Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)


I swear by the shelters of the stars (a mighty oath, if you but knew it) …


Of course, it is strange to inhabit the earth no longer,
To give up customs one barely had time to learn …


The prospect of scaling heaven, of ascending to the sky, of altering other worlds to suit our purposes—no matter how well intentioned we may be—sets the warning flags flying: We remember the human inclination toward overweening pride; we recall our fallibility and misjudgments when presented with powerful new technologies. We recollect the story of the Tower of Babel, a building “whose top may reach unto heaven,” and God’s fear about our species, that now “nothing will be restrained from them which they have imagined to do.”

We come upon Psalm 15, which stakes a divine claim to other worlds: “[T]he heavens are the Lord’s, but the Earth hath he given to the children of men.” Or Plato’s retelling of the Greek analogue of Babel—the tale of Otys and Ephialtes. They were mortals who “dared to scale heaven.” The gods were faced with a choice. Should they kill the upstart humans “and annihilate [their] race with thunderbolts”? On the one hand, “this would be the end of the sacrifices and worship which men offered” the gods and which gods craved. “But, on the other hand, the gods could not suffer [such] insolence to be unrestrained.”

If, in the long term, though, we have no alternative, if our choice really is many worlds or none, we are in need of other sorts of myth, myths of encouragement. They exist. Many religions, from Hinduism to Gnostic Christianity to Mormon doctrine, teach that—as impious as it may sound—it is the goal of humans to become gods. Or consider a story in the Jewish Talmud left out of the Book of Genesis. (It is in doubtful accord with the account of the apple, the Tree of Knowledge, the Fall, and the expulsion from Eden.) In the Garden, God tells Eve and Adam that He has intentionally left the Universe unfinished. It is the responsibility of humans, over countless generations, to participate with God in a “glorious” experiment—“completing the Creation.”

The burden of such a responsibility is heavy, especially on so weak and imperfect a species as ours, one with so unhappy a history. Nothing remotely like “completion” can be attempted without vastly more knowledge than we have today. But perhaps, if our very existence is at stake, we will find ourselves able to rise to this supreme challenge.

ALTHOUGH HE DID NOT quite use any of the arguments of the preceding chapter, it was Robert Goddard’s intuition that “the navigation of interplanetary space must be effected to ensure the continuance of the race.” Konstantin Tsiolkovsky made a similar judgment:

There are countless planets, like many island Earths … Man occupies one of them. But why could he not avail himself of others, and of the might of numberless suns?… When the Sun has exhausted its energy, it would be logical to leave it and look for another, newly kindled, star still in its prime.

This might be done earlier, he suggested, long before the Sun dies, “by adventurous souls seeking fresh worlds to conquer.”

But as I rethink this whole argument, I’m troubled. Is it too much Buck Rogers? Does it demand an absurd confidence in future technology? Does it ignore my own admonitions about human fallibility? Surely in the short term it’s biased against technologically less-developed nations. Are there no practical alternatives that avoid these pitfalls?

All our self-inflicted environmental problems, all our weapons of mass destruction are products of science and technology. So, you might say, let’s just back off from science and technology. Let’s admit that these tools are simply too hot to handle. Let’s create a simpler society, in which no matter how careless or short-sighted we are, we’re incapable of altering the environment on a global or even on a regional scale. Let’s throttle back to a minimal, agriculturally intensive technology, with stringent controls on new knowledge. An authoritarian theocracy is a tried-and-true way to enforce the controls.

Such a world culture is unstable, though, in the long run if not the short—because of the speed of technological advance. Human propensities for self-betterment, envy, and competition will always be throbbing subsurface; opportunities for short-term, local advantage will sooner or later be seized. Unless there are severe constraints on thought and action, in a flash we’ll be back to where we are today. So controlled a society must grant great powers to the elite that does the controlling, inviting flagrant abuse and eventual rebellion. It’s very hard—once we’ve seen the riches, conveniences, and lifesaving medicines that technology offers—to squelch human inventiveness and acquisitiveness. And while such a devolution of the global civilization, were it possible, might conceivably address the problem of self-inflicted technological catastrophe, it would also leave us defenseless against eventual asteroidal and cometary impacts.

Or you might imagine throttling back much further, back to hunter-gatherer society, where we live off the natural products of the land and abandon even agriculture. Javelin, digging stick, bow, arrow, and fire would then be technology enough. But the Earth could support at the very most a few tens of millions of hunter-gatherers. How could we get down to such low population levels without instigating the very catastrophes we are trying to avoid? Besides, we hardly know how to live the hunter-gatherer life anymore: We’ve forgotten their cultures, their skills, their tool-kits. We’ve killed off almost all of them, and we’ve destroyed much of the environment that sustained them. Except for a tiny remnant of us, we might not be able, even if we gave it high priority, to go back. And again, even if we could return, we would be helpless before the impact catastrophe that inexorably will come.

The alternatives seem worse than cruel: They are ineffective. Many of the dangers we face indeed arise from science and technology—but, more fundamentally, because we have become powerful without becoming commensurately wise. The world-altering powers that technology has delivered into our hands now require a degree of consideration and foresight that has never before been asked of us.

Science cuts two ways, of course; its products can be used for both good and evil. But there’s no turning back from science. The early warnings about technological dangers also come from science. The solutions may well require more of us than just a technological fix. Many will have to become scientifically literate. We may have to change institutions and behavior. But our problems, whatever their origin, cannot be solved apart from science. The technologies that threaten us and the circumvention of those threats both issue from the same font. They are racing neck and neck.

In contrast, with human societies on several worlds, our prospects would be far more favorable. Our portfolio would be diversified. Our eggs would be, almost literally, in many baskets. Each society would tend to be proud of the virtues of its world, its planetary engineering, its social conventions, its hereditary predispositions. Necessarily, cultural differences would be cherished and exaggerated. This diversity would serve as a tool of survival.

When the off-Earth settlements are better able to fend for themselves, they will have every reason to encourage technological advance, openness of spirit, and adventure—even if those left on Earth are obliged to prize caution, fear new knowledge, and institute Draconian social controls. After the first few self-sustaining communities are established on other worlds, the Earthlings might also be able to relax their strictures and lighten up. The humans in space would provide those on Earth with real protection against rare but catastrophic collisions by asteroids or comets on rogue trajectories. Of course, for this very reason, humans in space would hold the upper hand in any serious dispute with those on Earth.

The prospects of such a time contrast provocatively with forecasts that the progress of science and technology is now near some asymptotic limit; that art, literature, and music are never to approach, much less exceed, the heights our species has, on occasion, already touched; and that political life on Earth is about to settle into some rock-stable liberal democratic world government, identified, after Hegel, as “the end of history.” Such an expansion into space also contrasts with a different but likewise discernible trend in recent times—toward authoritarianism, censorship, ethnic hatred, and a deep suspicion of curiosity and learning. Instead, I think that, after some debugging, the settlement of the Solar System presages an open-ended era of dazzling advances in science and technology; cultural flowering; and wide-ranging experiments, up there in the sky, in government and social organization. In more than one respect, exploring the Solar System and homesteading other worlds constitutes the beginning, much more than the end, of history.

IT’S IMPOSSIBLE, for us humans at least, to look into our future, certainly not centuries ahead. No one has ever done so with any consistency and detail. I certainly do not imagine that I can. I have, with some trepidation, gone as far as I have to this point in the book, because we are just recognizing the truly unprecedented challenges brought on by our technology. These challenges have, I think, occasional straightforward implications, some of which I’ve tried briefly to lay out. There are also less straightforward, much longer-term implications about which I’m even less confident. Nevertheless, I’d like to present them too for your consideration:

Even when our descendants are established on near-Earth asteroids and Mars and the moons of the outer Solar System and the Kuiper Comet Belt, it still won’t be entirely safe. In the long run, the Sun may generate stupendous X-ray and ultraviolet outbursts; the Solar System will enter one of the vast interstellar clouds lurking nearby and the planets will darken and cool; a shower of deadly comets will come roaring out of the Oort Cloud threatening civilizations on many adjacent worlds; we will recognize that a nearby star is about to become a supernova. In the really long run, the Sun—on its way to becoming a red giant star—will get bigger and brighter, the Earth will begin to lose its air and water to space, the soil will char, the oceans will evaporate and boil, the rocks will vaporize, and our planet may even be swallowed up into the interior of the Sun.

Far from being made for us, eventually the Solar System will become too dangerous for us. In the long run, putting all our eggs in a single stellar basket, no matter how reliable the Solar System has been lately, may be too risky. In the long run, as Tsiolkovsky and Goddard long ago recognized, we need to leave the Solar System.

If that’s true for us, you might very well ask, why isn’t it true for others? And if it is true for others, why aren’t they here? There are many possible answers, including the contention that they have come here—although the evidence for that is pitifully slim. Or there may be no one else out there, because they destroy themselves, with almost no exceptions, before they achieve interstellar flight; or because in a galaxy of 400 billion suns ours is the first technical civilization.

A more likely explanation, I think, issues from the simple fact that space is vast and the stars are far apart. Even if there were civilizations much older and more advanced than we—expanding out from their home worlds, reworking new worlds, and then continuing onward to other stars—they would be unlikely, according to calculations performed by William I. Newman of UCLA and me, to be here. Yet. And because the speed of light is finite, the TV and radar news that a technical civilization has arisen on some planet of the Sun has not reached them. Yet.

Should optimistic estimates prevail and one in every million stars shelters a nearby technological civilization, and if as well they’re randomly strewn through the Milky Way—were these provisos to hold—then the nearest one, we recall, would be a few hundred light-years distant: at the closest, maybe 100 light-years, more likely a thousand light-years—and, of course, perhaps nowhere, no matter how far. Suppose the nearest civilization on a planet of another star is, say, 200 light-years away. Then, some 150 years from now they’ll begin to receive our feeble post-World War II television and radar emission. What will they make of it? With each passing year the signal will get louder, more interesting, perhaps more alarming. Eventually, they may respond: by returning a radio message, or by visiting. In either case, the response will likely be limited by the finite value of the speed of light. With these wildly uncertain numbers, the answer to our unintentional midcentury call into the depths of space will not arrive until around the year 2350. If they’re farther away, of course, it will take longer; and if much farther away, much longer. The interesting possibility arises that our first receipt of a message from an alien civilization, a message intended for us (not just an all-points bulletin), will occur in a time when we are well situated on many worlds in our solar system and preparing to move on.

With or without such a message, though, we will have reason to continue outward, seeking other solar systems. Or—even safer in this unpredictable and violent sector of the Galaxy—to sequester some of us in self-sufficient habitations in interstellar space, far from the dangers constituted by the stars. Such a future would, I think, naturally evolve, by slow increments, even without any grand goal of interstellar travel:

For safety, some communities may wish to sever their ties with the rest of humanity—uninfluenced by other societies, other ethical codes, other technological imperatives. In a time when comets and asteroids are being routinely repositioned, we will be able to populate a small world and then cut it loose. In successive generations, as this world sped outward, the Earth would fade from bright star to pale dot to invisibility; the Sun would appear dimmer, until it was no more than a vaguely yellow point of light, lost among thousands of others. The travelers would approach interstellar night. Some such communities may be content with occasional radio and laser traffic with the old home worlds. Others, confident of the superiority of their own survival chances and wary of contamination, may try to disappear. Perhaps all contact with them will ultimately be lost, their very existence forgotten.

Even the resources of a sizable asteroid or comet are finite, though, and eventually more resources must be sought elsewhere—especially water, needed for drink, for a breathable oxygen atmosphere, and for hydrogen to power fusion reactors. So in the long run these communities must migrate from world to world, with no lasting loyalty to any. We might call it “pioneering,” or “homesteading.” A less sympathetic observer might describe it as sucking dry the resources of little world after little world. But there are a trillion little worlds in the Oort Comet Cloud.

Living in small numbers on a modest stepmother world far from the Sun, we will know that every scrap of food and every drop of water is dependent on the smooth operation of a farsighted technology; but these conditions are not radically unlike those to which we are already accustomed. Digging resources out of the ground and stalking passing resources seem oddly familiar, like a forgotten memory of childhood: It is, with a few significant changes, the strategy of our hunter-gatherer ancestors. For 99.9 percent of the tenure of humans on Earth, we lived such a life. Judging from some of the last surviving hunter-gatherers just before they were engulfed by the present global civilization, we may have been relatively happy. It’s the kind of life that forged us. So after a brief, only partially successful sedentary experiment, we may become wanderers again—more technological than last time, but even then our technology, stone tools and fire, was our only hedge against extinction.

If safety lies in isolation and remoteness, then some of our descendants will eventually emigrate to the outer comets of the Oort Cloud. With a trillion cometary nuclei, each separated from the next by about as much as Mars is from Earth, there will be a great deal to do out there.*

The outer edge of the Sun’s Oort Cloud is perhaps halfway to the nearest star. Not every other star has am Oort Cloud, but many probably do. As the Sun passes nearby stars, our Oort Cloud will encounter, and partially pass through, other comet clouds, like two swarms of gnats interpenetrating but not colliding. To occupy a comet of another star will then be not much more difficult than to occupy one of our own. From the frontiers of some other solar system the children of the blue dot may peer longingly at the moving points of light denoting substantial (and well-lit) planets. Some communities—feeling the ancient human love for oceans and sunlight stirring within them—may begin the long journey down to the bright, warm, and clement planets of a new sun.

Other communities may consider this last strategy a weakness. Planets are associated with natural catastrophes. Planets may have pre-existing life and intelligence. Planets are easy for other beings to find. Better to remain in the darkness. Better to spread ourselves among many small and obscure worlds. Better to stay hidden.

ONCE WE CAN SEND our machines and ourselves far from home, far from the planets—once we really enter the theater of the Universe—we are bound to come upon phenomena unlike anything we’ve ever encountered. Here are three possible examples:

First: Starting some 550 astronomical units (AU) out—about ten times farther from the Sun than Jupiter, and therefore much more accessible than the Oort Cloud—there’s something extraordinary. Just as an ordinary lens focuses far-off images, so does gravity. (Gravitational lensing by distant stars and galaxies is now being detected.) Five hundred fifty AU from the Sun—only a year away if we could travel at 1 percent the speed of light—is where the focus begins (although when effects of the solar corona, the halo of ionized gas surrounding the Sun, are taken into account, the focus may be considerably farther out). There, distant radio signals are enormously enhanced, amplifying whispers. The magnification of distant images would allow us (with a modest radio telescope) to resolve a continent at the distance of the nearest star and the inner Solar System at the distance of the nearest spiral galaxy. If you are free to roam an imaginary spherical shell at the appropriate focal distance and centered on the Sun, you are free to explore the Universe in stupendous magnification, to peer at it with unprecedented clarity, to eavesdrop on the radio signals of distant civilizations, if any, and to glimpse the earliest events in the history of the Universe. Alternatively, the lens could be used the other way, to amplify a very modest signal of ours so it could be heard over immense distances. There are reasons that draw us to hundreds and thousands of AU. Other civilizations will have their own regions of gravitational focusing, depending on the mass and radius of their star, some a little closer, some a little farther away than ours. Gravitational lensing may serve as a common inducement for civilizations to explore the regions just beyond the planetary parts of their solar systems.

Second: Spend a moment thinking about brown dwarfs, hypothetical very low temperature stars, considerably more massive than Jupiter, but considerably less massive than the Sun. Nobody knows if brown dwarfs exist. Some experts, using nearer stars as gravitational lenses to detect the presence of more distant ones, claim to have found evidence of brown dwarfs. From the tiny fraction of the whole sky that has so far been observed by this technique, an enormous number of brown dwarfs is inferred. Others disagree. In the 1950s, it was suggested by the astronomer Harlow Shapley of Harvard that brown dwarfs—he called them “Lilliputian stars”—were inhabited. He pictured their surfaces as warm as a June day in Cambridge, with lots of area. They would be stars that humans could survive on and explore.

Third: The physicists B. J. Carr and Stephen Hawking of Cambridge University have shown that fluctuations in the density of matter in the earliest stages of the Universe could have generated a wide variety of small black holes. Primordial black holes—if they exist—must decay by emitting radiation to space, a consequence of the laws of quantum mechanics. The less massive the black hole, the faster it dissipates. Any primordial black hole in the final stages of decay today would have to weigh about as much as a mountain. All the smaller ones are gone. Since the abundance—to say nothing of the existence—of primordial black holes depends on what happened in the earliest moments after the Big Bang, no one can be sure that there are any to be found; we certainly can’t be sure that any lie nearby. Not very restrictive upper limits on their abundance have been set by the failure so far to find short gamma ray pulses, a component of the Hawking radiation.

In a separate study, G. E. Brown of Caltech and the pioneering nuclear physicist Hans Bethe of Cornell suggest that about a billion nonprimordial black holes are strewn through the Galaxy, generated in the evolution of stars. If so, the nearest may be only 10 or 20 light-years away.

If there are black holes within reach—whether they’re as massive as mountains or as stars—we will have amazing physics to study firsthand, as well as a formidable new source of energy. By no means do I claim that brown dwarfs or primordial black holes are likely within a few light-years, or anywhere. But as we enter interstellar space, it is inevitable that we will stumble upon whole new categories of wonders and delights, some with transforming practical applications.

I do not know where my train of argument ends. As more time passes, attractive new denizens of the cosmic zoo will draw us farther outward, and increasingly improbable and deadly catastrophes must come to pass. The probabilities are cumulative. But, as time goes on, technological species will also accrue greater and greater powers, far surpassing any we can imagine today. Perhaps, if we are very skillful (lucky, I think, won’t be enough), we will ultimately spread far from home, sailing through the starry archipelagos of the vast Milky Way Galaxy. If we come upon anyone else—or, more likely, if they come upon us—we will harmoniously interact. Since other space-faring civilizations are likely to be much more advanced than we, quarrelsome humans in interstellar space are unlikely to last long.

Eventually, our future may be as Voltaire, of all people, imagined:

Sometimes by the help of a sunbeam, and sometimes by the convenience of a comet, [they] glided from sphere to sphere, as a bird hops from bough to bough. In a very little time [they] posted through the Milky Way …

We are, even now, discovering vast numbers of gas and dust disks around young stars—the very structures out of which, in our solar system four and a half billion years ago, the Earth and the other planets formed. We’re beginning to understand how fine dust grains slowly grow into worlds; how big Earthlike planets accrete and then quickly capture hydrogen and helium to become the hidden cores of gas giants; and how small terrestrial planets remain comparatively bare of atmosphere. We are reconstructing the histories of worlds—how mainly ices and organics collected together in the chilly outskirts of the early Solar System, and mainly rock and metal in the inner regions warmed by the young Sun. We have begun to recognize the dominant role of early collisions in knocking worlds over, gouging huge craters and basins in their surfaces and interiors, spinning them up, making and obliterating moons, creating rings, carrying, it may be, whole oceans down from the skies, and then depositing a veneer of organic matter as the neat finishing touch in the creation of worlds. We are beginning to apply this knowledge to other systems.

In the next few decades we have a real chance of examining the layout and something of the composition of many other mature planetary systems around nearby stars. We will begin to know which aspects of our system are the rule and which the exception. What is more common—planets like Jupiter, planets like Neptune, or planets like Earth? Or do all other systems have Jupiters and Neptunes and Earths? What other categories of worlds are there, currently unknown to us? Are all solar systems embedded in a vast spherical cloud of comets? Most stars in the sky are not solitary suns like our own, but double or multiple systems in which the stars are in mutual orbit. Are there planets in such systems? If so, what are they like? If, as we now think, planetary systems are a routine consequence of the origin of suns, have they followed very different evolutionary paths elsewhere? What do elderly planetary systems, billions of years more evolved than ours, look like? In the next few centuries our knowledge of other systems will become increasingly comprehensive. We will begin to know which to visit, which to seed, and which to settle.

Imagine we could accelerate continuously at 1 g—what we’re comfortable with on good old terra firma—to the midpoint of our voyage, and decelerate continuously at 1 g until we arrive at our destination. It would then take a day to get to Mars, a week and a half to Pluto, a year to the Oort Cloud, and a few years to the nearest stars.

Even a modest extrapolation of our recent advances in transportation suggests that in only a few centuries we will be able to travel close to the speed of light. Perhaps this is hopelessly optimistic. Perhaps it will really take millennia or more. But unless we destroy ourselves first we will be inventing new technologies as strange to us as Voyager might be to our hunter-gatherer ancestors. Even today we can think of ways—clumsy, ruinously expensive, inefficient to be sure—of constructing a starship that approaches light speed. In time, the designs will become more elegant, more affordable, more efficient. The day will come when we overcome the necessity of jumping from comet to comet. We will begin to soar through the light-years and, as St. Augustine said of the gods of the ancient Greeks and Romans, colonize the sky.

Such descendants may be tens or hundreds of generations removed from anyone who ever lived on the surface of a planet. Their cultures will be different, their technologies far advanced, their languages changed, their association with machine intelligence much more intimate, perhaps their very appearance markedly altered from that of their nearly mythical ancestors who first tentatively set forth in the late twentieth century into the sea of space. But they will be human, at least in large part; they will be practitioners of high technology; they will have historical records. Despite Augustine’s judgment on Lot’s wife, that “no one who is being saved should long for what he is leaving,” they will not wholly forget the Earth.

But we’re not nearly ready, you may be thinking. As Voltaire put it in his Memnon, “our little terraqueous globe is the madhouse of those hundred thousand millions* of worlds.” We, who cannot even put our own planetary home in order, riven with rivalries and hatreds, despoiling our environment, murdering one another through irritation and inattention as well as on deadly purpose, and moreover a species that until only recently was convinced that the Universe was made for its sole benefit—are we to venture out into space, move worlds, re-engineer planets, spread to neighboring star systems?

I do not imagine that it is precisely we, with our present customs and social conventions, who will be out there. If we continue to accumulate only power and not wisdom, we will surely destroy ourselves. Our very existence in that distant time requires that we will have changed our institutions and ourselves. How can I dare to guess about humans in the far future? It is, I think, only a matter of natural selection. If we become even slightly more violent, shortsighted, ignorant, and selfish than we are now, almost certainly we will have no future.

If you’re young, it’s just possible that we will be taking our first steps on near-Earth asteroids and Mars during your lifetime. To spread out to the moons of the Jovian planets and the Kuiper Comet Belt will take many generations more. The Oort Cloud will require much longer still. By the time we’re ready to settle even the nearest other planetary systems, we will have changed. The simple passage of so many generations will have changed us. The different circumstances we will be living under will have changed us. Prostheses and genetic engineering will have changed us. Necessity will have changed us. We’re an adaptable species.

It will not be we who reach Alpha Centauri and the other nearby stars. It will be a species very like us, but with more of our strengths and fewer of our weaknesses, a species returned to circumstances more like those for which it was originally evolved, more confident, farseeing, capable, and prudent—the sorts of beings we would want to represent us in a Universe that, for all we know, is filled with species much older, much more powerful, and very different.

The vast distances that separate the stars are providential. Beings and worlds are quarantined from one another. The quarantine is lifted only for those with sufficient self-knowledge and judgment to have safely traveled from star to star.

ON IMMENSE TIMESCALES, in hundreds of millions to billions of years, the centers of galaxies explode. We see, scattered across deep space, galaxies with “active nuclei,” quasars, galaxies distorted by collisions, their spiral arms disrupted, star systems blasted with radiation or gobbled up by black holes—and we gather that on such timescales even interstellar space, even galaxies may not be safe.

There is a halo of dark matter surrounding the Milky Way, extending perhaps halfway to the distance of the next spiral galaxy (M31 in the constellation Andromeda, which also contains hundreds of billions of stars). We do not know what this dark matter is, or how it is arranged—but some* of it may be in worlds untethered to individual stars. If so, our descendants of the remote future will have an opportunity, over unimaginable intervals of time, to become established in intergalactic space, and to tiptoe to other galaxies.

But on the timescale for populating our galaxy, if not long before, we must ask: How immutable is this longing for safety that drives us outward? Will we one day feel content with the time our species has had and our successes, and willingly exit the cosmic stage? Millions of years from now—probably much sooner—we will have made ourselves into something else. Even if we do nothing intentionally, the natural process of mutation and selection will have worked our extinction or evolved us into some other species on just such a timescale (if we may judge by other mammals). Over the typical lifetime of a mammalian species, even if we were able to travel close to the speed of light and were dedicated to nothing else, we could not, I think, explore even a representative fraction of the Milky Way Galaxy. There’s just too much of it. And beyond are a hundred billion galaxies more. Will our present motivations remain unchanged over geological, much less cosmological, timescales—when we ourselves have been transfigured? In such remote epochs, we may discover outlets for our ambitions far grander and more worthy than merely populating an unlimited number of worlds.

Perhaps, some scientists have imagined, we will one day create new forms of life, link minds, colonize stars, reconfigure galaxies, or prevent, in a nearby volume of space, the expansion of the Universe. In a 1993 article in the journal Nuclear Physics, the physicist Andrei Linde—conceivably, in a playful mood—suggests that laboratory experiments (it would have to be quite a laboratory) to create separate, closed-off, expanding universes might ultimately be possible. “However,” he writes to me, “I myself do not know whether [this suggestion] is simply a joke or something else.” In such a list of projects for the far future, we will have no difficulty in recognizing a continuing human ambition to arrogate powers once considered godlike—or, in that other more encouraging metaphor, to complete the Creation.

FOR MANY PAGES NOW, we have left the realm of plausible conjecture for the heady intoxication of nearly unconstrained speculation. It is time to return to our own age.

My grandfather, born before radio waves were even a laboratory curiosity, almost lived to see the first artificial satellite beeping down at us from space. There are people who were born before there was such a thing as an airplane, and who in old age saw four ships launched to the stars. For all our failings, despite our limitations and fallibilities, we humans are capable of greatness. This is true of our science and some areas of our technology, of our art, music, literature, altruism, and compassion, and even, on rare occasion, of our statecraft. What new wonders undreamt of in our time will we have wrought in another generation? And another? How far will our nomadic species have wandered by the end of the next century? And the next millennium?

Two billion years ago our ancestors were microbes; a half-billion years ago, fish; a hundred million years ago, something like mice; ten million years ago, arboreal apes; and a million years ago, proto-humans puzzling out the taming of fire. Our evolutionary lineage is marked by mastery of change. In our time, the pace is quickening.

When we first venture to a near-Earth asteroid, we will have entered a habitat that may engage our species forever. The first voyage of men and women to Mars is the key step in transforming us into a multiplanet species. These events are as momentous as the colonization of the land by our amphibian ancestors and the descent from the trees by our primate ancestors.

Fish with rudimentary lungs and fins slightly adapted for walking must have died in great numbers before establishing a permanent foothold on the land. As the forests slowly receded, our upright apelike forebears often scurried back into the trees, fleeing the predators that stalked the savannahs. The transitions were painful, took millions of years, and were imperceptible to those involved. In our case the transition occupies only a few generations, and with only a handful of lives lost. The pace is so swift that we are still barely able to grasp what is happening.

Once the first children are born off Earth; once we have bases and homesteads on asteroids, comets, moons, and planets; once we’re living off the land and bringing up new generations on other worlds, something will have changed forever in human history. But inhabiting other worlds does not imply abandoning this one, any more than the evolution of amphibians meant the end of fish. For a very long time only a small fraction of us will be out there.

“In modern Western society,” writes the scholar Charles Lindholm,

the erosion of tradition and the collapse of accepted religious belief leaves us without a telos [an end to which we strive], a sanctified notion of humanity’s potential. Bereft of a sacred project, we have only a demystified image of a frail and fallible humanity no longer capable of becoming god-like.

I believe it is healthy—indeed, essential—to keep our frailty and fallibility firmly in mind. I worry about people who aspire to be “god-like.” But as for a long-term goal and a sacred project, there is one before us. On it the very survival of our species depends. If we have been locked and bolted into a prison of the self, here is an escape hatch—something worthy, something vastly larger than ourselves, a crucial act on behalf of humanity. Peopling other worlds unifies nations and ethnic groups, binds the generations, and requires us to be both smart and wise. It liberates our nature and, in part, returns us to our beginnings. Even now, this new telos is within our grasp.

The pioneering psychologist William James called religion a “feeling of being at home in the Universe.” Our tendency has been, as I described in the early chapters of this book, to pretend that the Universe is how we wish our home would be, rather than to revise our notion of what’s homey so it embraces the Universe. If, in considering James’ definition, we mean the real Universe, then we have no true religion yet. That is for another time, when the sting of the Great Demotions is well behind us, when we are acclimatized to other worlds and they to us, when we are spreading outward to the stars.

The Cosmos extends, for all practical purposes, forever. After a brief sedentary hiatus, we are resuming our ancient nomadic way of life. Our remote descendants, safely arrayed on many worlds through the Solar System and beyond, will be unified by their common heritage, by their regard for their home planet, and by the knowledge that, whatever other life may be, the only humans in all the Universe come from Earth.

They will gaze up and strain to find the blue dot in their skies. They will love it no less for its obscurity and fragility. They will marvel at how vulnerable the repository of all our potential once was, how perilous our infancy, how humble our beginnings, how many rivers we had to cross before we found our way.

*Even if we are not in any particular hurry, we may be able by then to make small worlds move faster than we can make spacecraft move today. If so, our descendants will eventually overtake the two Voyager spacecraft—launched in the remote twentieth century—before they leave the Oort Cloud, before they make for interstellar space. Perhaps they will retrieve these derelict ships of long ago. Or perhaps they will permit them to sail on.

*A value that nicely approximates modern estimates of the number of planets orbiting stars in the Milky Way Galaxy.

*Most of it may be in “nonbaryonic” matter, not made of our familiar protons and neutrons, and not anti-matter either. Over 90 percent of the mass of the Universe seems to be in this dark, quintessential, deeply mysterious stuff wholly unknown on Earth. Perhaps we will one day not only understand it, but also find a use for it.