-Caveat Lector- >From The New Scientist, http://www.newscientist.com/hottopics/quantum/quantum.jsp?id=22994400 - Taming the multiverse Parallel universes are no longer a figment of our imagination. They're so real that we can reach out and touch them, and even use them to change our world, says Marcus Chown FLICKING through New Scientist, you stop at this page, think "that's interesting" and read these words. Another you thinks "what nonsense", and moves on. Yet another lets out a cry, keels over and dies. Is this an insane vision? Not according to David Deutsch of the University of Oxford. Deutsch believes that our Universe is part of the multiverse, a domain of parallel universes that comprises ultimate reality. Until now, the multiverse was a hazy, ill-defined concept-little more than a philosophical trick. But in a paper yet to be published, Deutsch has worked out the structure of the multiverse. With it, he claims, he has answered the last criticism of the sceptics. "For 70 years physicists have been hiding from it, but they can hide no longer." If he's right, the multiverse is no trick. It is real. So real that we can mould the fate of the universes and exploit them. Why believe in something so extraordinary? Because it can explain one of the greatest mysteries of modern science: why the world of atoms behaves so very differently from the everyday world of trees and tables. The theory that describes atoms and their constituents is quantum mechanics. It is hugely successful. It has led to computers, lasers and nuclear reactors, and it tells us why the Sun shines and why the ground beneath our feet is solid. But quantum theory also tells us something very disturbing about atoms and their like: they can be in many places at once. This isn't just a crazy theory-it has observable consequences (see "Interfering with the multiverse"). But how is it that atoms can be in many places at once whereas big things made out of atoms-tables, trees and pencils-apparently cannot? Reconciling the difference between the microscopic and the macroscopic is the central problem in quantum theory. The many worlds interpretation is one way to do it. This idea was proposed by Princeton graduate student Hugh Everett III in 1957. According to many worlds, quantum theory doesn't just apply to atoms, says Deutsch. "The world of tables is exactly the same as the world of atoms." But surely this means tables can be in many places at once. Right. But nobody has ever seen such a schizophrenic table. So what gives? The idea is that if you observe a table that is in two places at once, there are also two versions of you-one that sees the table in one place and one that sees it in another place. The consequences are remarkable. A universe must exist for every physical possibility. There are Earths where the Nazis prevailed in the Second World War, where Marilyn Monroe married Einstein, and where the dinosaurs survived and evolved into intelligent beings who read New Scientist. However, many worlds is not the only interpretation of quantum theory. Physicists can choose between half a dozen interpretations, all of which predict identical outcomes for all conceivable experiments. Deutsch dismisses them all. "Some are gibberish, like the Copenhagen interpretation," he says-and the rest are just variations on the many worlds theme. For example, according to the Copenhagen interpretation, the act of observing is crucial. Observation forces an atom to make up its mind, and plump for being in only one place out of all the possible places it could be. But the Copenhagen interpretation is itself open to interpretation. What constitutes an observation? For some people, this only requires a large-scale object such as a particle detector. For others it means an interaction with some kind of conscious being. Worse still, says Deutsch, is that in this type of interpretation you have to abandon the idea of reality. Before observation, the atom doesn't have a real position. To Deutsch, the whole thing is mysticism-throwing up our hands and saying there are some things we are not allowed to ask. Some interpretations do try to give the microscopic world reality, but they are all disguised versions of the many worlds idea, says Deutsch. "Their proponents have fallen over backwards to talk about the many worlds in a way that makes it appear as if they are not." In this category, Deutsch includes David Bohm's "pilot-wave" interpretation. Bohm's idea is that a quantum wave guides particles along their trajectories. Then the strange shape of the pilot wave can be used to explain all the odd quantum behaviours, such as interference patterns. In effect, says Deutsch, Bohm's single universe occupies one groove in an immensely complicated multi-dimensional wave function. "The question that pilot-wave theorists must address is: what are the unoccupied grooves?" says Deutsch. "It is no good saying they are merely theoretical and do not exist physically, for they continually jostle each other and the occupied groove, affecting its trajectory. What's really being talked about here is parallel universes. Pilot-wave theories are parallel-universe theories in a state of chronic denial." Back and forth Another disguised many worlds theory, says Deutsch, is John Cramer's "transactional" interpretation in which information passes backwards and forwards through time. When you measure the position of an atom, it sends a message back to its earlier self to change its trajectory accordingly. But as the system gets more complicated, the number of messages explodes. Soon, says Deutsch, it becomes vastly greater than the number of particles in the Universe. The full quantum evolution of a system as big as the Universe consists of an exponentially large number of classical processes, each of which contains the information to describe a whole universe. So Cramer's idea forces the multiverse on you, says Deutsch. So do other interpretations, according to Deutsch. "Quantum theory leaves no doubt that other universes exist in exactly the same sense that the single Universe that we see exists," he says. "This is not a matter of interpretation. It is a logical consequence of quantum theory." Yet many physicists still refuse to accept the multiverse. "People say the many worlds is simply too crazy, too wasteful, too mind-blowing," says Deutsch. "But this is an emotional not a scientific reaction. We have to take what nature gives us." A much more legitimate objection is that many worlds is vague and has no firm mathematical basis. Proponents talk of a multiverse that is like a stack of parallel universes. The critics point out that it cannot be that simple-quantum phenomena occur precisely because the universes interact. "What is needed is a precise mathematical model of the multiverse," says Deutsch. And now he's made one. The key to Deutsch's model sounds peculiar. He treats the multiverse as if it were a quantum computer. Quantum computers exploit the strangeness of quantum systems-their ability to be in many states at once-to do certain kinds of calculation at ludicrously high speed. For example, they could quickly search huge databases that would take an ordinary computer the lifetime of the Universe. Although the hardware is still at a very basic stage, the theory of how quantum computers process information is well advanced. In 1985, Deutsch proved that such a machine can simulate any conceivable quantum system, and that includes the Universe itself. So to work out the basic structure of the multiverse, all you need to do is analyse a general quantum calculation. "The set of all programs that can be run on a quantum computer includes programs that would simulate the multiverse," says Deutsch. "So we don't have to include any details of stars and galaxies in the real Universe, we can just analyse quantum computers and look at how information flows inside them." If information could flow freely from one part of the multiverse to another, we'd live in a chaotic world where all possibilities would overlap. We really would see two tables at once, and worse, everything imaginable would be happening everywhere at the same time. Deutsch found that, almost all the time, information flows only within small pieces of the quantum calculation, and not in between those pieces. These pieces, he says, are separate universes. They feel separate and autonomous because all the information we receive through our senses has come from within one universe. As Oxford philosopher Michael Lockwood put it, "We cannot look sideways, through the multiverse, any more than we can look into the future." Sometimes universes in Deutsch's model peel apart only locally and fleetingly, and then slap back together again. This is the cause of quantum interference, which is at the root of everything from the two-slit experiment to the basic structure of atoms. Other physicists are still digesting what Deutsch has to say. Anton Zeilinger of the University of Vienna remains unconvinced. "The multiverse interpretation is not the only possible one, and it is not even the simplest," he says. Zeilinger instead uses information theory to come to very different conclusions. He thinks that quantum theory comes from limits on the information we get out of measurements (New Scientist, 17 February, p 26). As in the Copenhagen interpretation, there is no reality to what goes on before the measurement. But Deutsch insists that his picture is more profound than Zeilinger's. "I hope he'll come round, and realise that the many worlds theory explains where the information in his measurements comes from." Why are physicists reluctant to accept many worlds? Deutsch blames logical positivism, the idea that science should concern itself only with objects that can be observed. In the early 20th century, some logical positivists even denied the existence of atoms-until the evidence became overwhelming. The evidence for the multiverse, according to Deutsch, is equally overwhelming. "Admittedly, it's indirect," he says. "But then, we can detect pterodactyls and quarks only indirectly too. The evidence that other universes exist is at least as strong as the evidence for pterodactyls or quarks." Perhaps the sceptics will be convinced by a practical demonstration of the multiverse. And Deutsch thinks he knows how. By building a quantum computer, he says, we can reach out and mould the multiverse. "One day, a quantum computer will be built which does more simultaneous calculations than there are particles in the Universe," says Deutsch. "Since the Universe as we see it lacks the computational resources to do the calculations, where are they being done?" It can only be in other universes, he says. "Quantum computers share information with huge numbers of versions of themselves throughout the multiverse." Imagine that you have a quantum PC and you set it a problem. What happens is that a huge number of versions of your PC split off from this Universe into their own separate, local universes, and work on parallel strands of the problem. A split second later, the pocket universes recombine into one, and those strands are pulled together to provide the answer that pops up on your screen. "Quantum computers are the first machines humans have ever built to exploit the multiverse directly," says Deutsch. At the moment, even the biggest quantum computers can only work their magic on about 6 bits of information, which in Deutsch's view means they exploit copies of themselves in 26 universes-that's just 64 of them. Because the computational feats of such computers are puny, people can choose to ignore the multiverse. "But something will happen when the number of parallel calculations becomes very large," says Deutsch. "If the number is 64, people can shut their eyes but if it's 1064, they will no longer be able to pretend." What would it mean for you and me to know there are inconceivably many yous and mes living out all possible histories? Surely, there is no point in making any choices for the better if all possible outcomes happen? We might as well stay in bed or commit suicide. Deutsch does not agree. In fact, he thinks it could make real choice possible. In classical physics, he says, there is no such thing as "if"; the future is determined absolutely by the past. So there can be no free will. In the multiverse, however, there are alternatives; the quantum possibilities really happen. Free will might have a sensible definition, Deutsch thinks, because the alternatives don't have to occur within equally large slices of the multiverse. "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives," he says. "When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen." Let's hope that deciding to read this article was the right choice. Multi-universe Interfering with the multiverse You can see the shadow of other universes using little more than a light source and two metal plates. This is the famous double-slit experiment, the touchstone of quantum weirdness. Particles from the atomic realm such as photons, electrons or atoms are fired at the first plate, which has two vertical slits in it. The particles that go through hit the second plate on the far side. Imagine the places that are hit show up black and that the places that are not hit show up white. After the experiment has been running for a while, and many particles have passed through the slits, the plate will be covered in vertical stripes alternating black and white. That is an interference pattern. To make it, particles that passed through one slit have to interfere with particles that passed through the other slit. The pattern simply does not form if you shut one slit. The strange thing is that the interference pattern forms even if particles come one at a time, with long periods in between. So what is affecting these single particles? According to the many worlds interpretation, each particle interferes with another particle going through the other slit. What other particle? "Another particle in a neighbouring universe," says David Deutsch. He believes this is a case where two universes split apart briefly, within the experiment, then come back together again. "In my opinion, the argument for the many worlds was won with the double-slit experiment. It reveals interference between neighbouring universes, the root of all quantum phenomena." Further reading: The structure of the multiverse by David Deutsch, http://arxiv.org/abs/quant-ph/0104033 The Fabric of Reality by David Deutsch, Penguin (1997) Marcus Chown >From New Scientist magazine 14 July 2001. <A HREF="http://www.ctrl.org/";>www.ctrl.org</A> DECLARATION & DISCLAIMER ========== CTRL is a discussion & informational exchange list. Proselytizing propagandic screeds are unwelcomed. Substance—not soap-boxing—please! 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