Pale Blue Dot - Carl Sagan [13]
—VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL HISTORY (1752)
In the seventeenth century there was still some hope that, even if the Earth was not the center of the Universe, it might be the only “world.” But Galileo’s telescope revealed that “the Moon certainly does not possess a smooth and polished surface” and that other worlds might look “just like the face of the Earth itself.” The Moon and the planets showed unmistakably that they had as much claim to being worlds as the Earth does—with mountains, craters, atmospheres, polar ice caps, clouds, and, in the case of Saturn, a dazzling, unheard-of set of circumferential rings. After millennia of philosophical debate, the issue was settled decisively in favor of “the plurality of worlds.” They might be profoundly different from our planet. None of them might be as congenial for life. But the Earth was hardly the only one.
This was the next in the series of Great Demotions, down-lifting experiences, demonstrations of our apparent insignificance, wounds that science has, in its search for Galileo’s facts, delivered to human pride.
WELL, SOME HOPED, even if the Earth isn’t at the center of the Universe, the Sun is. The Sun is our Sun. So the Earth is approximately at the center of the Universe. Perhaps some of our pride could in this way be salvaged. But by the nineteenth century, observational astronomy had made it clear that the Sun is but one lonely star in a great self-gravitating assemblage of suns called the Milky Way Galaxy. Far from being at the center of the Galaxy, our Sun with its entourage of dim and tiny planets lies in an undistinguished sector of an obscure spiral arm. We are thirty thousand light years from the Center.
Well, our Milky Way is the only galaxy. The Milky Way Galaxy is one of billions, perhaps hundreds of billions of galaxies notable neither in mass nor in brightness nor in how its stars are configured and arrayed. Some modern deep sky photographs show more galaxies beyond the Milky Way than stars within the Milky Way. Every one of them is an island universe containing perhaps a hundred billion suns. Such an image is a profound sermon on humility.
Well, then, at least our Galaxy is at the center of the Universe. No, this is wrong too. When the expansion of the Universe was first discovered, many people naturally gravitated to the notion that the Milky Way was at the center of the expansion, and all the other galaxies running away from us. We now recognize that astronomers on any galaxy would see all the others running away from them; unless they were very careful, they would all conclude that they were at the center of the Universe. There is, in fact, no center to the expansion, no point of origin of the Big Bang, at least not in ordinary three-dimensional space.
Well, even if there are hundreds of billions of galaxies, each with hundreds of billions of stars, no other star has planets. If there are no other planets beyond our Solar System, perhaps there’s no other life in the Universe. Our uniqueness might then be saved. Since planets are small and feebly shine by reflected sunlight, they’re hard to find. Although applicable technology is improving with breathtaking speed, even a giant world like Jupiter, orbiting the nearest star, Alpha Centauri, would still be difficult to detect. In our ignorance, the geocentrists find hope.
There was once a scientific hypothesis—not just well received but prevailing—that supposed our solar system to have formed through the near collision of the ancient Sun with another star; the gravitational tidal interaction pulled out tendrils of sunstuff that quickly condensed into planets. Since space is mainly empty and near stellar collisions most rare, it was concluded that few other planetary systems exist—perhaps only one, around that other star that long ago co-parented the worlds of our solar system. Early in my studies,