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Minds, Machines, and the Multiverse: The Quest for the Quantum Computerby Julian R Brown
Chapter One: Late-Night Quantum Thoughts
Any technology that is sufficiently advanced is indistinguishable from magic.
Life in Other Universes
Gaunt, hair unkempt, and skin a ghostly shade, David Deutsch cuts a strange figure even by the standards of the eccentrics and oddballs that so often inhabit the upper reaches of science. Yet appearances can be deceptive. Talk to Deutsch and you won't find the withdrawn, head-in-the-clouds academic you might expect but someone who is articulate, personable, and exceedingly bright. You'll also quickly discover this is an individual whose ideas about life and the universe are bigger and more radical than those of anyone you are ever likely to meet.
As I drove from London to visit him at his home in Oxford, a fatal accident on the road alerted me to the bizarre implications of some of Deutsch's ideas, in particular his resolute belief in the existence of an infinite set of parallel universes — the multiverse, as he calls it. Within the multiverse there are endless alternate realities in which, supposedly, a huge army of mutant copies of everyone on this and other planets plays out an almost limitless variety of dramas. According to this world view, each time we make the smallest decision the entire assemblage divides along different evolutionary paths like some vast ant colony splitting off in divergent directions. In the process, large numbers of our copies suddenly embark on different lives in different universes.
Presumably, I thought, in some of those fabled other-universes the truck in front hadn't jackknifed, the accident hadn't happened, the crash victims were still alive, and I was now on my way instead of stuck in a jam. But did that mean the victims would therefore feel as though they had survived regardless of the accident because they would only be aware of the universes in which they had continued to live?
A foreshadow of this all-possible-worlds idea was intriguingly captured by Jorge Luis Borges in a short story now often cited by quantum physicists. In "The Garden of Forking Paths," written back in the 1950s, an illustrious Chinese governor is said to have written a strange kind of novel constructed as a kind of labyrinth. "In all fictional works, each time a man is confronted with several alternatives, he chooses one and eliminates the others; in the fiction of Ts'ui Pen, he chooses — simultaneously — all of them," Borges wrote. By writing a novel that pursued all possible story lines simultaneously, Borges's fictitious author, Ts'ui Pen, could have been writing a script for the multiverse.
But why, I wondered, should we care about other universes? There's surely enough to worry about in this one without concerning ourselves over the fate of the poor lost souls living in others. With such thoughts running through my mind, I was ready to be briefed by this world's leading advocate for the existence of those other worlds. When I finally arrived and entered Deutsch's house, it took me a moment to persuade myself that I hadn't stepped right into another universe. His place was an unbelievable mess. Papers, books, boxes, computer equipment, mugs, magazines, videos, and numerous other items were strewn everywhere. Even a trip to the bathroom entailed climbing over a mound of cardboard boxes. How does this man function, I wondered?
Okay, so this wasn't exactly a parallel universe, more a parallel mode of living. Yet out of the chaos there were definite, albeit eclectic, signs of order. A whiteboard in the corner of his living room was covered with equations, a reference to Oliver Stone's Natural Born Killers here, and notes about time travel there. On a piano in another corner of the room lay open a score of Beethoven's Waldstein Sonata. On the floor, littered among the detritus, were strange and elaborate drawings of spidery-looking aliens, designs for an animated film that Deutsch was making with some friends.
But it wasn't long before our conversation leapt far beyond the confines of Deutsch's domicile. "Somewhere in the multiverse there are Earths that were not hit by a comet 60 million years ago and on which the dinosaurs evolved into intelligent beings," Deutsch informed me. "Now those dinosaurs might look completely different from us, and their houses might look different from ours, and their cars probably look different from ours — but if you look at their airplanes and spaceships, they probably look much more similar to ours, and if you look at their microchips, they're going to look very similar to ours. The more knowledge that is embodied in a physical object, the more alike it will look in different universes because the more it will have been subjected to the same tests of efficiency, validity, truth, and so on."
"Do you really think there are such universes, in which dinosaurs have all those things?" I asked incredulously. "Undoubtedly," Deutsch replied without hesitation. "That's what the laws of physics tell us."
Deutsch is a physicist, winner of the 1998 Paul Dirac prize for theoretical physics and a researcher at the Centre for Quantum Computation at Oxford University's Clarendon Laboratory. Owing to his unusual lifestyle, though, he rarely sets foot in the university's buildings, preferring instead to inhabit his own private world. If you call Deutsch on the telephone before 2 p.m., you are unlikely to get an answer. He prefers to sleep during the day and work uninterrupted through the night.
During those long nocturnal hours Deutsch has deliberated on some extraordinary ideas. Among them is one of the biggest ideas in physics and mathematics since the development of quantum theory, a breakthrough that could also lead to the most significant invention since the digital computer. It is the concept of a quantum computer, a machine that has begun a revolution in computing — and far beyond — that looks set to be as momentous as the upheaval in Newtonian physics brought about in the early twentieth century. As Discover magazine once put it,4 a quantum computer "would in some sense be the ultimate computer, less a machine than a force of nature."
Why so? Because a quantum computer would make any existing computer — even the fastest Cray supercomputer — seem exceedingly puny. It would solve problems that will never be cracked by any conceivable nonquantum successors of current computers. It would make possible virtual-reality simulations of things we know can never be simulated with current technology, even in principle. But perhaps most important, it would throw open doors to a totally new kind of laboratory in which we could explore those alternate universes of which Deutsch is so fond. Imagine living in a small house for many years and then discovering in the basement a trapdoor that opened onto a colossal subterranean world of rooms that appeared to stretch on into infinity. For physicists and computer scientists that, in some sense, is what the arrival of the first quantum computer would be like.
From the early 1970s to the present day, Deutsch has seen his ideas grow from an apparently small ripple in the world of academic physics to what is now an intellectual maelstrom. By the nineties, the subject of quantum computing had become one of the most exciting and rapidly moving research disciplines in physics and computing. Chaos theory and superstring theory — the scientific gold rushes of the 1980s — suddenly seemed far less enticing in the light of the new Klondike: quantum computation. Theoretical
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