David Deutsch's research in quantum physics has been influential and highly acclaimed. His papers on quantum computation laid the foundations for that field, breaking new ground in the theory of computation as well as physics, and have triggered an explosion of research efforts worldwide. His work has revealed the importance of quantum effects in the physics of time travel, and he is an authority on the theory of parallel universes.

Born in Haifa, Israel, David Deutsch was educated at Cambridge and Oxford universities. After several years at the University of Texas at Austin, he returned to Oxford, where he now lives and works. He is a member of the Quantum Computation and Cryptography Research Group at the Clarendon Laboratory, Oxford University

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October 5, 1997
Shadow Worlds
Our universe, a physicist says, is but one of many.

A standard set piece in almost any laymen's introduction to modern physics is the musty old two-slit experiment. Aim a beam of photons at a photographic film. Then obstruct the path of the particles with a piece of cardboard with two holes punched through it. Close one hole and the photons that travel through the other hole will leave a single spot on the film. But open both holes at once and what do you see? Two spots side by side? Fans of popular physics know the answer is not so simple. Opening both holes causes the photons to trace a complex ''interference pattern'': an alternating configuration of light and dark bands representing the presence and absence of photons.

Opening both holes somehow prevents a particle from landing in places where it was previously free to go. The physicist John Bell said of this puzzling and now oddly familiar situation that it is as though ''the mere possibility of passing through the other hole'' affects the particle's motion ''and prevents it going in certain directions.'' Something funny is going on.

Since quantum theory was hammered together in the early part of the century, its architects have agonized over what it means. Human language doesn't seem up to the task. Niels Bohr put it like this: ''We must be clear that, when it comes to atoms, language can be used only as in poetry.'' By now a lot of physicists are tired of thinking about it. Quantum theory works, they say, whatever the hell it means. They prefer to communicate in the unambiguous language of mathematics.

David Deutsch, an Oxford physicist and the author of ''The Fabric of Reality: The Science of Parallel Universes -- and Its Implications,'' thinks this sort of attitude is a cop-out. Quantum mechanics, he insists, must be taken not just as a predictive tool but as an explanation for how the world really works. The price, which he seems quite happy to pay, is to accept that the universe is far stranger than it already appears. If we are to take quantum theory at face value, he argues, we are led to conclude that our universe is one of many in an ensemble of parallel universes that physicists have come to call the multiverse. Deutsch believes that the photons in the two-slit experiment are prevented from landing on certain parts of the film because they are being interfered with by invisible ''shadow'' photons from a parallel universe.

This ''many worlds'' interpretation of quantum mechanics was first put forward decades ago by the physicist Hugh Everett as a way of making sense of quantum mechanics. Suppose you want to measure a subatomic particle's position. According to quantum theory, the undisturbed particle is in a peculiar state of limbo in which all the possible positions it might assume ''exist'' (don't ask how) simultaneously. Only when the particle is measured does it snap into a precise location. There is no reason the particle chooses one place and not another. The choice is random. Everett proposed another, even more counterintuitive way to think about the situation. When the particle is measured, the universe splits into multiple copies. In each of these universes the electron takes on a different position. We just happen to be stuck in one -- and only one -- of those worlds.

Again, this is old stuff to physics fans. But Deutsch makes the most persuasive argument for the Everett interpretation that I've read. That, though, is only a small part of the book. Deutsch's grander, overriding aim is to argue that in making sense of the universe, science must take its theories not just as handy tools but as serious descriptions of how the world works. A true theory of everything, he says, will be woven from four strands. The multiverse version of quantum theory is the deepest of the theories, he writes. Also important is the theory of computation: the idea, developed by the mathematicians Alan Turing, Alonzo Church and others, that all physical processes can be simulated on a computer. Also crucial is the theory of evolution and, finally, an epistemology (theory of knowledge) that takes science not as a human construct but as an ever-improving map of the world.

''The Fabric of Reality'' is full of refreshingly oblique, provocative insights. But I came away from it with only the mushiest sense of how the strands in Deutsch's tapestry hang together. Early on he entertains the notion that, as he writes ''The Fabric of Reality,'' other David Deutsches in other universes are also writing books: ''Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea.'' I wish I could reach into one of those worlds and grab a clearer version of this perplexing book.

George Johnson, the author of ''Fire in the Mind: Science, Faith, and the Search for Order,'' is writing a biography of the physicist Murray Gell-Mann.

This article by David Deutsch appeared in Frontiers magazine, December 1998 and is copyright © by David Deutsch 1998.

David Deutsch’s Many Worlds

Our universe is just one of many, linked together by the astounding phenomena of the quantum world. David Deutsch believes this multiverse view of reality could hold the future of computing.

A growing number of physicists, myself included, are convinced that the thing we call ‘the universe’ — namely space, with all the matter and energy it contains — is not the whole of reality. According to quantum theory — the deepest theory known to physics — our universe is only a tiny facet of a larger multiverse, a highly structured continuum containing many universes.

Everything in our universe — including you and me, every atom and every galaxy — has counterparts in these other universes. Some counterparts are in the same places as they are in our universe, while others are in different places. Some have different shapes, or are arranged in different ways; some are so different that they are not worth calling counterparts. There are even universes in which a given object in our universe has no counterpart — including universes in which I was never born and you wrote this article instead.

On large scales, universes obey the laws of classical physics, and so each behaves as though the others were not there. But on microscopic scales, quantum mechanics becomes dominant and the universes are far from independent. Universes that are very alike are close together in the multiverse and affect each other strongly, though only in subtle, indirect ways — a phenomenon known as quantum interference.

Without quantum interference, electrons would spiral into atomic nuclei, destroying every atom literally in a flash. Solid matter would be unstable, and the phenomena of biological evolution and human thought would be impossible. And as I shall explain, it is quantum interference that provides our evidence for the existence of the multiverse.

Through interference, each particle in our universe can be affected by its counterparts in other universes. What we see as a single subatomic particle is really a sprawling trans-universe structure, spanning a large region of the multiverse. Although we cannot see the parts of this structure that are outside our universe, we can infer their presence from the results of experiments. Perhaps the most striking involve quantum computers — devices that collaborate with nearby universes to perform useful computations.

How do they do that? While conventional, non-quantum computers perform calculations on fundamental pieces of information called bits, which can take the values 0 or 1, quantum computers use objects called quantum bits, or qubits (pronounced queue-bits). A qubit can also either represent 0 or 1, but its value can vary from universe to universe. Hence in the time it takes a conventional computer to perform a given calculation, a quantum computer with its counterparts in other universes can perform many such calculations. In particular, they can each perform different pieces of a complex computation simultaneously. Using quantum interference, the computer in our universe can then combine its results with those of its counterparts, to arrive at the overall answer.

Not all types of computation are capable of being shared out among universes in this way. Within one universe we are free to shuffle information about from place to place, and to perform whatever logical operations we like on it, but in the multiverse, things are not so convenient. The laws of physics severely restrict the operations that we can perform. Nevertheless, quantum computers offer fundamentally new capabilities, including absolutely secure methods of communication, ways of breaking the best existing codes, and seemingly miraculous algorithms for solving mathematical problems that are currently intractable.

For instance, Deep Blue, IBM’s chess-playing supercomputer, can examine about 200 million chess positions per second by sharing the work among its 256 processors, each of which examines almost one million positions per second. A quantum computer, running a search algorithm discovered by Lov Grover of AT&T’s Bell Laboratories in New Jersey, could outclass Deep Blue by sharing the work among many universes. Grover proved that if there were time to search N items using a conventional computer in one universe, his algorithm could exploit the multiverse to search a total of N2 items in the same time. Thus a single quantum processor, with the same clock rate as one of Deep Blue’s processors, could examine a trillion chess positions in one second — and in two seconds it could examine four trillion, in three seconds nine trillion, and so on.

Research groups worldwide are now racing to build the first practical quantum computer. Any physical object that can store a bit can in principle also serve as a qubit, but in practice, because interference is harder to control in larger systems, qubits have to be microscopic objects such as individual ions or atomic nuclei. The most powerful prototype quantum computers in existence have only a handful of qubits each, but they can already demonstrate modes of computation that no existing computer can match.

To predict that future quantum computers, made to a given specification, will work in the ways I have described, one need only solve a few uncontroversial equations. But to explain exactly how they will work, some form of multiple-universe language is unavoidable. Thus quantum computers provide irresistible evidence that the multiverse is real. One especially convincing argument is provided by quantum algorithms — even more powerful than Grover’s — which calculate more intermediate results in the course of a single computation than there are atoms in the visible universe. When a quantum computer delivers the output of such a computation, we shall know that those intermediate results must have been computed somewhere, because they were needed to produce the right answer. So I issue this challenge to those who still cling to a single-universe world view: if the universe we see around us is all there is, where are quantum computations performed? I have yet to receive a plausible reply.
Oxford physicist David Deutsch laid the theoretical foundations of quantum computing. He examines the multiverse, quantum computers and other topics in his book The Fabric of Reality, published by Penguin.

The Fabric of Reality : The Science of Parallel Universes -- And Its Implications

One major school of quantum theory posits a multiplicity of universes; but what does that imply about the reality we live in? A simple experiment, familiar to every student of physics, involves light passing through slits in a barrier; its results, according to Oxford physicist Deutsch, lead inevitably to the idea that there are countless universes parallel to our own, through which some of the light must pass. This "many worlds" interpretation of quantum theory has gained advocates in recent years, and Deutsch argues that it is time for scientists to face the full implications of this idea. (After all, the entire point of science is to help us understand the world we live in--the "fabric of reality" of his title.) To that end, he outlines a new view of the multiverse (the total of all the parallel universes).

He argues that quantum computation, a discipline in which he is a pioneering thinker, has the potential for building computers that draw on their counterparts in parallel universes; this could make artificial intelligence a reality, despite Roger Penrose's objections (which Deutsch deals with in some detail). Likewise, time travel into both the future and the past should be possible, though not in quite the form envisioned by science fiction writers; the trips would almost certainly be one-way, and they would likely take the travelers into different universes from the one they began in.

Deutsch takes particular pains to refute Thomas Kuhn's "paradigm" model of science, which essentially denies progress. A final chapter looks at the long-range implications of his views, including the place of esthetic and moral values (areas more scientists now seem willing to confront).

Not easy going by any means, but worth the work for anyone interested in the thought processes of a scientist on the leading edge of his discipline.

For scientists and lay readers alike, this complete and rational synthesis of disciplines offers a new, optimistic message about existence. Deutsch discusses, demystifies and connects such topics as quantum computers; the physics of time travel; the comprehensibility of nature and the physical limits of virtual reality; the significance of human life; and the ultimate fate of the universe. Charts & figures throughout.

"Deutsch presents his vision of reality by combining ideas from four "strands" of science: quantum physics, epistemology, the theory of computation, and modern evolutionary theory. The implications of Richard Dawkins' work is also discussed in the book. I highly recommend it." -- John Catalano

New Horizons
By Mark K. Anderson

2:00 a.m. July 2, 2001 PDT

Seventy-five years have passed since the physicist Neils Bohr said, "Anyone who isn't shocked by quantum theory has not understood it."

Of course, back then people could still remember a time when just a glimpse of stocking was considered something shocking. Today, the University of Michigan hosts a quantum conference that brings Bohr's old saw into the shock-jock age.

The first Quantum Applications Symposium proposes to address the question, "Will quantum effects dominate the course of technology development in the 21st century?"Judging from the lineup of speakers slated to address this issue at the three-day event, the only real question appears to be how many exclamation points should be put after the answer "Yes!"

Quantum computing will dominate the discussion in roughly half of the talks presented -- from such distinguished researchers as David Deutsch of Oxford University's Centre for Quantum Computation, Brian Josephson of Cambridge University and Phil Platzman of Lucent Bell Labs.

Platzman will speak on one of the most promising new hardware ideas in the field, a quantum computer with a processing engine that consists of an array of electrons floating above the surface of liquid helium.

Although the proposal of Platzman and his collaborator Mark Dykman appeared only two years ago in the journal Science, teams in both the United States and the U.K. have already begun designing the guts of this new breed of machine. Such progress indeed raises the possibility of showcasing a quantum computer's unrivaled speed and power within the decade.

Also on tap is Paul Benioff of Argonne National Labs. His work has introduced the prospect of merging immobile quantum computing hardware with very mobile microscopic robots. So far, Benioff has considered the quantum robot as a minuscule bloodhound, a device that could perform physical searches of molecular or cellular sets, such as a real-world Google at a sub-micron scale.

Benioff stressed, however, that his work is presently more theoretical than many of the applications being discussed at the conference. "Let's first do quantum computers," he said. "And then when we get those things up and running, we can look at the other ideas."

Lute Maleki of the Jet Propulsion Laboratory, will be speaking about quantum applications in space exploration and aerospace engineering. First, he said, quantum technologies are already being developed for ultra-sensitive gravity meters used in the mapping of planetary features. These can, for instance, allow scientists to hunt for subsurface oceans on Jupiter's moon, Europa, as a space probe zips by.

Maleki also plans to discuss atomic lasers, a technology involving a coherent and tightly focused beam of atoms that was first developed by MIT researchers in 1997. "We're now where photon lasers were in the late 1950s," Maleki said. "But one of the things that they could lead to is making a 3-D matter hologram using atom lasers." Such a strange technology would not just make a 3-D image of something, but rather an actual replicate object out of the atoms in the laser.

14 July 2001

Taming The Multiverse

by Marcus Chown

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 ultiverse 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,
The Fabric of Reality by David Deutsch, Penguin (1997)

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