Mirror Earth (8 page)

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 under
gravity 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 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 fire
were, 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 mostly
invisible 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 worlds
if he wanted? Multiple worlds might not arise naturally, acknowledged John Buridan, the rector of the University of Paris, in the early 1300s, but God could do things that wouldn't happen naturally. A half century later, another French scholar and cleric, Nicole Oresme, went a step further, arguing that other worlds
could
arise naturally. Aristotle's assertion that “down” must mean “toward the Earth” wasn't the only way to look at it. If you allowed “down” to mean “toward a heavy body,” the problem went away. Leonardo da Vinci suggested that this might be the case for the Moon. The fact that the Moon doesn't fall to Earth suggests, he wrote in his notebooks, that it was its own center of attraction, with its own complement of water, air, and fire. The German theologian and philosopher Nicholas of Cusa, who influenced Leonardo's thinking, wrote:

Rather than think that so many stars and parts of the heavens are uninhabited and that this earth of ours alone is peopled … we will suppose that in every region there are inhabitants, differing in nature by rank and all owing their origin to God, who is the centre and circumference of all stellar regions.

Even as theologians and scholars and natural philosophers were wrestling with Aristotle's ideas, his competition, the atomists, showed up as well, after a millennium in obscurity. The writings of the original atomists had disappeared with the rise of Aristotle, but a Roman philosopher named Lucretius, writing in the first century B.C., had preserved their ideas.
In 1417, Lucretius's atomist manuscript, titled
De rerum natura
(On the nature of things) was translated into Latin as well.

This was something of a problem. Having worked themselves into an outrage that some dead Greek had dared put a limit on how many worlds God could create, here came another dead Greek who rejected such a limit, but who was also pretty explicitly an atheist. Even worse, Lucretius's lyrical writing meant that his work was thought of primarily as a work of poetic literature at first. He sneaked onto everyone's must-read list before anyone could focus on his heresy.

Still, by the late 1500s, the notion of a plurality of worlds had become respectable, as long as it was expressed properly. God could make as many worlds as he wanted, in principle. But in practice, he'd just made ours (this was the line a young Galileo took, decades before he turned his telescope on the heavens). The arguments, as always, were purely theological, with no practical implications at all.

This wouldn't last long, however. In 1543, a Polish cleric and astronomer named Mikołaj Kopernik or Niklas Koppernigk, later Latinized to Nicolaus Copernicus, died and posthumously published a manuscript. Titled
De revolutionibus orbium caelestium
(On the revolutions of the heavenly spheres), it argued that the motions of the planets through the night sky could be best explained if the Earth and the other planets orbited the Sun, rather than everything orbiting the Earth. A preface to the manuscript suggested that this was a purely mathematical exercise—it didn't mean the Earth
actually
orbited the Sun. But a man named Andreas Osiander, who was overseeing the printing of the book, may well have added the
preface without Copernicus's permission. Osiander was evidently appalled at the challenge to Church doctrine this new model of the solar system implied. The Earth must be central because it was in the very first sentence of the Bible, and because humans were the focus of God's closest and most loving attention.

As a result of both the preface and his death, Copernicus never got in trouble with Church authorities. The same can't be said for those who followed him—especially an ex-monk named Giordano Bruno, who was burned at the stake in 1600 by the Inquisition for a long list of heresies. Among them were his loudly proclaimed belief in magic, his rejection of orthodox Christianity in favor of a version incorporating Egyptian mysticism, his insistence that Copernicus's Sun-centered cosmology was
not
just a mathematical exercise, and his further insistence that other worlds weren't merely possible but they existed. “There are,” he wrote in 1584 in the treatise
De l'infinito universo e mondi
(On the infinite universe and worlds),

countless suns and countless earths all rotating round their suns in exactly the same way as the seven planets of our system [the Moon was considered a planet at the time]. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth … Destroy the theories that the Earth is the center of the Universe!

In 1608, less than a decade after Bruno's execution, Galileo got wind of a Dutch invention called the perspective glass, which used lenses to make faraway objects look nearer. He built his own, improved version, which he called the perspicillum. The Dutch used their instrument to look at distant objects on the ground. Galileo, whose intellectual life was caught up in the heavens, turned his perspicillum in that direction as well. (The word
telescope
was coined in 1611 by the astronomer Giovanni Demisiani.)

Looking upward, Galileo immediately made a series of discoveries that made it impossible to think of the Copernican, Sun-centered solar system as just a mathematical fiction. He saw four unsuspected moons orbiting Jupiter, which meant they weren't orbiting the Earth. He saw moons around Saturn—or thought he did. The blurry bulges on either side of the planet that he believed were moons were actually Saturn's magnificent rings. He saw Venus go through phases, just like the Moon, proving that it, too, went around the Sun. He saw that our own Moon wasn't a smooth, perfect sphere, which was another of Aristotle's ideas the Church liked (things in the heavens must be perfect, they reasoned, because God is perfect). Giordano Bruno could reasonably be seen as something of a lunatic; all he had to offer were his beliefs. Galileo had evidence.

The Church resisted that evidence however. Finally, in 1633, the Inquisition forced Galileo to recant his belief in a Sun-centered universe and put him under house arrest for the rest of his life. But anyone who had access to a telescope could
look through it and see everything Galileo had seen. Evidently, not everyone was willing to try. “My Dear Kepler,” Galileo wrote to the German astronomer Johannes Kepler in 1610, “what do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an asp and do not want to look at either the planets, the moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times?”

Many did try, however, and for those in non-Catholic countries such as Germany and England, the Inquisition wasn't a threat. Within a century, thanks to Galileo, Kepler, Isaac Newton, John Flamsteed, Edmond Halley, and many others, the Earth had been all but completely displaced from the center of the universe in Western thinking. Thanks to Newton in particular, people who studied the skies now understood that the same physical laws applied in the heavens and on the Earth.

But Kepler, especially, had taken things much further. The Moon, he argued, wasn't just superficially like the Earth in being spherical and mountainous. Even before the telescope had been invented, he was arguing that the Moon also had an atmosphere and weather. He also suspected—surprisingly, given that the idea of extraterrestrial life feels relatively modern—that it was inhabited. Even more surprisingly, he wasn't the first. As far back as the sixth century B.C., the Pythagorean school of philosophy held (according, at least, to secondhand writings) that “the moon is … inhabited as our earth is, and contains animals of a larger size and plants of a rarer beauty than our globe affords. The animals in their virtue
and energy are fifteen degrees superior to ours, emit nothing excrementitious, and the days are fifteen times longer.”

Kepler himself imagined two races of Moon people in his 1634 work
Somnium
(The dream), a fictional account of a trip to the Moon. The Subvolvans lived on the familiar side of the Moon that faces the Earth; the Privolvans lived on the far side. But it wasn't just the Moon that was inhabited: Kepler also concluded there must be intelligent beings on Jupiter. The moons Galileo had found proved it: Since they weren't visible to the naked eye from Earth, God must have created them for the Jovians to look at.