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.[41]

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:

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.