Free Mirror Earth by Michael D. Lemonick

The world included the Earth plus the sky and everything in it. So when the philosopher Epicurus wrote in the fourth century B.C. that “there are infinite worlds both like and unlike this world of ours … There nowhere exists an obstacle to the infinite number of worlds,” he was talking about what we would now call parallel universes, places forever inaccessible to us that would consist of a parallel Earth whose skies were dotted with glittering stars.
    Epicurus and his rough contemporaries Democritus and Leucippus were known as “atomists,” because they believed the world and everything in it were made of tiny, invisible, and indivisible units they called atoms. These atoms, they argued, gathered together in different ways to form the stars, the Moon, the Sun, the planets, and the Earth, along with everything on it. When physical objects changed—the wood in a fire turninginto ashes, flames, and smoke; the wood in a tree growing larger and bulkier every year; the wood in a downed tree slowly rotting away—it was simply a matter of the atoms rearranging themselves into another form.
    It all sounds impossibly modern, until you realize that atomism was a purely philosophical idea. The atomists had no evidence of any kind to support their assertions. They did no experiments that could test the hypothesis, so there was no way to refute it if it was wrong. That made atomism unscientific by definition, and it remained so until the early 1800s, when the English chemist John Dalton revived the idea to explain the nature of the chemical elements. Unlike the Greeks, Dalton and the scientists who came afterward did all sorts of experiments that could have demolished the atomic theory. The theory survived, obviously, although it wasn’t fully confirmed until 1908 by the French chemist Jean Baptiste Perrin.
    With infinite numbers of atoms swirling around an infinite void, the atomists reasoned that it was inevitable for them to form into infinite numbers of worlds. Writing six hundred years later, the philosopher Diogenes Laertius paraphrased the atomist Leucippus as believing that “the worlds come into being as follows: many bodies of all sorts and shapes move by abscission from the infinite into a great void; they come together there and produce a single whirl, in which, colliding with one another and revolving in all manner of ways, they begin to separate like to like.”
    Again, it seems remarkably sophisticated. Leucippus might have been describing the modern theory of planet formation, in which a cloud of interstellar gas and dust collapses undergravity to form a dense knot of matter—a future star—surrounded by a swirling disk of material that will turn into planets. But again, this wasn’t a scientific idea but rather a philosophical one. And Leucippus’s worlds, like those of Epicurus, were entire, self-contained universes, entirely separate from one another and constructed in different ways. (In another seemingly uncanny foreshadowing of modern science, this concept predates by more than two millennia the multiple-universe theories that have arisen independently from string theory, quantum theory, and inflationary cosmology.)
    Given the resonance of their ideas with those of twenty-first-century physics and cosmology, you might expect the names of Epicurus, Democritus, and Leucippus to be more familiar than they are. Instead, we’re familiar with Aristotle, who lived at about the same time. That’s because his theory of the universe won out over theirs, and in Aristotle’s theory, only a single world was possible. In Aristotle’s universe, everything was made of just four elements, not an infinite number of atoms. The elements didn’t swirl around, turning from one thing into another: They moved, inexorably, to their natural resting places and (mostly) stayed there. The heaviest of the four was earth—perfectly sensible, since the ground always