a strategic planner at the Pentagon before dying in 1982. According to the many-worlds interpretation, our universe is merely one among a vast ensemble of alternate universes—a multiverse—all of them interacting in a ghostly way to produce otherwise inexplicable quantum phenomena.
What would happen, Deutsch wondered, if quantum mechanics was applied to computer science? Might all the different parallel universes in the multiverse be coaxed into collaborating on a single computation?
Deutsch took as his starting point the classical theory of computability, which had been pioneered in the years before the Second World War by the Englishman Alan Turing. Among Turing’s discoveries was a program for a “universal” computer, one that would be capable of mimicking to perfection the output of any special-purpose machine. Deutsch set about recasting Turing’s work in quantum terms. In doing so, he managed to construct a quantum version of Turing’s universal computer—that is, a single quantum operator (or “Hamiltonian,” as it is known in the trade) that can do the work of any conceivable computing machine, whether a conventional computer of the kind now in use or a quantum computer as envisaged by Feynman. And Deutsch’s universal quantum computer had another marvelous property: in principle, it could simulate any physically possible environment. It was the ultimate “virtual reality” machine.
Deutsch, who was in his early twenties at the time (he was born in Israel in 1953), later downplayed his proof of the existence of a universal quantum computer as “fairly straightforward.” He went to Caltech to present it to Richard Feynman, who was already suffering from the cancer that would kill him in 1988. After Deutsch had written the first bits of his proof on the blackboard, the ailing Feynman startled him by jumping out of his seat, grabbing the chalk, and finishing it himself.
For Deutsch, a universal computer had become nothing less than the key to understanding reality. Such a machine, being able to generate all physically possible worlds, would be the consummation of physical knowledge. It would be a single, buildable physical object that could describe or mimic with perfect accuracy any part of the quantum multiverse. And since it was possible to build a universal computer, Deutsch concluded, such a machine must actually be built somewhere in the multiverse. Omniscience exists!
Such speculative flights come quite naturally to Deutsch, who, after returning to England from the United States, was appointed to be a research physicist at Oxford’s Clarendon Laboratory. In 1997, he laid out his worldview in a book titled The Fabric of Reality. To achieve a deep scientific understanding of reality, he argued therein, we must use not only quantum mechanics and the theory of computation, but also the theory of evolution. (He credits Richard Dawkins as one of his intellectual heroes.) Life and thought, he declared, determine the very warp and woof of the quantum multiverse. Whereas physical structures, like constellations and galaxy clusters, vary randomly from one universe to the next, knowledge-bearing structures—embodied in physical minds—arise from evolutionary processes that ensure they are nearly identical across different universes. From the perspective of the quantum multiverse as a whole, mind is a pervasive ordering principle, like a giant crystal.
Clearly, here was a man who aspired to a complete understanding of what he was pleased to call the “fabric of reality.” Would that complete understanding encompass the mystery of existence itself? Would it yield an answer to the question Why is there something rather than nothing? I ardently hoped to find out. I had reviewed Deutsch’s book years ago in the Wall Street Journal—favorably, as I dimly recalled. Surely, I thought, he would be willing to talk to an admirer such as myself, especially one who had taken the trouble to come all the way to Oxford. So I e-mailed him, introducing myself and mentioning the nice review I had given his book in the United States more than a decade ago.
“I just checked on Google,” Deutsch e-mailed me back. “Arrogant in tone and marred by leaps of logic—is that the one?”
Oh dear. My memory seemed to have played me false. I googled the review myself. The full sentence he had quoted read, “Arrogant in tone and marred by leaps of logic, his book nonetheless bristles with subversive insights about virtual reality, time and time travel, mathematical certainty, and free will.” That didn’t sound so bad. In the review I had also called Deutsch “mad, bad, and dangerous to know”—a description originally applied to Lord Byron. E-mailing him again, I pointed out that this was