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.1
The outer edge of the Sun's Oort Cloud is perhaps halfway to the nearest star. Not every other star has an 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 non-primordial black holes are strewn through the Galaxy, generated in the evolution of stars. If so, the nearest may be only 10 or 20 lightyears 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 spacefaring 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 millions1 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, reengineer 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 some2 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.