Pale Blue Dot - Carl Sagan [142]
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