Free Mirror Earth by Michael D. Lemonick Page A

lies under our feet and objects made entirely or partly of earth (rocks, trees, people) tend to fall to the ground unless something prevents them from doing so.
    The lightest element in Aristotle’s cosmology was fire. That makes sense as well: Things made of fire (lightning, shooting stars) are high in the sky, and the flames of an actual fire try to leap upward, to their natural place. In between earth and firewere, from the bottom up, water (it sits on top of the ground in the form of puddles, lakes, rivers, and oceans, and it falls to the ground as rain) and air (bubbles rise in water, and the air quite clearly sits above lakes and oceans but below the Sun).
    If Aristotle’s cosmology was right, the atomists’ notion of other worlds had to be wrong. The natural home for the element earth was under our feet; if there happened to be any substantial amount of it up in the heavens, it would long since have returned home. To imagine other worlds, you’d have to imagine multiple natural homes, which made no sense. Even if those homes existed, for the sake of argument, how would the air know where to go? If it migrated up to its natural place above our Earth, it would be moving down toward an unnatural location with respect to another Earth. It was—assuming Aristotle was right—crazy.
    Aristotle’s theories won out not only because they were intuitively persuasive, but also because they described an orderly universe, not a swirling chaos of invisible atoms randomly coming together. They were also comprehensive, not only describing the makeup of the world but also explaining the motions of the Sun, Moon, and planets (they were embedded in nested crystalline spheres that rotated majestically around the Earth at different rates). It was a neat, self-consistent cosmology, based on real things people could see. This turns out to be another foreshadowing of the modern controversy over string theory. It’s the hottest idea in modern physics: The building blocks of matter aren’t subatomic particles, but rather vibrating loops of “string,” far smaller than the particles we know, which live in an eleven-dimensional space that’s mostlyinvisible to us. It’s a mathematically powerful theory, but there’s no clear way to test it, aside from building a particle accelerator the size of the Milky Way. Some physicists argue that this makes it pure philosophy, not science.
    In any case, Aristotle’s cosmology, along with the rest of his science, won over enough of his colleagues and disciples and their descendants that the atomist theory was stored away in a musty intellectual drawer. The idea of a plurality of worlds went with it, where it remained more or less dormant in European thought for more than a thousand years.
    What revived the idea of multiple worlds was not, as you might expect, the emergence of modern scientific thinking. It didn’t come from Galileo or Newton or Copernicus. Instead, a burst of renewed interest came from the Catholic Church. The trigger was the rediscovery of Aristotle’s great work
De caelo
(On the heavens). He’d written it before the dawn of Christianity, but the book wasn’t translated into Latin until the late 1100s, long after the Church had become the dominant intellectual and social force in Europe. When
De caelo
reappeared, Catholic scholars—or simply, scholars, since there was virtually no other kind—scrutinized it to figure out whether the legendary philosopher’s ideas were compatible with Church doctrine. Many of those ideas passed the test. The Earth, for example, was at the center of biblical creation, so Aristotle’s putting it at the center of the world was perfectly appropriate.
    But when Aristotle declared that other worlds couldn’t exist, many believers thought he was going too far. Who was Aristotle, they asked, to say God couldn’t make other worldsif he wanted? Multiple worlds might not