At three o'clock on a warm summer afternoon, I arrive as scheduled at David Deutsch's home in Oxford, England. Deutsch, one of the world's leading theoretical physicists, a distinguished fellow of the British Computer Society and champion of what must certainly be the strangest scientific worldview ever created, is something of a recluse. He likes to sleep late and warned me not to come too early. Although I'm on time, my knocks on his door go unanswered. The house is dark and quiet. The doorbell doesn't seem to be working. After about 10 minutes a light goes on in an upstairs window, followed by the sound of running water. I knock harder, which at last triggers activity on the other side of the door. I hear feet pounding down stairs; the door opens, and Deutsch asks me to come in.
Deutsch believes the only way to make sense of the seemingly baffling equations of quantum mechanics, including this mathematical description of the workings of a quantum computer, is to assume the existence of parallel universes.
Piles of precariously stacked books line the route to his office, rising from the floor like stalagmites. A large poster of a brooding Albert Einstein hangs on one wall. Deutsch sits, sipping orange juice. He is slender, with birdlike attentiveness, and for someone who hardly ever leaves his home, surprisingly friendly and open. He looks much younger than 48. If his arguments, which have won over more than a few of his colleagues, turn out to be correct, our meeting is also occurring countless times in innumerable parallel universes, all in perfect accord with the uncanny laws of quantum theory.
Few physicists deny the validity of these laws, although they might not agree with Deutsch's interpretation of them. The laws insist that the fundamental constituents of reality, such as protons, electrons, and other subatomic particles, are not hard and indivisible. They behave like both waves and particles. They can appear out of nothing— a pure void— and disappear again. Physicists have even managed to teleport atoms, to move them from one place to another without passing through any intervening space. On the quantum scale, objects seem blurred and indistinct, as if created by a besotted god. A single particle occupies not just one position but exists here, there, and many places in between. "That quantum theory is outlandish, everyone agrees," says Deutsch. It seems completely in conflict with the world of big physics according to Newton and Einstein.
To grapple with the contradictions, most physicists have chosen an easy way out: They restrict the validity of quantum theory to the subatomic world. But Deutsch argues that the theory's laws must hold at every level of reality. Because everything in the world, including ourselves, is made of these particles, and because quantum theory has proved infallible in every conceivable experiment, the same weird quantum rules must apply to us. We, too, must exist in many states at once, even if we don't realize it. There must be many versions of late-rising David Deutsches, Earth, and the entire universe. All possible events, all conceivable variations on our lives, must exist, says Deutsch. We live not in a single universe, he says, but in a vast and rich "multiverse."
