feature in literature and film, especially in speculative genres such as science fiction, dystopian and post-apocalyptic fiction.

Likelihood of nuclear war

See also: World War III § Historical close calls

As of 2016, humanity has about 15,000 nuclear weapons, thousands of which are on hair-trigger alert. While stockpiles have been on the decline following the end of the Cold War, every nuclear country is currently undergoing modernization of its nuclear arsenal. Some experts believe this modernization may increase the risk of nuclear proliferation, nuclear terrorism, and accidental nuclear war.

John F. Kennedy estimated the probability of the Cuban Missile Crisis escalating to nuclear conflict as between 33% and 50%.

In a poll of experts at the Global Catastrophic Risk Conference in Oxford (17‐20 July 2008), the Future of Humanity Institute estimated the probability of complete human extinction by nuclear weapons at 1% within the century, the probability of 1 billion dead at 10% and the probability of 1 million dead at 30%. These results reflect the median opinions of a group of experts, rather than a probabilistic model; the actual values may be much lower or higher.

Scientists have argued that even a small-scale nuclear war between two countries could have devastating global consequences and such local conflicts are more likely than full-scale nuclear war.

Likelihood of complete human extinction

See also: Cobalt bomb and Nuclear winter

Many scholars have posited that a global thermonuclear war with Cold War-era stockpiles, or even with the current smaller stockpiles, may lead to human extinction. This position was bolstered when nuclear winter was first conceptualized and modelled in 1983. However, models from the past decade consider total extinction very unlikely, and suggest parts of the world would remain habitable. Technically the risk may not be zero, as the climactic effects of nuclear war are uncertain and could theoretically be larger than current models suggest, just as they could theoretically be smaller than current models suggest. There could also be indirect risks, such as a societal collapse following nuclear war that can make humanity much more vulnerable to other existential threats.

A related area of inquiry is: if a future nuclear arms race someday leads to larger stockpiles or more dangerous nuclear weapons than existed at the height of the Cold War, at what point could a war with such weapons result in human extinction? Physicist Leo Szilard warned in the 1950s that a deliberate "doomsday device" could be constructed by surrounding powerful hydrogen bombs with a massive amount of cobalt. Cobalt has a half-life of five years, and its global fallout might, some physicists have posited, be able to clear out all human life via lethal radiation intensity. The main motivation for building a cobalt bomb in this scenario is its reduced expense compared with the arsenals possessed by superpowers; such a doomsday device does not need to be launched before detonation, and thus does not require expensive missile delivery systems, and the hydrogen bombs do not need to be miniaturized for delivery via missile. The system for triggering it might have to be completely automated, in order for the deterrent to be effective. A modern twist might be to also lace the bombs with aerosols designed to exacerbate nuclear winter. A major caveat is that nuclear fallout transfer between the northern and southern hemispheres is expected to be small; unless a bomb detonates in each hemisphere, the effect of a bomb detonated in one hemisphere on the other is diminished.

Effects of nuclear war

Historically, it has been difficult to estimate the total number of deaths resulting from a global nuclear exchange because scientists are continually discovering new effects of nuclear weapons, and also revising existing models.

Early reports considered direct effects from nuclear blast and radiation and indirect effects from economic, social, and political disruption. In a 1979 report for the U.S. Senate, the Office of Technology Assessment estimated casualties under different scenarios. For a full-scale countervalue/counterforce nuclear exchange between the U.S. and the Soviet Union, they predicted U.S. deaths from 35 to 77 percent (70 million to 160 million dead at the time), and Soviet deaths from 20 to 40 percent of the population.

Although this report was made when nuclear stockpiles were at much higher levels than they are today, it also was made before the risk of nuclear winter was discovered in the early 1980s. Additionally, it did not consider other secondary effects, such electromagnetic pulses (EMP), and the ramifications they would have on modern technology and industry.

Nuclear winter

Main article: Nuclear winter

In the early 1980s, scientists began to consider the effects of smoke and soot arising from burning wood, plastics, and petroleum fuels in nuclear-devastated cities. It was speculated that the intense heat would carry these particulates to extremely high altitudes where they could drift for weeks and block out all but a fraction of the sun's light. A landmark 1983 study by the so-called TTAPS team (Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack and Carl Sagan) was the first to model these effects and coined the term "nuclear winter."

More recent studies make use of modern global circulation models and far greater computer power than was available for the 1980s studies. A 2007 study examined consequences of a global nuclear war involving moderate to large portions of the current global arsenal. The study found cooling by about 12–20 °C in much of the core farming regions of the US, Europe, Russia and China and as much as 35 °C in parts of Russia for the first two summer growing seasons. The changes they found were also much longer lasting than previously thought, because their new model better represented entry of soot aerosols in the upper stratosphere, where precipitation does not occur, and therefore clearance was on the order of 10 years. In addition, they found that global cooling caused a weakening of the global hydrological cycle, reducing global precipitation by about 45%.

The authors did not discuss the implications for agriculture in depth, but noted that a 1986 study which assumed no food production for a year projected that "most of the people on the planet would run out of food and starve to death by then" and commented that their own results show that, "This period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse than previously thought."

In contrast to the above investigations of global nuclear conflicts, studies have shown that even small-scale, regional nuclear conflicts could disrupt the global climate for a decade or more. In a regional nuclear conflict scenario where two opposing nations in the subtropics would each use 50 Hiroshima-sized nuclear weapons (about 15 kiloton each) on major populated centres, the researchers estimated as much as five million tons of soot would be released, which would produce a cooling of several degrees over large areas of North America and Eurasia, including most of the grain-growing regions. The cooling would last for years, and according to the research, could be "catastrophic". Additionally, the analysis showed a 10% drop in average global precipitation, with the largest losses in the low latitudes due to failure of the monsoons.

Regional nuclear conflicts could also inflict significant damage to the ozone layer. A 2008 study found that a regional nuclear weapons exchange could create a near-global ozone hole, triggering human health problems and impacting agriculture for at least a decade. This effect on the ozone would result from heat absorption by soot in the upper stratosphere, which would modify wind currents and draw in ozone-destroying nitrogen oxides. These high temperatures and nitrogen oxides would reduce ozone to the same dangerous levels we now experience below the ozone hole above Antarctica every spring.

Nuclear famine

Main article: Nuclear famine

It is difficult to estimate the number of casualties that would result from nuclear winter, but it is likely that the primary effect would be global famine (known as Nuclear Famine), wherein mass starvation occurs due to disrupted agricultural production and distribution. In a 2013 report, the International Physicians for the Prevention of Nuclear War (IPPNW) concluded that more than two billion people, about a third of the world's population, would be at risk of starvation in the event of a regional nuclear exchange between India and Pakistan, or by the use of even a small proportion of nuclear arms held by the U.S. and Russia. Several independent studies show corroborated conclusions that agricultural outputs will be significantly reduced for years by climatic changes driven by nuclear wars. Reduction of food supply will be further exacerbated by rising food prices, affecting hundreds of millions of vulnerable people, especially in the poorest nations of the world.

Electromagnetic pulse

See also: Nuclear electromagnetic pulse and High-altitude nuclear explosion

An electromagnetic pulse (EMP) is a burst of electromagnetic radiation. Nuclear explosions create a pulse of electromagnetic radiation called a nuclear EMP or NEMP. Such EMP interference is known to be generally disruptive or damaging to electronic equipment. If a single nuclear weapon "designed to emit EMP were detonated 250 to 300 miles up over the middle of the country it would disable the electronics in the entire United States."

Given that many of the comforts and necessities we enjoy in the 21st century are predicated on electronics and their functioning, an EMP would disable hospitals, water treatment facilities, food storage facilities, and all electronic forms of communication. An EMP blast threatens the foundation which supports the existence of the modern human condition. Certain EMP attacks could lead to large loss of power for months or years Currently, failures of the power grid are dealt with using support from the outside. In the event of an EMP attack, such support would not exist and all damaged components, devices, and electronics would need to be completely replaced.

In 2013, the US House of Representatives considered the "Secure High-voltage Infrastructure for Electricity from Lethal Damage Act" that would provide surge protection for some 300 large transformers around the country. The problem of protecting civilian infrastructure from electromagnetic pulse has also been intensively studied throughout the European Union, and in particular by the United Kingdom. While precautions have been taken, James Woolsey and the EMP Commission suggested that an EMP is the most significant threat to the U.S. The greatest threat to human survival in the aftermath of an EMP blast would be the inability to access clean drinking water. For comparison, in the aftermath of the 2010 Haitian earthquake, the water infrastructure had been devastated and led to at least 3,333 deaths from cholera in the first few months after the earthquake. Other countries would similarly see the resurgence of previously non-existent diseases as clean water becomes increasingly scarce.

The risk of an EMP, either through solar or atmospheric activity or enemy attack, while not dismissed, was suggested to be overblown by the news media in a commentary in Physics Today. Instead, the weapons from rogue states were still too small and uncoordinated to cause a massive EMP, underground infrastructure is sufficiently protected, and there will be enough warning time from continuous solar observatories like SOHO to protect surface transformers should a devastating solar storm be detected.

Nuclear fallout

Main article: Nuclear fallout

Nuclear fallout is the residual radioactive dust and ash propelled into the upper atmosphere following a nuclear explosion. Fallout is usually limited to the immediate area, and can only spread for hundreds of miles from the explosion site if the explosion is high enough in the atmosphere. Fallout may get entrained with the products of a pyrocumulus cloud and fall as black rain (rain darkened by soot and other particulates).

This radioactive dust, usually consisting of fission products mixed with bystanding atoms that are neutron activated by exposure, is a highly dangerous kind of radioactive contamination. The main radiation hazard from fallout is due to short-lived radionuclides external to the body. While most of the particles carried by nuclear fallout decay rapidly, some radioactive particles will have half-lives of seconds to a few months. Some radioactive isotopes, like strontium 90 and cesium 137, are very long lived and will create radioactive hot spots for up to 5 years after the initial explosion. Fallout and black rain may contaminate waterways, agriculture, and soil. Contact with radioactive materials can lead to radiation poisoning through external exposure or accidental consumption. In acute doses over a short amount of time radiation will lead to prodromal syndrome, bone marrow death, central nervous system death and gastrointestinal death. Over longer periods of exposure to radiation, cancer becomes the main health risk. Long term radiation exposure can also lead to in utero effects on human development and transgenerational genetic damage.

Origins and analysis of extinction hypotheses

As a result of the extensive nuclear fallout of the 1954 Castle Bravo nuclear detonation, author Nevil Shute wrote the popular novel On the Beach which was released in 1957, in this novel so much fallout is generated in a nuclear war that all human life is extinguished. However the premise that all of humanity would die following a nuclear war and only the "cockroaches would survive" is critically dealt with in the 1988 book Would the Insects Inherit the Earth and Other Subjects of Concern to Those Who Worry About Nuclear War by nuclear weapons expert Philip J. Dolan.

In 1982 nuclear disarmament activist Jonathan Schell published The Fate of the Earth, which is regarded by many to be the first carefully argued presentation that concluded that extinction is a significant possibility from nuclear war. However, the assumptions made in this book have been thoroughly analyzed and determined to be "quite dubious". The impetus for Schell's work, according to physicist Brian Martin, was to argue that "if the thought of 500 million people dying in a nuclear war is not enough to stimulate action, then the thought of extinction will. Indeed, Schell explicitly advocates use of the fear of extinction as the basis for inspiring the "complete rearrangement of world politics".

The belief in "overkill" is also commonly encountered, with an example being the following statement made by nuclear disarmament activist Philip Noel-Baker in 1971 – "Both the US and the Soviet Union now possess nuclear stockpiles large enough to exterminate mankind three or four – some say ten – times over". Brian Martin suggested that the origin of this belief was from "crude linear extrapolations", and when analyzed it has no basis in reality. Similarly, it is common to see stated that the combined explosive energy released in the entirety of World War II was about 3 megatons, while a nuclear war with warhead stockpiles at Cold War highs would release 6000 WWII's of explosive energy. An estimate for the necessary amount of fallout to begin to have the potential of causing human extinction is regarded by physicist and disarmament activist Joseph Rotblat to be 10 to 100 times the megatonnage in nuclear arsenals as they stood in 1976; however, with the world megatonnage decreasing since the Cold War ended this possibility remains hypothetical.

According to the 1980 United Nations report General and Complete Disarmament: Comprehensive Study on Nuclear Weapons: Report of the Secretary-General, it was estimated that there were a total of about 40,000 nuclear warheads in existence at that time, with a potential combined explosive yield of approximately 13,000 megatons.

By comparison, in the Timeline of volcanism on Earth when the volcano Mount Tambora erupted in 1815 – turning 1816 into the Year Without A Summer due to the levels of global dimming sulfate aerosols and ash expelled – it exploded with a force of roughly 800 to 1,000 megatons, and ejected 160 km³ (38 cu mi) of mostly rock/tephra, which included 120 million tonnes of sulfur dioxide as an upper estimate. A larger eruption, approximately 74,000 years ago, in Mount Toba produced 2,800 km³ (670 cu mi) of tephra, forming lake Toba, and produced an estimated 6,000 million tonnes (6.6×10⁹ short tons) of sulfur dioxide. The explosive energy of the eruption may have been as high as equivalent to 20,000,000 megatons (Mt) of TNT, while the Chicxulub impact, connected with the extinction of the dinosaurs, corresponds to at least 70,000,000 Mt of energy, which is roughly 7000 times the maximum arsenal of the US and Soviet Union.

However, it must be noted that comparisons with supervolcanos are more misleading than helpful due to the different aerosols released, the likely air burst fuzing height of nuclear weapons and the globally scattered location of these potential nuclear detonations all being in contrast to the singular and subterranean nature of a supervolcanic eruption. Moreover, assuming the entire world stockpile of weapons were grouped together, it would be difficult due to the nuclear fratricide effect to ensure the individual weapons would detonate all at once. Nonetheless, many people believe that a full-scale nuclear war would result, through the nuclear winter effect, in the extinction of the human species, though not all analysts agree on the assumptions inputted into these nuclear winter models.

• 

      “You left me.” Those no sooner had been voiced than she would have given all she possessed to have not broken silence at all.
     “Yes.”
     His assent again threw her off balance. She had so expected some lie, some explanation, even some sign of shame or need to assure her that what she knew to be true was not. Now she simply stared at him.
     She could be as blunt and she had to know. When he added nothing to that she demanded: “Why?”
     “There has been death here.” Now he held that weapon in his left hand only, with his right he touched one of those many things hanging from his belt—this a narrow strip of some dark material—which looked like metal such as she had … of course, it was like that substance which enclosed the dead guardians!
     “There is still death here.” She regained much of her self-confidence, now she was able to nod at the frozen figure behind him as if it were no more than a carving of stone.
     “That is not what I meant,” he returned.
     “This,” he unhooked the strip of metal (Geiger counter) from his belt and held it up. Her eyes were keen enough to catch a play of color across it. “This indicates radiation. My people are immune to a high degree. It is part of our history. There was once a war fought on my world (Terra), such a war as,” Thorn looked around him as if he needed some inspiration, something he could draw upon to make things clear to her, “such a war as luckily this planet has never seen.
     “There were weapons used which killed—”
     She recalled the blasted tree, “By shooting fire? Such a thing maybe as that?” Releasing her grip on the carrier, she pointed to the rod (laser sidearm) he carried. “I saw—the burnt leaves, the withering.”
     “This is only a small, a very small example of such weapons.” He did not explain, she noted, how he had come by what he held; she was very sure he could not have brought it with him through the desert. The thing was too large to have been concealed anywhere among their belongings.
     “No,” he was continuing, “there were other fire throwers, such as could consume all of Kuxortal within a flash of thought (nuclear warheads). Much of my world died so. There were left only small pockets which held life. And the few of my own species who survived—they changed—or their children did (mutation). Some died because the changes were such as they became monsters who could not live. A few, so very few, were still human in form. Only they were now born armored against the force of weapons (the survivors had a mutation for increased resistance to radiation) such as those that had killed their world—unless the fire touched them directly.
     “For it is also the curse of such a war that the very air was poisoned. Those who breathed it, ventured into certain places, died, not quickly as in the fire, but slowly and with great pain and suffering. Only there is something also near here which kills!”
     Now he did begin to move closer to her, but this time Simsa did not shrink away. He was holding out that strip of metal he had worn, in open invitation for her to look at it.
     “When I came out of the pool—after I had drawn you in, since you were not conscious, nor able to help yourself—I found this thing you see. So, I pulled you part-way out and came to explore because of what can be read here.”
     Thorn placed his rod weapon on the pavement, pointed now with his free hand to the strip. There was a distinct line of red upward along it.
     “This showed me danger—the very danger which my kind know well from their own past. It might have meant that before us was death—perhaps not for me, but for you and your creatures. I had to find the source, know whether or not there was a deadly radiation.”
     “You took the carrier,” Simsa pointed out.
     Again he nodded. “If there was the degree of radiation which this indicated, then the food, the water on it might already be poisoned (neutron activation) for you. I had to make sure that you did not eat or drink before you left the pool chamber and I was not there to warn.”

• 

(ed note: Marvin and his father live in the Lunar colony. When Marvin turns ten years old, his father takes him on a trip in a moon bus to go see something...)

It seemed to Marvin that the mountains stretched on forever: but at last, many hours later, the range ended in a towering, precipitous headland that rose steeply from a cluster of little hills. They drove down into a shallow valley that curved in a great arc toward the far side of the mountains: and as they did so, Marvin slowly realized that something very strange was happening in the land ahead.

The sun was now low behind the hills on the right: the valley before them should be in total darkness. Yet it was awash with a cold white radiance that came spilling over the crags beneath which they were driving.

Then, suddenly, they were out in the open plain, and the source of the light lay before them in all its glory.

It was very quiet in the little cabin now that the motors had stopped. The only sound was the faint whisper of the oxygen feed and an occasional metallic crepitation as the outer walls of the vehicle radiated away their heat. For no warmth at all came from the great silver crescent that floated low above the far horizon and flooded all this land with pearly light. It was so brilliant that minutes passed before Marvin could accept its challenge and look steadfastly into its glare, but at last he could discern the outlines of continents, the hazy border of the atmosphere, and the white islands of cloud. And even at this distance, he could see the glitter of sunlight on the polar ice.

It was beautiful, and it called to his heart across the abyss of space. There in that shining crescent were all the wonders that he had never known—the hues of sunset skies, the moaning of the sea on pebbled shores, the patter of falling rain, the unhurried benison of snow. These and a thousand others should have been his rightful heritage, but he knew them only from the books and ancient records, and the thought filled him with the anguish of exile.

Why could they not return? It seemed so peaceful beneath those lines of marching cloud. Then Marvin, his eyes no longer blinded by the glare, saw that the portion of the disk that should have been in darkness was gleaming faintly with an evil phosphorescence: and he remembered. He was looking upon the funeral pyre of a world—upon the radioactive aftermath of Armageddon. Across a quarter of a million miles of space, the glow of dying atoms was still visible, a perennial reminder of the ruinous past. It would be centuries yet before that deadly glow died from the rocks and life could return again to fill that silent, empty world.

And now Father began to speak, telling Marvin the story which until this moment had meant no more to him than the fairy tales he had once been told. There were many things he could not understand: it was impossible for him to picture the glowing, multicolored pattern of life on the planet he had never seen. Nor could he comprehend the forces that had destroyed it in the end, leaving the Colony, preserved by its isolation, as the sole survivor. Yet he could share the agony of those final days, when the Colony had learned at last that never again would the supply ships come flaming down through the stars with gifts from home. One by one the radio stations had ceased to call: on the shadowed globe the lights of the cities had dimmed and died, and they were alone at last, as no men had ever been alone before, carrying in their hands the future of the race.

Then had followed the years of despair, and the long-drawn battle for survival in this fierce and hostile world. That battle had been won, though barely: this little oasis of life was safe against the worst that Nature could do. But unless there was a goal, a future toward which it could work, the Colony would lose the will to live, and neither machines nor skill nor science could save it then.

So, at last, Marvin understood the purpose of this pilgrimage. He would never walk beside the rivers of that lost and legendary world, or listen to the thunder raging above its softly rounded hills. Yet one day—how far ahead?—his children’s children would return to claim their heritage. The winds and the rains would scour the poisons from the burning lands and carry them to the sea, and in the depths of the sea they would waste their venom until they could harm no living things. Then the great ships that were still waiting here on the silent, dusty plains could lift once more into space, along the road that led to home.

That was the dream: and one day, Marvin knew with a sudden flash of insight, he would pass it on to his own son, here at this same spot with the mountains behind him and the silver light from the sky streaming into his face.

He did not look back as they began the homeward journey. He could not bear to see the cold glory of the crescent Earth fade from the rocks around him, as he went to rejoin his people in their long exile.

• 

For three hundred years, while its fame spread across the world, the little town had stood here at the river’s bend. Time and change had touched it lightly; it had heard from afar both the coming of the Armada and the fall of the Third Reich, and all Man’s wars had passed it by.

Now it was gone, as though it had never been. In a moment of time the toil and treasure of centuries had been swept away. The vanished streets could still be traced as faint marks in the vitrified ground, but of the houses, nothing remained. Steel and concrete, plaster and ancient oak—it had mattered little at the end. In the moment of death they had stood together, transfixed by the glare of the detonating bomb. Then, even before they could flash into fire, the blast waves had reached them and they had ceased to be. Mile upon mile the ravening hemisphere of flame had expanded over the level farmlands, and from its heart had risen the twisting totem-pole that had haunted the minds of men for so long, and to such little purpose.

The rocket had been a stray, one of the last ever to be fired. It was hard to say for what target it had been intended. Certainly not London, for London was no longer a military objective. London, indeed, was no longer anything at all. Long ago the men whose duty it was had calculated that three of the hydrogen bombs would be sufficient for that rather small target. In sending twenty, they had been perhaps a little overzealous.

This was not one of the twenty that had done their work so well. Both its destination and its origin were unknown: whether it had come cross the lonely Arctic wastes or far above the waters of the Atlantic, no one could tell and there were few now who cared. Once there had been men who had known such things, who had watched from afar the flight of the great projectiles and had sent their own missiles to meet them. Often that appointment had been kept, high above the Earth where the sky was black and sun and stars shared the heavens together. Then there had bloomed for a moment that indescribable flame, sending out into space a message that in centuries to come other eyes than Man’s would see and understand.

• 

(ed note: Central Control is the galactic federation of alien nations. Terra is a second-class star nation, their only source of interstellar trade revenue is by hiring out Terran warriors as mercenaries. On primitive planets are hired Terran "Archs" {for archons}, who are only allowed swords and simple rifles. On advanced worlds are hired Terran "Mechs" {for mechanized}, who are allowed aircraft, tanks, and blasters.)

      They saw but little of the Venturi city, being taken along passages chiseled through the native rock to a room near the top of the cliff, one side of which was transparent. Their guide withdrew and Kana went over to that window, craving the feeling of freedom it gave.
     “Volcano crater,” Hansu observed.
     The center of the island was a cup, its walls terraced and planted, a grove of trees extending into a miniature woodland in the depth of the hollow. But there were no signs of buildings.
     “But where—”
     The Blademaster looked beyond the peaceful carpet of vegetation to the crater walls.
     “We’re in their city now,” he explained. “They’ve hollowed out the cliffs—”
     In a moment Kana saw the evidences of that—the regular openings in the rock which must equal such windows as the one before which he now stood.
     “What a scheme!” he marveled. “Even a bomber would have a hard time putting this out of commission—unless it dropped hot stuff—”

     At the corner of the Blademaster’s jaw a tiny muscle pulled tight.
     “When the law is broken once, it can be easily fractured again.”

     “Use hot stuff?” Kana’s horrified amazement was genuine. He could accept the enmity of the Mechs, even the struggle for power backed in some mysterious way by Central Control Agents, but the thought of turning to nuclear weapons against—! Terra had learned too bitter a lesson in the Big Blow-up and the wars which followed. Those had occurred a thousand years ago but they had scarred the memories of his species for all time. He could not conceive of a Terran using nukes—it was so unnatural that it made his head reel.
     “We’ve had evidence enough that this is not just a Mech (Terran) plot,” Hansu pointed out relentlessly. “We may be conditioned against hot stuff because of our past history—but others (such as non-Terran aliens) aren’t. And we daren’t overlook any possibility—”
     That was an axiom of the corps he should have remembered. Never overlook any possibility, be prepared for any change in prospects—in the balance of force against force.

• 

"But they're Terran settlers, or at least from Terran stock, aren't they?"

"Sure," Tau sipped his coffee slowly. "But there are settlers and settlers, son. And a lot depends upon when they left Terra and why, and who they were--also what happened to them after they landed out here."

"And Khatkans are really special?"

"Well, they have an amazing history. The colony was founded by escaped prisoners—and just one racial stock. They took off from Earth close to the end of the Second Atomic War. That was a race war, remember? Which made it doubly ugly." Tau's mouth twisted in disgust. "As if the color of a man's skin makes any difference in what lies under it! One side in that line-up tried to take over Africa—herded most of the natives into a giant concentration camp and practiced genocide on a grand scale. Then they were cracked themselves, hard and heavy. During the confusion some survivors in the camp staged a revolt, helped by the enemy. They captured an experimental station hidden in the center of the camp and made a break into space in two ships which had been built there. That voyage must have been a nightmare, but they were desperate. Somehow they made it out here to the rim and set down on Khatka without power enough to take off again—and by then most of them were dead.

"But we humans, no matter what our race, are a tough breed. The refugees discovered that climatically their new world was not too different from Africa, a lucky chance which might happen only once in a thousand times. So they thrived, the handful who survived.

"They reverted to the primitive for survival. Then, about two hundred years ago, long before the first Survey Scout discovered them, something happened. Either the parent race mutated, or, as sometimes occurs, a line of people of superior gifts emerged—not in a few isolated births, but with surprising regularity in five family clans. There was a short period of power struggle until they realized the foolishness of civil war and formed an oligarchy, heading a loose tribal organization. With the Five Families to push and lead, a new civilization developed, and when Survey came to call they were no longer savages.

• 

• 

(ed note: The free trader crew of the Solar Queen purchase the rights to a planet in a Survey Auction. And quickly discover they've bought a pig in a poke.)

      THEY WERE ALL in the mess cabin again, the only space in the Queen large enough for the crew to assemble. Tang Ya set a reader on the table while Captain Jellico slit the packet and brought out the tiny roll of film it contained. Dane believed afterwards that few of them drew a really deep breath until it was fitted into place and the machine focused on the wall in lieu of the regular screen.
     “Planet—Limbo—only habitable one of three in a yellow star system—” the impersonal voice of some bored Survey clerk droned through the cabin.
     On the wall of the Queen appeared a flat representation of a three world system with the sun in the centre. Yellow sun—perhaps the planet had the same climate as Terra! Dane’s spirits soared. Maybe they were in luck—real luck.
     “Limbo—” that was Rip wedged beside him. “Man, oh, man, that’s no lucky name—that sure isn’t!”
     But Dane could not identify the title. Half the planets on the trade lanes had outlandish names didn’t they—any a Survey man slapped on them.
     “Co-ordinates—” the voice rippled out lines of formulae which Wilcox took down in quick notes. It would be his job to set the course to Limbo.
     “Climate—resembling colder section of Terra. Atmosphere—” more code numbers which were Tau’s concern. But Dane gathered that it was one in which human beings could live and work.

     The image in the screen changed. Now they might be hanging above Limbo, looking at it through their own view ports. And that vision was greeted with at least one exclamation of shocked horror.

     For there was no mistaking the cause of those brown-grey patches disfiguring the land masses. It was the leprosy of war—a war so vast and terrible that no Terran could be able to visualize its details.
     “A burnt off!” that was Tau, but above his voice rose that of the Captain’s.
     “It’s a filthy trick!”

     “Hold it!” Van Rycke’s rumble drowned out both outbursts, his big hand shot out to the reader’s control button. “Let’s have a close up. North a bit, along those burn scars—”
     The globe on the screen shot towards them, enlarging so that its limits vanished and they might have been going in for a landing. The awful waste of the long ago war was plain, earth burned and tortured into slag, maybe still even poisonous with radioactive wastes. But the Cargo-Master had not been mistaken, along the horrible scars to the north was a band of strangely tinted green which could only be vegetation. Van Rycke gave a sigh of satisfaction.
     “She isn’t a total loss—” he pointed out.
     “No,” retorted Jellico bitterly, “probably shows just enough life so we can’t claim fraud and get back our money.”

     “Forerunner ruins?” the suggestion came from Rip, timidly as if he felt he might be laughed down.
     Jellico shrugged. “We aren’t museum men,” he snapped. “And where would we have to go to make a deal with them—off Naxos anyway. And how are we going to lift from here now without cash for the cargo bond?”
     He had hammered home every bad point of their present situation. They owned ten-year trading rights to a planet which obviously had no trade—they had paid for those rights with the cash they needed to assemble a cargo. They might not be able to lift from Naxos. They had taken a Free Trader’s gamble and had lost.

     Only the Cargo-Master showed no dejection. He was still studying the picture of Limbo.
     “Let’s not go off with only half our jets spitting,” he said mildly. “Survey doesn’t sell worlds which can’t be exploited—”
     ”Not to the Companies, no,” Wilcox commented, “but who’s going to listen to a kick from a Free Trader—unless he’s Cofort!”
     “I still say,” Van Rycke continued in the same even tone, “that we ought to explore a little farther—”
     “Yes?” Jellico’s eyes held a spark of smouldering anger. “You want us to go there and be stranded? She’s burnt off—so she’s got to be written off our books. You know there’s never any life left on a Forerunner planet that was assaulted—”
     “Most of them are just bare rock now,” Van Rycke said reasonably. “It looks to me as if Limbo didn’t get the full treatment. After all—what do we know about the Forerunners—precious little! They were gone centuries, maybe even thousands of years, before we broke into space. They were a great race, ruling whole systems of planets, and they went out in a war which left dead worlds and even dead suns swinging in its wake. All right.
     “But maybe Limbo was struck in the last years of that war, when their power was on the wane. I’ve seen the other blasted worlds—Hades and Hel, Sodom, and Satan, and they’re nothing but cinders. This Limbo still has vegetation. And because it isn’t as badly hit as those others I think we might just have something—”

     He is winning his point, Dane told himself—noticing the change of expression on the faces around the table. Maybe it’s because we don’t want to believe that we’ve been taken so badly, because we want to hope that we can win even yet. Only Captain Jellico looked stubbornly unconvinced.
     “We can’t take the chance,” he repeated, his lips in an obstinate line. “We can fuel this ship for one trip—one trip. If we make it to Limbo and there’s no return cargo—well,” he slapped his hand on the table, “you know what that will mean—dirt-side for us!”
     Steen Wilcox cleared his throat with a sharp rasp which drew their attention. “Any chance of a deal with Survey?” he wanted to know.
     Kamil laughed, scorn more than amusement in the sound. ”Do the Feds ever give up any cash once they get their fingers on it?” he inquired.

     No one answered him until Captain Jellico got to his feet, moving heavily as if some of the resilience had oozed out of his tough body.
     “We’ll see them in the morning. You willing to try it, Van?”
     The Cargo-Master shrugged. “All right, I’ll tag along. Not that it’ll do us any good.”
     “Blasted—right off course—”
     Dane stood again at the open hatch looking out into a night made almost too bright by Naxos’ twin moons. Kamil’s words were not directed to him, he was sure. And a moment later that was confirmed by an answer from Rip.

     “I don’t call luck bad, man, ’til it up and slaps me in the face. Van had an idea—that planet wasn’t blasted black. You’ve seen pictures of Hel and Sodom, haven’t you? They’re cinders, as Van said. This Limbo, now—it shows green. Did you ever think, Ali, what might happen if we walked on to a world where some of the Forerunners’ stuff was lying around?”
     “Hm—” the idea Rip presented struck home. “But would trading rights give us ownership of such a find?”
     “Van would know—that’s part of his job. Why—” for the first time Rip must have sighted Dane at the hatch, “here’s Thorson. How about it, Dane? If we found Forerunner material, could we claim it legally?”
     Dane was forced to admit that he didn’t know. But he determined to hunt up the answer in the Cargo-Master’s tape library of rules and regulations.
     “I don’t think that the question has ever come up,” he said dubiously. “Have they ever found usable Forerunner remains—anything except empty ruins? The planets on which their big installations must have been are the burnt off ones—”

     “I wonder,” Kamil leaned back against the hatch door and looked at the winking lights of the town, “what they were like. All of the strictly human races we have encountered are descended from Terran colonies. And the five non-human ones we know are all as ignorant of the Forerunners as we are. If they left any descendants we haven’t contacted them yet. And—” he paused for a long moment before he added, “did you ever think it is just as well we haven’t found any of their installations? It’s been exactly ten years since the Crater War—”
     His words trailed off into a thick silence which had a faint menacing quality Dane could not identify, though he understood what Kamil must be aiming at. Terrans fought, viciously, devastatingly. The Crater War on Mars had been only the tail end of a long struggle between home planet and colonist across the void. The Federation kept an uneasy peace, the men of Trade worked frantically to make that permanent before another and more deadly conflict might wreck the whole Service and perhaps end their own precarious civilization.
     What would happen if weapons such as the Forerunners had wielded in their last struggle, or even the knowledge of such weapons, fell into the wrong Terran hands? Would Sol become a dead star circled by burnt off cinder worlds?
     “Sure, it might cause trouble if we found weapons,” Rip had followed the same argument. “But they had other things besides arms. And maybe on Limbo—”

• 

It was called Case Ragnarok, and it was insane. Yet in a time when madness had a galaxy by the throat, it was also inevitable.

It began as a planning study over a century earlier, when no one really believed there would be a war at all, and perhaps the crowning irony of the Final War was that a study undertaken to demonstrate the lunatic consequences of an unthinkable strategy became the foundation for putting that strategy into effect. The admirals and generals who initially undertook it actually intended it to prove that the stakes were too high, that the Melconian Empire would never dare risk a fight to the finish with the Concordiat—or vice versa—for they knew it was madness even to consider. But the civilians saw it as an analysis of an "option" and demanded a full implementation study once open war began, and the warriors provided it. It was their job to do so, of course, and in fairness to them, they protested the order . . . at first. Yet they were no more proof against the madness than the civilians when the time came.

And perhaps that was fitting, for the entire war was a colossal mistake, a confluence of misjudgments on a cosmic scale. Perhaps if there had been more contact between the Concordiat and the Empire it wouldn't have happened, but the Empire slammed down its non-intercourse edict within six standard months of first contact. From a Human viewpoint, that was a hostile act; for the Empire, it was standard operating procedure, no more than simple prudence to curtail contacts until this new interstellar power was evaluated. Some of the Concordiat's xenologists understood that and tried to convince their superiors of it, but the diplomats insisted on pressing for "normalization of relations." It was their job to open new markets, to negotiate military and political and economic treaties, and they resented the Melconian silence, the no-transit zones along the Melconian border . . . the Melconian refusal to take them as seriously as they took themselves. They grew more strident, not less, when the Empire resisted all efforts to overturn the non-intercourse edict, and the Emperor's advisors misread that stridency as a fear response, the insistence of a weaker power on dialogue because it knew its own weakness.

Imperial Intelligence should have told them differently, but shaping analyses to suit the views of one's superiors was not a purely Human trait. Even if it had been, Intelligence's analysts found it difficult to believe how far Human technology outclassed Melconian. The evidence was there, especially in the Dinochrome Brigade's combat record, but they refused to accept that evidence. Instead, it was reported as disinformation, a cunning attempt to deceive the Imperial General Staff into believing the Concordiat was more powerful than it truly was and hence yet more evidence that Humanity feared the Empire.

And Humanity should have feared Melcon. It was Human hubris, as much as Melconian, which led to disaster, for both the Concordiat and the Empire had traditions of victory. Both had lost battles, but neither had ever lost a war, and deep inside, neither believed it could. Worse, the Concordiat's intelligence organs knew Melcon couldn't match its technology, and that made it arrogant. By any rational computation of the odds, the Human edge in hardware should have been decisive, assuming the Concordiat had gotten its sums right. The non-intercourse edict had succeeded in at least one of its objectives, however, and the Empire was more than twice as large as the Concordiat believed . . . with over four times the navy.

So the two sides slid into the abyss—slowly, at first, one reversible step at a time, but with ever gathering speed. The admirals and generals saw it coming and warned their masters that all their plans and calculations were based on assumptions which could not be confirmed. Yet even as they issued their warning, they didn't truly believe it themselves, for how could so many years of spying, so many decades of analysis, so many computer centuries of simulations, all be in error? The ancient data processing cliché about "garbage in" was forgotten even by those who continued to pay it lip service, and Empire and Concordiat alike approached the final decisions with fatal confidence in their massive, painstaking, painfully honest—and totally wrong—analyses.

No one ever knew for certain who actually fired the first shot in the Trellis System. Losses in the ensuing engagement were heavy on both sides, and each navy reported to its superiors—honestly, so far as it knew—that the other had attacked it. Not that it mattered in the end. All that mattered was that the shot was fired . . . and that both sides suddenly discovered the terrible magnitude of their errors. The Concordiat crushed the Empire's frontier fleets with contemptuous ease, only to discover that they'd been only frontier fleets, light forces deployed to screen the true, ponderous might of the Imperial Navy, and the Empire, shocked by the actual superiority of Humanity's war machines, panicked. The Emperor himself decreed that his navy must seek immediate and crushing victory, hammering the enemy into submission at any cost and by any means necessary, including terror tactics. Nor was the Empire alone in its panic, for the sudden revelation of the Imperial Navy's size, coupled with the all-or-nothing tactics it adopted from the outset, sparked the same desperation within the Concordiat leadership.

And so what might have been no more than a border incident became something more dreadful than the galaxy had ever imagined. The Concordiat never produced enough of its superior weapons to defeat Melcon outright, but it produced more than enough to prevent the Empire from defeating it. And if the Concordiat's deep strikes prevented the Empire from mobilizing its full reserves against Human-held worlds, it couldn't stop the Melconian Navy from achieving a numerical superiority sufficient to offset its individual technical inferiorities. War raged across the light-centuries, and every clash was worse than the last as the two mightiest militaries in galactic history lunged at one another, each certain the other was the aggressor and each convinced its only options were victory or annihilation. The door to madness was opened by desperation, and the planning study known as Case Ragnarok was converted into something very different. It may be the Melconians had conducted a similar study—certainly their operations suggested they had—but no one will ever know, for the Melconian records, if any, no longer exist.

Yet the Human records do, and they permit no self-deception. Operation Ragnarok was launched only after the Melconian "demonstration strike" on New Vermont killed every one of the planet's billion inhabitants, but it was a deliberately planned strategy which had been developed at least twelve standard years earlier. It began at the orders of the Concordiat Senate . . . and ended thirty-plus standard years later, under the orders of God alone knew what fragments of local authority.

There are few records of Ragnarok's final battles because, in all too many cases, there were no survivors . . . on either side. The ghastly mistakes of diplomats who misread their own importance and their adversaries' will to fight, of intelligence analysts who underestimated their adversaries ability to fight, and of emperors and presidents who ultimately sought "simple" resolutions to their problems, might have bred the Final War, yet it was the soldiers who finished it. But then, it was always the soldiers who ended wars—and fought them, and died in them, and slaughtered their way through them, and tried desperately to survive them—and the Final War was no different from any other in that respect.

Yet it was different in one way. This time the soldiers didn't simply finish the war; this time the war finished them, as well.

—Kenneth R. Cleary, Ph.D.
From the introduction to Operation Ragnarok: Into the Abyss
Cerberus Books, Ararat, 4056

---

• 

Jackson had seen the visual records of the approach to the world which had been renamed Ararat. They retained enough tech base for that, though no one was certain how much longer the old tri-vids would continue to function, and a much younger Jackson had watched in awe as Ararat swelled against the stars in the bridge view screens of Commodore Isabella Perez's flagship, the transport Japheth.

Of course, calling any of the expedition's ships a "transport" was a bit excessive. For that matter, no one was certain Perez had actually ever been an officer in anyone's navy, much less a commodore. She'd never spoken about her own past, never explained where she'd been or what she'd done before she arrived in what was left of the Madras System with Noah and Ham and ordered all two hundred uninfected survivors of the dying planet of Sheldon aboard. Her face had been flint steel-hard as she refused deck space to anyone her own med staff couldn't guarantee was free of the bio weapon which had devoured Sheldon. She'd taken healthy children away from infected parents, left dying children behind and dragged uninfected parents forcibly aboard, and all the hatred of those she saved despite themselves couldn't turn her from her mission.

It was an impossible task from the outset. Everyone knew that. The two ships with which she'd begun her forty-six-year odyssey had been slow, worn out bulk freighters, already on their last legs, and God only knew how she'd managed to fit them with enough life support and cryo tanks to handle the complements she packed aboard them. But she'd done it. Somehow, she'd done it, and she'd ruled those spaceborne deathtraps with an iron fist, cruising from system to system and picking over the Concordiat's bones in her endless quest for just a few more survivors, just a little more genetic material for the Human race.

She'd found Japheth, the only ship of the "squadron" which had been designed to carry people rather than cargo, at the tenth stop on her hopeless journey. Japheth had been a penal transport before the War. According to her log, Admiral Gaylord had impressed her to haul cold-sleep infantry for the Sarach Campaign, although how she'd wound up three hundred light-years from there at Zach's Hundred remained a mystery. There'd been no one alive, aboard her or on the system's once-habitable world, to offer explanations, and Commodore Perez hadn't lingered to seek any, for Noah's com section had picked up faint transmissions in Melconian battle code.

She'd found Shem in Battersea, the same system in which her ground parties had shot their way into the old sector zoo to seize its gene bank. The Empire had used a particularly ugly bio weapon on Battersea. The sector capital's population of two billion had been reduced to barely three hundred thousand creatures whose once-Human ancestry was almost impossible to recognize, and the half-mad, mutant grandchildren of the original zoo staff had turned the gene bank into a holy relic. The Commodore's troopers had waded through the blood of its fanatic defenders and taken thirty percent casualties of their own to seize that gathered sperm and ova, and without it, Ararat wouldn't have had draft or food animals . . . or eagles.

Like every child of Ararat, Jackson could recite the names of every system Perez had tried in such dreary succession. Madras, Quinlan's Corner, Ellerton, Second Chance, Malibu, Heinlein, Ching-Hai, Cordoba, Breslau, Zach's Hundred, Kuan-Yin . . . It was an endless list of dead or dying worlds, some with a few more survivors to be taken aboard the Commodore's ships, some with a little salvageable material, and most with nothing but dust and ash and bones or the background howl of long-life radioactives. Many of the squadron's personnel had run out of hope. Some had suicided, and others would have, but Commodore Perez wouldn't let them. She was a despot, merciless and cold, willing to do anything it took—anything at all—to keep her creaky, ill-assorted, overcrowded rust buckets crawling towards just one more planetfall.

Until they hit Ararat.

No one knew what Ararat's original name had been, but they knew it had been Melconian, and the cratered graves of towns and cities and the shattered carcasses of armored fighting vehicles which littered its surface made what had happened to it dreadfully clear. No one had liked the thought of settling on a Melconian world, but the expedition's ships were falling apart, and the cryo systems supporting the domestic animals—and half the fleet's Human passengers—had become dangerously unreliable. Besides, Ararat was the first world they'd found which was still habitable. No one had used world burners or dust or bio agents here. They'd simply killed everything that moved—including themselves—the old-fashioned way.

And so, despite unthinkable challenges, Commodore Perez had delivered her ragtag load of press-ganged survivors to a world where they could actually live. She'd picked a spot with fertile soil and plentiful water, well clear of the most dangerously radioactive sites, and overseen the defrosting of her frozen passengers—animal and human alike—and the successful fertilization of the first generation of animals from the Battersea gene bank. And once she'd done that, she'd walked out under Ararat's three moons one spring night in the third local year of the colony's existence and resigned her command by putting a needler to her temple and squeezing the trigger.

She left no explanation, no diary, no journal. No one would ever know what had driven her to undertake her impossible task. All the colony leaders found was a handwritten note which instructed them never to build or allow any memorial to her name.

• 

      The machine was a vast fortress, containing no life, set by its long-dead masters to destroy anything that lived. It and many others like it were the inheritance of Earth from some war fought between unknown interstellar empires, in some time that could hardly be connected with any Earthly calendar.
     ;One such machine could hang over a planet colonized by men and in two days pound the surface into a lifeless cloud of dust and steam, a hundred miles deep. This particular machine had already done just that.
     It used no predictable tactics in its dedicated, unconscious war against life. The ancient, unknown gamesmen had built it as a random factor, to be loosed in the enemy's territory to do what damage it might. Men thought its plan of battle was chosen by the random disintegrations of atoms in a block of some long-lived isotope buried deep inside it, and so was not even in theory predictable by opposing brains, human or electronic.

     Men called it a berserker.

     The two Earth ships and the berserker were half a light year from the nearest sun. The berserker could not leap out of normal space, toward the defenseless colonies of the planets of that sun, while the two ships stayed close to it. There were only two men aboard Foxglove. They had more machinery working for them than did Del, but both manned ships were mites compared to their opponent.
     Del's radar showed him an ancient ruin of metal, not much smaller in cross section than New Jersey. Men had blown holes in it the size of Manhattan Island, and melted puddles of slag as big as lakes upon its surface.
     But the berserker's power was still enormous. So far no man had fought it and survived. Now, it could squash Del's little ship like a mosquito; it was wasting its unpredictable subtlety on him. Yet there was a special taste of terror in the very difference of it. Men could never frighten this enemy, as it frightened them.

     Earthmen's tactics, worked out from bitter experience against other berserkers, called for a simultaneous attack by three ships. Foxglove and Murray made two. A third was supposedly on the way, but still about eight hours distant, moving at C-plus velocity, outside of normal space. Until it arrived, Foxglove and Murray must hold the berserker at bay, while it brooded unguessable schemes.
     It might attack either ship at any moment, or it might seek to disengage. It might wait hours for them to make the first move—though it would certainly fight if the men attacked it. It had learned the language of Earth's spacemen—it might try to talk with them. But always, ultimately, it would seek to destroy them and every other living thing it met. That was the basic command given it by the ancient warlords.
     A thousand years ago, it would easily have swept ships of the type that now opposed it from its path, whether they carried fusion missiles or not. Now, it was in some electrical way conscious of its own weakening by accumulated damage. And perhaps in long centuries of fighting its way across the galaxy it had learned to be wary.

• 

• 

      Kirk nodded. "Commodore, I'm Jim Kirk, in command of the Enterprise. Do you understand?"
     "Enterprise?" Decker said. "We couldn't contact—couldn't run—had to do it—no choice at all..."
     "No choice about what?"
     "I had to beam them down. The only chance they had..."
     "Do you mean your crew?"
     Decker nodded. "I was—last aboard. It attacked again—knocked out the transporter. I was stranded aboard."
     "But where was the crew?"
     "The third planet."
     "There is no third planet now."
"There was," Decker said. "There was. That thing ... destroyed it ... I heard them ... four hundred of my men ... calling for help ... begging me ... and I couldn't ..." The Commodore's voice went slower and slower, as though he were an ancient clockwork mechanism running down, and faded out entirely.

     "Fantastic," Scott said, almost to himself. "What kind of a weapon could do that?"
     "If you had seen it—you'd know," Decker said, rousing himself with obvious effort. "The whole thing is a weapon. It must be."
     Kirk said, "What does it look like, Commodore?"
     "A hundred times the size of a starship—a mile long, with a maw big enough to swallow a dozen ships. It destroys planets—cuts them to rubble."
     "Why? Is it an alien ship—or is it alive?"
     "Both—neither—I don't know."
     "Where is this thing now?"
     "I—don't know that either."

     Kirk lifted his communicator. "Mr. Spock, how is the tape analysis going?"
     "We're ready now, Captain. We find that the Constellation was attacked by what seems to be essentially a robot—an automated weapon of great size and power. Its apparent function is to smash planets to rubble, and then 'digest' the debris for fuel. It is, therefore, self-maintaining as long as there are planetary bodies to feed it."
     "Origin?"
     "Mr. Sulu has computed the path of the machine, using the destroyed solar systems detected by ourselves and the Constellation as a base course. We find the path leads out of the galaxy at a sharp angle. Projected in the opposite direction, and assuming that the machine does not alter its course, it will go through the most densely populated section of our galaxy."
     Kirk said "Mr. Spock, have the Transporter Room beam Dr. McCoy and Commodore Decker aboard immediately."

     "They're aboard, Captain," Spock's voice said from the communicator. And then, without any transition at all, "Red alert! Red Alert! Mr. Sulu, out of the plane of the ecliptic at sixty degrees north! Warp One!"
     "Mr. Spock!" Kirk shouted, although of course Spock could have heard him equally well if he had whispered. "Why the alert? Why are you running? I'm blind here."
     "Commodore Decker's planet-killer, Captain. It just popped out of subspace. Metallic body, a large funnel-mouth, at least a mile long. It is pursuing us, but we seem to be able to maintain our distance at Warp One. No, it's gaining on us. Sensors indicate some kind of total conversion drive. No evidence of life aboard. Which is not surprising, since isotope dating indicates that it is at least three billion years old."
     "Three billion!" Kirk said. "Mr. Spock, since it's a robot, what are our chances of deactivating it?"
     "I would say none, Captain. I doubt that we would be able to maneuver close enough without drawing a direct attack upon ourselves. We could of course beam men aboard in spacesuits, but since the thing is obviously designed to be a doomsday machine, its control mechanisms would be inaccessible on principle."

     "A doomsday machine, sir?" Scott said.
     "A calculated bluff, Scotty. A weapon so powerful that it will destroy both sides in a war if it's used. Evidently some race in another galaxy built one—this one—and its bluff was called. The machine is now all that's left of the race—and it's evidently programmed to keep on destroying planets as long as it's functioning."

---

     "Very well. Now I need an engineering assessment. What would happen if the reactor were to go critical?"
     "Why, Captain, you know as well as I do—a fusion explosion, of course."
     "Yes, Scotty, but if this reactor were to do so, how big would the explosion be?"
     "Oh," Scott's voice said. "That's easily answered, the potential is always on the faceplate of a ship's reactor; I'll just check it ... The figure is 97.8 megatons."
     "Would the resulting fireball be sufficient to disrupt a neutronium hull?"
     "Neutronium, sir? You mean the planet-killer? What makes you think the hull is neutronium?"
     "Because from this distance the Enterprise could have cut it into scrap metal by now if it weren't."
     "Hmm—aye, that follows. Well, Captain, neutronium is formed in the cores of white dwarf stars, with fusion going on all around it. So I'd say the fireball would just push the machine away, rather than collapsing the hull. And sir, in a vacuum the fireball would be something like a hundred and fifty miles in diameter. That means it would envelop the Enterprise too—and we don't have a neutronium hull."
     "That's true, but it isn't what I have in mind. Scotty, I want you to rig a thirty-second delayed detonation switch, so the reactor can be blown from up here on the engineers' bridge. Can do?"
     "Aye, sir," Scott said. "But why ..."

     "Just rig it, fast. Then get yourself and the damage control party up here. Kirk to Enterprise."
     "Spock here."
     "Mr. Spock, I don't have any sensors over here worth mentioning, so I won't know when I'm in transporter range. The instant I am, let me know."
     "Acknowledge. May I ask your intent, Captain?"
     "Scotty is rigging a thirty-second delayed detonation switch on the impulse power reactor of the Constellation. I am going to pilot the vessel right down the planet-killer's throat—and you'll have thirty seconds to beam the five of us aboard the Enterprise before the reactor blows."
     There was a brief pause. When Spock's voice returned, there actually seemed to be a faint trace of human concern in it. "Jim, thirty seconds is very fine timing. The transporter is not working at a hundred per cent efficiency; our repairs were necessarily rather hasty."
     "That's a chance I'll have to take. However, it does change things a little. I'll want you to beam Mr. Scott and the damage control party over as soon as we are in range. I'll be the only one to stay aboard until the last minute."

     "Acknowledge. Sir, may I point out two possible other flaws? First, we cannot know the composition of the interior workings of the planet-killer. If they too are neutronium, nothing will happen except that it will get very hot inside there."
     " 'Very hot' is certainly a mild way of putting it," Kirk said drily. "All right, Mr. Spock, to use logic right back at you, Proposition A: The planet-killer operates in a vacuum, which means most of its circuits are cryogenic. Heating them a few million degrees may be quite enough to knock it out. Proposition B: Pure neutronium cannot carry an elecrical current, because its electron shells are collapsed. Hence, many important parts of the planet-killer's interior cannot be neutronium. Conclusion: an interior fusion explosion will kill it. How is that for a syllogism?"
     "It is not a syllogism at all, Captain, but a sorites; however, I agree that it is a sound one. My second objection is more serious. The planet-killer is open to space at one end, and that is the end facing us. The neutronium hull will confine the fireball and shoot it directly out of the funnel at the Enterprise in a tongue of flame hundreds of miles long. This is an undesirable outcome."
     Kirk almost laughed, although there was nothing in the least funny about the objection itself. "Mr. Spock, if that happens, we will all die. But the planet-killer will have been destroyed. Our mandate is to protect Federation lives, property and Interests. Hence this outcome, as you call it, is in fact more desirable than undesirable."
     "Now that," Spock said, "is a syllogism, and a sound one. Very well, Captain, I withdraw my objections."

---

(ed note: after lots of drama, the planet-killer is defeated)

     "Mr. Spock, you strike me as a man who still has some reservations."
     "Only one, Captain; and it is pure speculation."
     "Nevertheless, let's hear it."
     "Well, Captain, when two powers prepare forces of such magnitude against each other, it almost always means that they are at a state of technological parity; otherwise they would not take such risks of self-destruction."
     "Meaning?"
     "Meaning, sir, that the existence of one such doomsday machine implies the existence of another."
     "I suppose that's possible," Kirk said slowly, repressing a shudder. "Though the second one may not have been launched in time. Well, Mr. Spock, supposing we were to hear of another? What would you do?"
     Spock's eyebrows went up. "That is no problem, sir. I would feed it a fusion bomb disguised as a ship, or better still, an asteroid; that is not what concerned me. The danger, as such, can now be regarded as minimal, even if there is another such machine."

• 

“I suspect that your assumptions are basically correct,” the ship said when Tarrel had concluded her account. “The Dreadnought, as you call it, is almost certainly only a machine, and not an especially clever one at that. Those times when it seemed only to be playing with convoy, destroying ships in an almost lazy manner, it was probably responding at a low-priority attack status. There was no need for it to be in any hurry.”

“What manner of machine?” Daerran asked.

“A ship-killing machine, of course,” Trendaessa explained. “It apparently scans large areas of space for the presence of artificial power sources and any machine that is intact and potentially functional. The attack on the station did show it destroying one larger ship that was not powered up, while shuttles escaped unharmed, so there must be some targeting priority other than just the sources of active power. My belief is that this is an automated weapon of unknown alien origin, designed to destroy a civilization’s ability to make war by decimating the ships and supporting devices that make interplanetary flight possible.”

“Did someone aim this damned thing at us?” Tarrel asked.

“That is possible, but I doubt it,” Trendaessa said. “Most likely, this one was set loose and just never got turned off when the war was over, if there was anyone left to turn it off. The fact that it responded to your attempt to communicate is interesting. It was probably asking you if you were friend or foe. Needless to say, you did not know how to answer. Given much more information, which I doubt that I will be able to obtain, I might be able to learn the codes that identify a ship as a friend, or perhaps even tell it to shut itself down. It would probably take far less time to simply find a way to destroy it.”

“Then you have not yet seen a way to destroy it?” Daerran asked.

“No, I am afraid not. That monster is protected by the most powerful shield that I have ever seen. My own shield can of course be set at the proper frequency and level of power that will make me invisible to scanners by simply absorbing the active scanner signals, and by containing all emissions from the ship itself. The Dreadnought has a shield powerful enough that it contains light, like a black hole, although gravity is not the agent of this process. It is not actually invisible; if it was between you and a planet of a very close star, then you would see that area blacked out by its shields. But it does not reflect or allow light to escape, and that gives it functional invisibility in open space. Scanner invisibility is much more useful. From a defensive standpoint, we are about even.”

---

“My interior scans of the Dreadnought, and the analysis of debris we collected, indicated that it was at least a quarter of a million years old. We were able to make sense of the few transmissions it made. It seems that it would have never attacked anyone except your people. It confused you with the enemy that it was designed to destroy, for the simple chance that the level of Union technology and ship design almost exactly matched, and your physical appearance was very much the same. When it tried to talk to the Starwolf carriers early on, it was simply telling us to mind our own business.”

• 

As Seaton assumed, the near-collision of suns which had affected so disastrously the planet Valeron did not come unheralded to overwhelm a world unwarned, since for many hundreds of years her civilization had been of a high order indeed. Her astronomers were able, her scientists capable, the governments of her nations strong and just. Years before its occurrence the astronomers had known that the catastrophe was inevitable and had calculated dispassionately its every phase — to the gram, the centimeter, and the second.

With all their resources of knowledge and of power, however, it was pitifully little that the people of Valeron could do; for of what avail are the puny energies of man compared to the practically infinite forces of cosmic phenomena? Any attempt of the humanity of the doomed planet to swerve from their courses the incomprehensible masses of those two hurtling suns was as surely doomed to failure as would be the attempt of an ant to thrust from its rails an onrushing locomotive.

But what little could be done was done; done scientifically and logically; done, if not altogether without fear, at least in as much as was humanly possible without favor. With mathematical certainty were plotted the areas of least strain, and in those areas were constructed shelters. Shelters buried deeply enough to be unaffected by the coming upheavals of the world's crust; shelters of unbreakable metal, so designed, so latticed and braced as to withstand the seismic disturbances to which they were inevitably to be subjected.

Having determined the number of such shelters that could be built, equipped, and supplied with the necessities of life in the time allowed, the board of selection began its cold-blooded and heartless task. Scarcely one in a thousand of Valeron's teeming millions was to be given a chance for continued life, and they were to be chosen only from the children who would be in the prime of young adulthood at the time of the catastrophe.

These children were the pick of the planet: flawless in mind, body, and heredity. They were assembled in special schools near their assigned refuges, where they were instructed intensively in everything that they would have to know in order that civilization should not disappear utterly from the universe.

Such a thing could not be kept a secret long, and it is best to touch as lightly as possible upon the scenes which ensued after the certainty of doom became public knowledge.

Humanity both scaled the heights of self-sacrificing courage and plumbed the very depths of cowardice and depravity.

Characters already strong were strengthened, but those already weak went to pieces entirely in orgies to a normal mind unthinkable. Almost overnight a peaceful and lawabiding world went mad — became an insane hotbed of crime, rapine, and pillage unspeakable. Martial law was declared at once, and after a few thousand maniacs had been ruthlessly shot down, the soberer inhabitants were allowed to choose between two alternatives. They could either die then and there before a firing squad, or they could wait and take whatever slight chance there might be of living through what was to come — but devoting their every effort meanwhile to the end that through those selected few the civilization of Valeron should endure.

Many chose death and were executed summarily and without formality, without regard to wealth or station. The rest worked. Some worked devotedly and with high purpose, some worked hopelessly and with resignation. Some worked stolidly and with thoughts only of the present, some worked slyly and with thoughts only of getting themselves, by hook or by crook, into one of those shelters. All, however, from the highest to the lowest, worked.

Since the human mind cannot be kept indefinitely at high tension, the new condition of things came in time to be regarded almost as normal, and as months lengthened into years the routine was scarcely broken. Now and then, of course, one went mad and was shot; another refused to continue his profitless labor and was shot; still another gave up the fight and shot himself. And always there were the sly the self-seekers, the bribers, the corruptionists — willing to go to any lengths whatever to avoid their doom. Not openly did they carry on their machinations, but like loathsome worms eating at the heart of an outwardly fair fruit. But the scientists, almost to a man, were loyal. Trained to think, they thought clearly and logically, and surrounded themselves with soldiers and guards of the same stripe. Old men or weaklings would have no place in the post-cataclysmic world and there were accommodations for only the exactly predetermined number; therefore only those selected children and no others could be saved or would. And as for bribery, threats, blackmail, or any possible form of racketry or corruption — of what use is wealth or power to a man under sentence of death? And what threat or force could sway him?

Wherefore most of the sly were discovered, exposed, and shot.

Time went on. The shelters were finished. Into them were taken stores, libraries, tools and equipment of every sort necessary for the rebuilding of a fully civilized world. Finally the "children," now in the full prime of young manhood and young womanhood, were carefully checked in. Once inside those massive portals they were of a world apart.

They were completely informed and completely educated; they had for long governed themselves with neither aid nor interference; they knew precisely what they must face; they knew exactly what to do and exactly how to do it. Behind them the mighty, multiply seals were welded into place and broken rock by the cubic mile was blasted down upon their refuges.

Day by day the heat grew more and more intense. Cyclonic storms raged ever fiercer, accompanied by an incessant blaze of lightning and a deafeningly continuous roar of thunder. More and ever more violent became the seismic disturbances as Valeron's very core shook and trembled under the appalling might of the opposing cosmic forces.

Work was at an end and the masses were utterly beyond control. The devoted were butchered by their frantic fellows; the hopeless were stung to madness; the stolid were driven to frenzy by the realization that there was to be no future; the remaining sly ones deftly turned the unorganized fury of the mob into a purposeful attack upon the shelters, their only hope of life.

But at each refuge the rabble met an unyielding wall of guards loyal to the last, and of scientists who, their work now done, were merely waiting for the end. Guards and scientists fought with rifles, ray-guns, swords, and finally with clubs, stones, fists, feet and teeth. Outnumbered by thousands they fell and the howling mob surged over their bodies. To no purpose. Those shelters had been designed and constructed to withstand the attacks of Nature gone berserk, and futile indeed were the attempts of the frenzied hordes to tear a way into their sacred recesses.

Thus died the devoted and high-souled band who had saved their civilization; but in that death each man was granted the boon which, deep in his heart, he craved. They had died quickly and violently, fighting for a cause they knew to be good. They did not die as did the members of the insanely terror-stricken, senseless mob... in agony... lingeringly... but it is best to draw a kindly veil before the horrors attendant upon that riving, that tormenting, that cosmic outraging of a world.

---

The suns passed, each upon his appointed way. The cosmic forces ceased to war and to the tortured and ravaged planet there at last came peace. The surviving children of Valeron emerged from their subterranean retreats and undauntedly took up the task of rebuilding their world. And to such good purpose did they devote themselves to the problems of rehabilitation that in a few hundred years there bloomed upon Valeron a civilization and a culture scarcely to be equaled in the universe.

For the new race had been cradled in adversity. In its ancestry there was no physical or mental taint or weakness, all dross having been burned away by the fires of cosmic catastrophe which had so nearly obliterated all the life of the planet. They were as yet perhaps inferior to the old race in point of numbers, but were immeasurably superior to it in physical, mental, moral, and intellectual worth.

Immediately after the Emergence it had been observed that the two outermost planets of the system had disappeared and that in their stead revolved a new planet. This phenomenon was recognized for what it was, an exchange of planets; something to give concern only to astronomers.

No one except sheerest romancers even gave thought to the possibility of life upon other worlds, it being an almost mathematically demonstrable fact that the Valeronians were the only life in the entire universe. And even if other planets might possibly be inhabited, what of it? The vast reaches of empty ether intervening between Valeron and even her nearest fellow planet formed an insuperable obstacle even to communication, to say nothing of physical passage. Little did anyone dream, as generation followed generation, of what hideously intelligent life that interloping planet bore, nor of how the fair world of Valeron was to suffer from it.

• 

      Pa handled the pail of air in a twist of cloth. Now that it was inside the Nest, you could really feel its coldness. It just seemed to suck the heat out of everything. Even the flames cringed away from it as Pa put it down close by the fire.
     Yet it’s that glimmery white stuff in the pail that keeps us alive. It slowly melts and vanishes and refreshes the Nest and feeds the fire. The blankets keep it from escaping too fast. Pa’d like to seal the whole place, but he can’t — building’s too earthquake-twisted, and besides he has to leave the chimney open for smoke.
     Pa says air is tiny molecules that fly away like a flash if there isn’t something to stop them. We have to watch sharp not to let the air run low. Pa always keeps a big reserve supply of it in buckets behind the first blankets, along with extra coal and cans of food and other things, such as pails of snow to melt for water. We have to go way down to the bottom floor for that stuff, which is a mean trip, and get it through a door to outside.
     You see, when the Earth got cold, all the water in the air froze first and made a blanket ten feet thick or so everywhere, and then down on top of that dropped the crystals of frozen air, making another white blanket sixty or seventy feet thick maybe.
     Of course, all the parts of the air didn’t freeze and snow down at the same time.
     First to drop out was the carbon dioxide — when you’re shoveling for water, you have to make sure you don’t go too high and get any of that stuff mixed in, for it would put you to sleep, maybe for good, and make the fire go out. Next there’s the nitrogen, which doesn’t count one way or the other, though it’s the biggest part of the blanket. On top of that and easy to get at, which is lucky for us, there’s the oxygen that keeps us alive. Pa says we live better than kings ever did, breathing pure oxygen, but we’re used to it and don’t notice. Finally, at the very top, there’s a slick of liquid helium, which is funny stuff. All of these gases in neat separate layers. Like a pussy caffay, Pa laughingly says, whatever that is.

---

     You see, it’s this way. We know that everything is dead out there. Pa heard the last radio voices fade away years ago, and had seen some of the last folks die who weren’t as lucky or well-protected as us. So we knew that if there was something groping around out there, it couldn’t be anything human or friendly.
     Besides that, there’s a feeling that comes with it always being night, cold night. Pa says there used to be some of that feeling even in the old days, but then every morning the Sun would come and chase it away. I have to take his word for that, not ever remembering the Sun as being anything more than a big star. You see, I hadn’t been born when the dark star snatched us away from the Sun, and by now it’s dragged us out beyond the orbit of the planet Pluto, Pa says, and taking us farther out all the time.

---

     Something had to be done, and almost before I knew what I was going to say, I heard myself asking Pa to tell us about the old days, and how it all happened.
     He sometimes doesn’t mind telling that story, and Sis and I sure like to listen to it, and he got my idea. So we were all settled around the fire in a wink, and Ma pushed up some cans to thaw for supper, and Pa began. Before he did, though, I noticed him casually get a hammer from the shelf and lay it down beside him.
     It was the same old story as always — I think I could recite the main thread of it in my sleep — though Pa always puts in a new detail or two and keeps improving it in spots.

     He told us how the Earth had been swinging around the Sun ever so steady and warm, and the people on it fixing to make money and wars and have a good time and get power and treat each other right or wrong, when without warning there comes charging out of space this dead star, this burned out sun, and upsets everything.
     You know, I find it hard to believe in the way those people felt, any more than I can believe in the swarming number of them. Imagine people getting ready for the horrible sort of war they were cooking up. Wanting it even, or at least wishing it were over so as to end their nervousness. As if all folks didn’t have to hang together and pool every bit of warmth just to keep alive. And how can they have hoped to end danger, any more than we can hope to end the cold?
     Sometimes I think Pa exaggerates and makes things out too black. He’s cross with us once in a while and was probably cross with all those folks. Still, some of the things I read in the old magazines sound pretty wild. He may be right.
     The dark star, as Pa went on telling it, rushed in pretty fast and there wasn’t much time to get ready. At the beginning they tried to keep it a secret from most people, but then the truth came out, what with the earthquakes and floods — imagine, oceans of unfrozen water! — and people seeing stars blotted out by something on a clear night. First off they thought it would hit the Sun, and then they thought it would hit the Earth. There was even the start of a rush to get to a place called China, because people thought the star would hit on the other side. But then they found it wasn’t going to hit either side, but was going to come very close to the Earth.
     Most of the other planets were on the other side of the Sun and didn’t get involved. The Sun and the newcomer fought over the Earth for a little while — pulling it this way and that, like two dogs growling over a bone, Pa described it this time — and then the newcomer won and carried us off. The Sun got a consolation prize, though. At the last minute he managed to hold on to the Moon.
     That was the time of the monster earthquakes and floods, twenty times worse than anything before. It was also the time of the Big Jerk, as Pa calls it, when all Earth got yanked suddenly, just as Pa has done to me once or twice, grabbing me by the collar to do it, when I’ve been sitting too far from the fire.
     You see, the dark star was going through space faster than the Sun, and in the opposite direction, and it had to wrench the world considerably in order to take it away.
     The Big Jerk didn’t last long. It was over as soon as the Earth was settled down in its new orbit around the dark star. But it was pretty terrible while it lasted. Pa says that all sorts of cliffs and buildings toppled, oceans slopped over, swamps and sandy deserts gave great sliding surges that buried nearby lands. Earth was almost jerked out of its atmosphere blanket and the air got so thin in spots that people keeled over and fainted — though of course, at the same time, they were getting knocked down by the Big Jerk and maybe their bones broke or skulls cracked.
     We’ve often asked Pa how people acted during that time, whether they were scared or brave or crazy or stunned, or all four, but he’s sort of leery of the subject, and he was again tonight. He says he was mostly too busy to notice.
     You see, Pa and some scientist friends of his had figured out part of what was going to happen — they’d known we’d get captured and our air would freeze — and they’d been working like mad to fix up a place with airtight walls and doors, and insulation against the cold, and big supplies of food and fuel and water and bottled air. But the place got smashed in the last earthquakes and all Pa’s friends were killed then and in the Big Jerk. So he had to start over and throw the Nest together quick without any advantages, just using any stuff he could lay his hands on.
     I guess he’s telling pretty much the truth when he says he didn’t have any time to keep an eye on how other folks behaved, either then or in the Big Freeze that followed — followed very quick, you know, both because the dark star was pulling us away very fast and because Earth’s rotation had been slowed in the tug-of-war, so that the nights were ten old nights long.
     Still, I’ve got an idea of some of the things that happened from the frozen folk I’ve seen, a few of them in other rooms in our building, others clustered around the furnaces in the basements where we go for coal.

• 

      "You see, we knew for nearly ten generations what was coming. Our scientists struggled to learn either the secret of flight through space at speed greater than that of light, or the secret of the energy of matter. Some worked on means of living through the cold.
     "Marlan found the secret of the Sleep three generations before the Breaking. We pinned our faith on it. We built this city, always with the thought that so great an undertaking took time, and that if the other secrets were discovered, our refuge here could be abandoned. These chambers are far below the solid rock of the city. The dams — they were to protect our machines. We had a vast bank of photo-cells up there.

     "But I must tell you in order. The astronomers first saw and recognized the danger. Tharl, a great red sun, was moving toward us. We did not know it till the spectroscope was invented, Then it was years later that we learned it was not only moving toward us, but at us. Our sun Shaln was a yellow normal star, We knew what would happen. Our planet revolved well out from its sun, and Tharl would pass close to us.
     "It was a terrible time. Tharl gleamed like a red eye, and grew imperceptibly during our lives. But we could tell in the end that it was growing. Our scientists did all in their power. Men were born, lived, had children, and died, knowing the doom of our world. Many men turned to physics and chemistry then.
     "Tharl was so close in my youth, I could study it through a small telescope. It had no planets. Already it was beginning to affect our orbit, for it was gigantic, and its gravitational arms reached out hundreds of billions of miles. Nearly two light years, in fact. I took up astro-physics, and when I was still a young man the exact calculations were finally made. Before, it had not been definitely known just what would happen. Some thought our planets would be thrown into Shaln. Some, that we would be captured by Tharl. Some said we would be hurled into it, others that our own sun would explode and deluge us with fire.

     "We knew finally. Tharl would pass at a distance of hundreds of billions of miles. Myrya would be torn free of Shaln in an ever-widening orbit as Tharl approached, but so slowly that only as Tharl retreated would Myrya be free of Shaln. Then the retreating Tharl would leave Myrya free in space. All but the two innermost planets would be torn from Shaln. The three outer planets of the system would be attached to Than in enormous, enormously eccentric orbits. We, and two other planets would simply be torn free of both. Our system of ten moons would stay with us, and would scarcely be perturbed.
     "The process had already begun when I started my observatory work. I could watch it myself, as the planets began to elongate their orbits toward Than, so gradually it was barely perceptible.
     "Tarnsor — the Refuge — was completed, and the engineers began the greater work beneath the city and above. The dams were built. The pits were dug. There were no quakes to fear, for the effects were so gentle, and so wide-spread in time that no shocks would result. The physicists and astro-physicists had calculated, and calculated well. We knew that our greatest dangers would be from two sources — cosmic rays and meteors. Our re-awakening depended on the machines which we must leave at the surface. But if a meteor struck them — or the slow action of cosmic rays disintegrated them — therefore the dams.
     "We ourselves were to sleep in these deep chambers. Marlan had found the system which would induce suspended animation. No better preserver of life could be found than the absolute cold of space. Men experimented with themselves for fifty years, and were awakened in perfect health. That meant little, of course, save that the drug was good.
     "Seventy-nine chambers were prepared, each capable of housing ten thousand men. This one was the room of the scientists. You see our hope. As Myrya wandered through illimitable space and infinite time, somewhere, somehow, she would blunder into the gravitational field of a star, circle near, and be warmed once more. Eventually it would be captured. We had calculated that one star in four was a multiple, and one in a thousand had planets."

     "One in a thousand! We calculated one in a hundred thousand."

     "One in a thousand is right. We studied the question rather — carefully." Karshan paused momentarily. "We hoped to be captured by some such star. If no heat reached us, if the cosmic rays did not destroy us — we might. Many would not accept the system. It was better so, since we could care for but a small portion of the people.
     "I was mature when Shaln slipped back and back into space, and Tharl raced on ahead. The two stars began to give less and less heat. The people all over Myrya began to feel the first icy clutch of the eternal winter. They had dug deep caverns, and in them set up heating apparatus. They had huge energy-concentrators, using the power-system of all Myrya to heat and light their caverns. You see, they were certain the glowing gas of the galaxy would shine wherever the planet wandered. They depended on it. Their calculations showed that this energy would always be available. But we knew their calculations were wrong.
     "That was the sky the cold came into. The ones who tried to fight it held out for nearly ten thousand days. They really lived on the internal heat of the planet. But even that was exhausted eventually, and the chill ate in. Air became a problem, for it had frozen in lakes about them, They could no longer leave their caverns.
     "We waited till that time, when the planet was growing cold, and there was plentiful frozen air to start our process. Long before, when the cold had just begun, we had filled the dams, leaving only a passage out. The solution — it had taken a generation to prepare it. There was the lower dam, and the upper dam, and beyond that, the reservoir. In that were millions on millions of gallons of solution. Lead nitrate in water. Saturated. We ran a layer over our city and over the tubular tunnel we had built to the outside. It took cold to freeze it, of course. We flowed in layer after layer, letting each freeze in place. We built up successive layer upon layer until at last over us, and our machines, and our city of refuge was a block of ice and lead-salts half a mile thick.
     "Then we blasted loose the lower gates, and opened them. The solution was frozen hard, and of course did not flow out. The time passed, and when the liquid air was plentiful, we retired, and all our people save the scientists were put in the Sleep, in tubs of liquid air.
     "We waited till the ten thousand days had passed, and finally — we slept. The last man was put in his place and prepared by a special machine," Karshan paused. He had gotten a new reel from the cabinet, a reel of fine wire, and now he put this in the projector. Abruptly Aarn started. From the projector came mental impulses, the impulses of Karshan himself.

     "This is the record of Kanshan of Myrya on the First Awakening. Our wandering planet has drawn near a new star. The apparatus has functioned perfectly. The heat of the sun has melted the Great Seal, and the solution has flowed through the lower gates, into the lower reservoir. The primary direct photocell started operation on being cleared, and the concentrator field built up till it commenced operation. The clocks indicate the Seal was broken three thousand days ago. The atmosphere is gaseous once more. But we are retreating from the sun into space. This star has no planetary system, and we have not been captured.
     "I am the third member of the first group. Two others of the seven in the group have awakened, and four failed to return to life. They have been removed.
     "Examination shows that we are now headed diagonally across the edge of the galaxy, through the thickest of the star-fringe. We may hope for a second near passage soon. The first passage has been unavailing."
     There followed a long technical discussion of their condition, of the condition of the machines left to awaken them automatically, and a report that all was well with them; they were uninjured.

• 

     "The other two members of the Awakened Party have set out to investigate. It is difficult, for the world is terribly cold. We are forced to live in the Records Vault, the only place habitable. The foods are plentiful and good." There was a slight pause in the record, as though a period of time had elapsed.
     "The explorers have returned with the records of those who attempted to live in the heat of the galaxy. They were unsuccessful, and the last perished after a period of two generations. The bodies of the last survivors were found huddled in a single small cavern, using the heat of the Myryan power system for warmth. The plant decayed due to the impossibility of their giving it proper attention, and the extremely low temperature. It is no longer capable of functioning. Our plant alone remains in operation.
     "The cold is growing intense. The solution has been pumped into the upper reservoir, and the Seal has been re-established. The air is beginning to condense. With some difficulty we have succeeded in establishing an observatory on Mount Kior and have taken many photographs. We have been forced to retreat, as the stored power is almost exhausted, and the power plant is no longer in operation.
     "We will again go into the sleep within twenty-two hours."

     Silently Karshan inserted a reel of film into the camera, and the picture appeared on the wall as mental impulses came from the machine in a concluding thought.
     "The radio-active ores left as clocks indicate the passage of one half the half life of the ninety-second element — " and a phrase of time that to Aarn suddenly becomes intelligible — "a period of two and one half billions of years." Simultaneously Aarn saw the picture.
     Across a field of white waste, illimitable in expanse, glowed a faint, minute disc. A far-distant sun. Boiling, steaming pools of deep blue liquid lay in puddles and ponds among great masses of white, fluffy snow. A crawling, grotesque figure of a man moved in the dim light, a man in a cumbersome, cold-proof suit. The scene vanished. Across a deep-cleft valley jutted a white mountain, and beyond it the great rising arc of the gas-cloud galaxy. It appeared exactly the same. "A period of two and one half billions of years." The thought seemed to cling in Aarn's mind. The galaxy was unchanged in that period —

     How old was Karshan?

     The picture faded just as Aarn recognized the deep-cleft valley as the valley of the city. But a tremendous mass of solid white filled it from brim to brim. The Great Seal.
     Again the machine sent out its impulses.
     "This is the record of Karshan of Myrya on the Second Awakening. I am the third man of the first group. One other has awakened with me, and one of the second group. The others we did not call. Three are enough for the work.
     "Myrya is passing a giant red star, at a distance of nearly 100,000,000,000 miles. Even at this distance the planet has been entirely freed of the cold. There is no planetary system, and we will not be captured. The passage will consume nearly two thousand days. Much research work is to be done in this time in an attempt to find some better radioactive ore. The refined samples left at the last awakening have been 49% transmuted, indicating an elapsed period of approximately five billions of years. Myrya shows little change. The cities have not been destroyed, and stand practically perfect, torn a bit, however, by the terrific storms of the thaw.
     "The seal was broken after 6000 days of light above intensity 10. Another one thousand days elapsed before our awakening. All the apparatus was found in perfect condition. It was perfectly designed. The solution has been pumped back from the lower to the upper reservoir.

     "Calculation and observation from the observatory indicates that a terrible change has taken place in our course. This giant sun has deflected us, and accelerated us with its own proper motion so that we shall be hurled almost directly out of the galaxy. If this course is not deflected, we shall wander out into intergalactic space for an inconceivable time.
     "There are slight but noticeable changes in the make-up of the galaxy now. The central gas-cloud seems to have contracted, and many new stars have appeared. The stellar fringe is noticeably broader. Perhaps it is best that we circle outward, for should we enter the gas-cloud itself, our fate would be certain. Myrya would become the nucleus of a new sun.
     "We are leaving this star now. We have already pumped the solution back into the upper reservoir. I fear we will be hard-put this time, for the enormous radiation of this sun declines so slowly, our stored power may not last till we are safely out in the cold regions where we may enter the Sleep. We have discovered a new radioactive material whose half-life is twenty times that of element ninety-two, We are going again into the Sleep. The atmosphere above has not yet frozen, but our energy is exhausted. We have been able to freeze sufficient air in the cold rooms of the sleeping quarters to freeze our bodies. We will probably be safe enough.

     "Investigation of those who are sleeping indicates that many of them have died. Tharsarn suggests a two-fold reason for this. Many of course did not survive the original action of the drugs. This was indetectable at the time. Many more have possibly been killed by the atom-smashing rays from space. Even under our Great Seal, and half a mile more of solid rock, in the enormous times that have passed, these rays might well have been deadly. They do not influence the machines, since machines are not as delicate as body chemistry.
     "That I have survived, Tharsarn believes to be due to a peculiar susceptibility on my part to the action of the drugs, and to the fact that during the periods of awakening I have renewed the entire chemical structure of my body, replacing the destroyed atoms with fresh material from the foods I have eaten. He says that we who awaken have a better chance of ultimate survival.
     "Our latest calculations minimize this. We are very near the edge of the galaxy. And our parabolic orbit will lead us far out into intergalactic space. "This concludes the second report. We have been awake three thousand two hundred and twenty-one days."

• 

• 

(ed note: everybody on Terra becomes quite upset when astronomers notice two planets entering our solar system. Bronson Alpha is about the size of Uranus, it has the moon Bronson Beta which is the size and composition of Terra. In what is a highly unlikely and and suspicious situation, Bronson Alpha is going to make a direct hit on Terra and splatter the world like a bug on a windshield then leave the solar system never to be seen again. Oh, and Bronson Beta is going insert itself into Terra's former orbit in some outlandish act of orbital circularization performed with no reaction mass. Well, what do you expect from a novel written in 1932?)

      "That's what the League of the Last Days is working on, Tony—the chance of escape that's offered by the world like ours, which will pass so close and go on. We may transfer to it, Tony, if we have the will and the skill and the nerve! We could send a rocket to the moon to-day, if it would do us any good, if any one could possibly live on the moon after he got there. Well, Bronson Beta will pass us closer than the moon. Bronson Beta is the size of the earth, and therefore can have an atmosphere. It is perfectly possible that people—who are able to reach it—can live there.
     "It's a world, perhaps very like ours, which has been in immutable cold and dark for millions of years, probably, and which now will be coming to life again.
     "Think of it, Tony! The tremendous, magnificent adventure of making a try for it! It was a world once like ours, circling around some sun. People lived on it; and animals and plants and trees. Evolution had occurred there too, and progress. Civilization had come. Thousands of years of it, maybe. Tens of thousands of years—perhaps much more than we have yet known. Perhaps, also, much less. It's the purest speculation to guess in what stage that world was when it was torn from its sun and sent spinning into space.
     "But in whatever stage it was in, you may be sure it is in exactly that stage now; for when it left its sun, life became extinct. The rivers, the lakes, the seas, the very air, froze and became solid, encasing and keeping everything just as it was, though it wandered through space for ten million years.
     "But as it approaches the sun, the air and then the seas will thaw. The people cannot possibly come to life, nor the animals or birds or other things; but the cities will stand there unchanged, the implements, the monuments, their homes—all will remain and be uncovered again.
     "If this world were not doomed, what an adventure to try for that one, Tony! And a possible adventure—a perfectly possible adventure, with the powers at our disposal to-day!"

---

     "Exactly how," asked Eliot James, "do they look through the telescope?"
     "The big one—Bronson Alpha," replied Jack Taylor, as they all looked up from the street, "not very different from before. It seems to be gaseous, chiefly—it always was chiefly gaseous, unlike the earth and Mars, but like Jupiter and Saturn and Neptune. Its approach to the sun has increased the temperature of its envelope, but has brought out no details of its geography, if you could call it that. Bronson Alpha offers us no real surface, as such. It seems to be a great globe with a massive nucleus surrounded by an immense atmosphere. What we see is only the outer surface of the atmosphere."
     "Could it ever have been inhabited?" the poet asked.
     "In no such sense as we understand the word. For one thing, if we found ourselves on Bronson Alpha, we would never find any surface to live on. There is probably no sudden alteration of material such as exists on the earth when air stops and land and water begin."
     "But the other world—Bronson Beta—is different."
     "Very different from its companion up there, but not so different from our world, it seems. It has a surface we can see, with air and clouds in its atmosphere. The clouds shift or disappear and form again; but there are fixed details which do not change, and which prove a surface crust exists. The atmosphere was frozen solid in the long journey through space, but the sun has thawed out the air and has started, at least, on thawing out the seas."
     "You're sure there are seas too?"
     "There are great spaces that seem to be seas, that satisfy every visual and spectroscopic test of seas."

     "Have you seen," asked the poet, "anything like—cities?"
     "Cities?"
     "The ruins of cities, I mean. That globe seems to be so much like the earth; and sometimes it has had its sun. It lived in the sunshine of a star that was an octillion, octillion miles away (10⁵⁴ miles, which is a whopping 1.7×10⁴¹ light years. Methinks the authors got a little carried away). I thought just now, looking at it, that perhaps on it were cities like this, where people once watched the coming of whatever pulled them loose from their sun, and dropped them into the black mouth of space."
     Some of the company about him were looking up and listening; others paid no attention to him. He did not care; a few had shared his feeling; and among them was Eve, who stood near him.

     "Would you rather we went that way?" she said to him.
     "Slipping into space, falling away, all of us in the world together retreating farther and farther away from our sun, gradually freezing as we went into darkness?" Eliot James shook his handsome head. "No; if I had my choice, I think I'd elect our way. Yet I wonder how they faced it—what they did?"
     "I wonder," said Eve, her eyes upon the yellow orb, "if we'll ever know."