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David Deutsch is a distinguished quantum physicist and a member of the Centre for Quantum Computation at the Clarendon Laboratory, Oxford University. He has received the Paul Dirac Prize and Medal from the Institute of Physics for ‘outstanding contributions to theoretical physics’. He recently talked with Filiz Peach about his work and hopes.
David Deutsch’s book The Fabric of Reality, offers a startling new worldview which combines quantum physics, evolution, epistemology and computation. He also deals with quantum computation, a new field of physics in which he has been a pioneer. His explanation of the nature of the universe in terms of quantum physics is inspiring and thought-provoking. However, his favoured interpretation of quantum theory in terms of there being many parallel universes (or a ‘multiverse’ as he calls it) is not widely accepted in the scientific community, or at least not yet. But it may well be part of a new unifying theory of the universe in the 21st century. The Fabric of Reality is a clearly-written book, intelligible even for those of us who are not scientists. It was shortlisted for the 1997 Los Angeles Times Book Prize, and the 1998 Rhône- Poulenc Prize for Science Books.
Professor Deutsch, could you please tell our readers why you became interested in quantum physics?
I am interested in anything that is fundamental. Quantum physics and the General Theory of Relativity are the two most fundamental theories that physics has. They are the theories within which other theories are formulated; they provide the framework for all of physics.
So how did you first become involved?
When I was a graduate student, for my thesis I studied quantum field theory in curved space-time – a topic that is on the boundary between quantum theory and the General Theory of Relativity. It was hoped – it still is hoped – that one day these two theories will be unified. Logically, they are in deep conflict with each other, and this conflict is not within the reach of present day experiments to resolve. We know that a unification isn’t going to be easy. That unified theory would be called quantum gravity. The reason for studying quantum field theory in curved space-time was that it was hoped that when we understood that well, it would provide a clue to quantum gravity. We did eventually understand it well, and it did not provide a clue to quantum gravity. But it did convince me that quantum theory is at present the deeper of the two, and also, for the moment at any rate, provides more promising lines of research. Peter Medawar said once that science is the art of the soluble. You cannot necessarily solve the most profound problems right away. You have to go for the most profound soluble problem. And in that respect, I thought that quantum theory was the more promising.
So you now believe that quantum mechanics will provide a unifying theory of the universe?
‘Provide’ is not quite the right word. Quantum theory will be a pathway, a component of some future more unifying theory which will involve among other things the General Theory of Relativity. But also I think it will involve areas which are now not even considered part of physics. Certain areas of epistemology, certain parts of philosophy and mathematics, and the theory of evolution will also be part of the new unifying theory, of which we do have glimpses but which has not yet been formulated.
Quantum mechanics is very complex. And there are still unresolved areas. Do you think the mystery of it may be resolved, say, within 20 years or so? Or is that too optimistic?
One hears a lot about the ‘mysteries’ of quantum mechanics but I do not think that there are any. Although there are still open areas of research within quantum mechanics I do not think that they are fundamental mysteries provided that one adopts the many-worlds interpretation of quantum physics [see page 22]. There are mysteries in physics, principally the unification of quantum theory with General Relativity. We really have only clues at the moment, and I would be rash to predict that this would be solved in the next 20 years, although this is one of those areas where the solution could come at any time. And then there would be a frantic rush to work out its meaning. Even that frantic rush might take decades. So, I do not know.
How will this theory help to explain man’s existence in the world?
Again, we don’t know yet. We only have some tantalising clues. It seems likely to me that the 400 year old consensus in science that human beings are insignificant in the fundamental scheme of things in the universe has to break down. It is not that we know what the true role of humans is. It is that the arguments that humans don’t have a fundamental role in the scheme of things, which used to seem so selfevidently true, have all fallen away. I mean, it is no longer true that human beings are necessarily destined to have a negligible effect on physical events, because there is the possibility that humans will spread and colonize the galaxy. If they do, they will necessarily have to affect its physical constitution in some ways. It is no longer true that the fundamental quantities of nature – forces, energies, pressures – are independent of anything that humans do, because the creation of knowledge (or ‘adaptation’ or ‘evolution’ and so on) now has to be understood as one of the fundamental processes in nature; that is, they are fundamental in the sense that one needs to understand them in order to understand the universe in a funda-mental way. So, in this and other ways, ‘human’ quantities – human considerations, human affairs and so on – are fundamental after all. But we do not yet understand the details of how they fit in with the more familiar fundamental processes that we know about from physics.
What scientists or philosophers have most influenced your own work?
Let us deal with the philosophers first because that is a shorter list. I think it is principally Karl Popper, and to a lesser extent Jacob Bronowski (through The Ascent of Man) and William Godwin, who is a very underrated 18th century philosopher, with a broader, more integrated and more sophisticated perspective that, say, Locke or Hume. He is underrated because he made serious mistakes too. For instance, he completely misunderstood economics and that led him to advocate a sort of communistic lifestyle. Yet many of his political ideas are actually spot on, and very modern.
As far as the scientists go, one can divide them into two categories, that is scientists who personally influenced me, and those whose work influenced my work. The ones who personally influenced me were Dennis Sciama, the cosmologist and astrophysicist who sadly died last year, and John Wheeler. Both had the very rare attribute of being able to choose and nurture excellent students. Sciama, for example, was the supervisor of Martin Rees, Stephen Hawking and, in all, over a dozen of the foremost physicists and cosmologists in Britain. And the same is true in America with John Wheeler. The third person I should mention is Bryce de Witt, who I worked under when I was in Texas as a student. He was the one who introduced me to Everett’s manyworlds interpretation of quantum mechanics, and to the wider implications of quantum field theory, and it was because of his take on both the formalism and interpretation of quantum mechanics that I got interested in quantum computers.
In the context of the current interest in human consciousness how do you see the relationship between the material explanation of the human being and consciousness? How does consciousness fit into the quantum world?
First of all, I do not believe in the supernatural, so I take it for granted that consciousness has a material explanation. I also do not believe in insoluble problems, therefore I believe that this explanation is accessible in principle to reason, and that one day we will understand consciousness just as we today understand what life is, whereas once this was a deep mystery.
Are you saying that human consciousness can be reduced to neural activities in the brain?
No, no. ‘Reduction’ to an underlying level is just one possible mode of explanation. For instance, although we know that living processes, at the reductionist level are nothing more than physical and chemical processes, we also know that their explanation cannot be made at that underlying level. That is, although the physics of life is not different from the physics of anything else, the explanation of life requires a substantive new theory, namely the theory of evolution. That is the kind of relationship, I think, that consciousness has with physics; the explanation of consciousness again needs a different mode of explanation, except that for consciousness it has not yet been invented, that is the problem. I am completely unsatisfied with modes of explanation such as Daniel Dennett’s which try to say that the problem is already solved. In general I think that it is rare for a situation to exist where a lot of people think there is a problem and in fact it is already solved. In the case of consciousness I think that there are genuine problems, for instance the problem of what are qualia (such as the subjective experience of seeing red). This is clearly unsolved and Dennett’s proposals don’t solve it.
In your book, The Fabric of Reality, you are challenging the single universe conception of reality. In Chapter II, you clearly explain quantum theory which tells us about the behaviour of microscopic particles. You also explain the ‘single particle interference’ experiment and argue that there are intangible shadow particles, and then that there are parallel universes each of which is similar to the tangible one. This is a difficult step for many of us. Could you please clarify how you proceed from intangible particles to many universes (or multiverse as you call it)?
Let’s start with the microscopic world, because it is only at the microscopic level that we have direct evidence of parallel universes. The first stage in the argument is to note that the behaviour of particles in the single slit experiment reveals there are processes going on that we do not see but which we can detect because of their interference effects on things that we do see. The second step is to note that the complexity of this unseen part of the microscopic world is much greater than that which we do see. And the strongest illustration of that is in quantum computation where we can tell that a moderate-sized quantum computer could perform computations of enormous complexity, greater complexity than the entire visible universe with all the atoms that we see, all taking place within a quantum computer consisting of just a few hundred atoms. So there is a lot more in reality than what we can see. What we can see is a tiny part of reality and the rest of it most of the time does not affect us. But in these special experiments some parts of it do affect us, and even those parts are far more complicated than the whole of what we see. The only remaining intermediate step is to see that quantum mechanics, as we already have it, describes these other parts of reality, the parts that we don’t see, just as much as the parts we do see. It also describes the interaction of the two, and when we analyse the structure of the unseen part we see that to a very good approximation, it consists of many copies of the part that we can see. It is not that there is a monolithic ‘other universe’ which is very complicated and has different rules or whatever. The unseen part behaves very like the seen part, except that there are many copies.
It is rather like the discovery of other planets or other galaxies. Having previously known only the Milky Way,we did not just find that there are vast numbers of stars out there, far more than in the Milky Way. There are more galaxies out there than there are stars in the Milky Way. We also found that most of the stars outside the Milky Way are actually arranged in other little Milky Ways themselves. And that is exactly what happens with parallel universes. It is of course only an analogy but quite a good one; just like the stars and galaxies, the unseen parts of reality are arranged in groups that resemble the seen part. Within one of these groups, which we call a parallel universe, the particles all can interact with each other, even though they barely interact with particles in other universes. They interact in much the same way as the ones in our seen universe interact with each other. That is the justification for calling them universes. The justification for calling them parallel is that they hardly interact with each other, like parallel lines that do not cross. That is an approximation, because interference phenomena do make them interact slightly. So, that is the sequence of arguments that leads from the parallelism, which by the way is much less controversial at the microscopic level than the macroscopic level, right up to parallel universes. Philosophically, I would like to add to that that it simply does not make sense to say that there are parallel copies of all particles that participate in microscopic interactions, but that there are not parallel copies of macroscopic ones. It is like saying that someone is going to double the number of pennies in a bank account without doubling the number of Pounds.
But couldn’t this interference phenomenon be due to a yet unknown law of physics within this universe?
Well, there are very sweeping theorems that tell us that no singleuniverse explanation can account for quantum phenomena in the same way that the full quantum theory does. Quantum theory explains all these phenomena to the limits of present day experiment perfectly, and it is, according to some measures anyway, the best corroborated theory in the history of science. And there are no rival theories known except slight variants of quantum theory itself. We know that an alternative explanation could not be made along single-universe lines, unless perhaps it is a completely new kind of theory. So, the answer is ‘no’.
A few years ago, BBC Horizon did a documentary on time travel in which you explained the parallel universes theory and suggested that there was ‘hard evidence’ for it. Well, it is a controversial theory and is accepted only by a minority of physicists, as you yourself acknowledge in your book. Why do you think there is such a strong reaction to this theory in the scientific community? And how do you reply to their criticism?
I must confess that I am at a loss to understand this sociological phenomenon, the phenomenon of the slowness with which the many universes interpretation has been accepted over the years. I am aware of certain processes and events that have contributed to it. For instance Niels Bohr, who was the inventor of the Copenhagen interpretation, had a very profound influence over a generation of physicists and one must remember that physics was a much smaller field in those days. So, the influence of a single person, especially such a powerful personality as Niels Bohr, could make itself felt much more than it would be today. So that is one thing – that Niels Bohr’s influence educated two generations of physicists to make certain philosophical moves of the form “we must not ask such and such a question.” Or, “a particle can be a wave and a wave can be a particle,” became a sort of mantra and if one questioned it one was accused of not understanding the theory fully. Another thing is that quantum theory happened to arise in the heyday of the logical positivists. Many physicists – perplexed by the prevailing interpretations of quantum physics – realised that they could do their day-to-day job without ever addressing that issue, and then along came a philosophy which said that this day-to-day job was, as a matter of logic, all that there is in physics. This is a very dangerous and stultifying approach to science but many physicists took it and it is a very popular view within physics even to this day. Nobody will laugh at you if, in reply to the question “are there really parallel universes or not?”, you answer “that is a meaningless question; all that matters is the shapes of the traces in the bubble chamber, that is all that actually exists.” Whereas philosophers have slowly realised that that is absurd, physicists still adopt it as a way out. It is certainly no more than ten percent, or probably fewer, of physicists talking many universes language. But it is heartening that the ones who do tend to be the ones working in fields where that question is significant, which are quantum cosmology and quantum theory of computation. By no means all, even in those fields, but those are the strongholds of the many-worlds interpretation. Those also tend to be the physicists who have thought most about that issue. But why it has taken so long, why there is such resistance, and why people feel so strongly about this issue, I do not fully understand.
I know that you are also working on a quantum computer. Given the counterintuitive character of the quantum world, it must be a very challenging project.
It is a very hard technical task, and the science is in its infancy. I am not involved in any of the experimental work, except as a spectator. I work only on the theory. I can only say that I am extremely impressed by the power of the experimental techniques that are now available. These people routinely manipulate individual atoms and individual photons, and engineer interactions between them and measure them with extraordinary precision, and they are very optimistic about the possibility of building working quantum computers. At the moment the most powerful quantum computer in the world probably has 3 or 4 qubits. One would probably need several hundred to perform any quantum computation that was useful as such.
How close are you to achieving your objective?
There are many intermediate objectives, but speaking of the objective of a quantum computer that can actually perform useful quantum computations, we are decades away. But there are many intermediate objectives of great theoretical and philosophical interest which will happen before that.
Could you perhaps tell us how a quantum computer can contribute to our understanding of quantum mechanics? And what kind of effect can it have, if any, on our everyday lives?
Those are two questions. For the first one, I think quantum computers will contribute in two separate ways. One is that the theory of quantum computation appears to be a very elegant and powerful way of looking at quantum mechanics in general, and quantum mechanics in general is arguably the deepest theory in physics along with General Relativity. Expressing the theories of physics in the language and notation of quantum computation makes them clearer and gives us a deeper understanding of what they mean. The other way that it helps us understand physics is by helping us to understand the many universes theory. Before quantum computation the prototype experiments which would demonstrate the existence of parallel universes were things like the two-slit experiment where the number of universes involved is small. The interaction between them is very crude. A particle is deflected into another direction, and not much else happens. When you finish the interference has ended. But in quantum computation the complexity of what is happening is very high so that philosophically, it becomes an unavoidable obligation to try to explain it. It is not just a correction to something else; it is the overwhelmingly dominant effect. It is not just crude; the outcome is a complex and subtle function of how the experiment is set up, and of what happens in the hidden parts of the multiverse. One can then take those results and as with any other computation one can put them into a further quantum computation and the second one will work only if the first one produced all the right results in all the universes. It really cries out for explanation rather than simply prediction. This will have philosophical implications in the long run, just in the way that the existence of Newton’s laws profoundly affected the debate on things like determinism. It is not that people actually used Newton’s laws in that debate, but the fact that they existed at all coloured a great deal of philosophical discussions subsequently. That will happen with quantum computers I am sure.
In our everyday lives, that is still an open question, because that rather depends on how feasible it is to build quantum computers and how cheap they will be when we do build them. It also depends on, theoretically, how many useful types of quantum algorithm are invented. The only general-purpose useful algorithm so far is Grover’s algorithm, which is a search algorithm. If quantum computers can be built economically then they will have an impact because of Grover’s algorithm. Search is a component of almost every computer program because searching through a list of possibilities is what you do in every case where there is not a clever mathematical algorithm to get what you want. An obvious example is chess playing; there is no formula for the best chess move given a certain position. All you do is search through all the possibilities of how the given position can continue. And the fastest known algorithms are simply search algorithms. They take one move after another and just search down to whatever depth they can in a given time.
Can an ordinary computer do the same job?
Yes, ordinary computers can perform searches; the best existing chess computers are ordinary computers which do normal searches. But Grover’s algorithm does searching much faster than any classical algorithm could do. It is a feature of classical searching that if you are searching through n possibilities, the time taken is proportional to n – that is, basically it is n times the time taken to look at one possibility. Quantum computing using Grover’s algorithm uses the square root of n steps, so it needs only the time to look at the square root of the total number of possibilities, and it shares the work among the square root of n universes. To put it another way, in the time a classical computer can perform a thousand search steps, a quantum computer can perform a million. In the time the classical one can perform a million, a quantum computer can perform a trillion. You soon get to the region where the classical computer is outclassed even if the quantum computer is slower in terms of the actual steps. I think the existing computers perform hundreds of millions of search steps per second and to play a chess move takes a few seconds. So, a quantum computer doing the same kind of thing would be able to perform some trillions of times more analyses and therefore would completely outclass Deep Blue, the best existing chess machine. But it is not just chess, it is any problem where one has to search through possible solutions: cryptography, design, where you are trying different wing shapes for an aeroplane or whatever. Anywhere where there is not a formula to the answer. Probably most of computer time that is currently devoted to solving problems, is devoted to searching of some kind or other.
In your research do you get support from your colleagues or is there a general scepticism around?
I would say that I am sceptical myself about, for instance, the speed of progress that we can expect in quantum theory and experiments. I am sceptical but optimistic at the same time. As regards the subject of quantum computers, it is generally regarded as an exciting growth area. The Centre for Quantum Computation has been formed at the Clarendon Laboratory in Oxford and it is attracting world class researchers and they seem to get some outstanding research students too. We are making a remarkable progress and the field is regarded by the physics community at large as very promising. Of course, we cannot predict the future growth of knowledge, as Popper would say. So, we do not know that our progress in the future will continue to be as exciting and as rapid as it has been. That is the way it is looking at present.
What is the most frustrating part of your research?
I think perhaps, if I am to pick out some frustrating part, it is that the field has now grown so much and become so complex that I cannot follow it all. There are whole areas, for instance, in the mathematical theory of quantum computers, quantum complexity theory, where I just do not know enough to follow the latest research in detail. So I have to pick and choose. For many years I was in the fortunate position of being in a very, very new field, and everybody knew everybody. Everybody understood everyone else’s research. That is no longer the case. It is just too big and too diverse. We do still have, though, the atmosphere of camaraderie where we all help each other that we had originally. I think what tends to happen when fields get big is that competition and rivalry set in, and people tend to hide their results from each other. So far, that is not happening in our field and it is wonderful.
In view of scientific developments in areas like biochemistry, DNA research, genetic engineering, information technology, are you optimistic about the 21st century?
Or do you see a dark side? Oh yes. I am optimistic about tecnological progress, but there is bound to be a dark side. There are bound to be many horrible unintended consequences of new knowledge. That always happens. I think rationalism, the whole philosophical stance of advocating reason and progress would do much better to glorify problems than theories. It is problems that are inherently wonderful; solutions are merely useful. And the fact that solutions always create new problems is not, on balance, a drawback but their most useful attribute. Science ought to be regarded as a transition from one problem situation to the next. The theory – the means by which we make a transition – is secondary. It is the problem that is primary. In fact, I even sometimes say, only half jokingly, that theories ought to be renamed ‘misconceptions’, and that progress consists of moving from one misconception to a preferable misconception. That is, from a misconception that contains a great deal of falsehood to one that contains less falsehood. Then perhaps we would not be tempted to hubris when we make a great discovery. Also the public would not gain the mistaken impression that science claims to know everything and to solve everything and to insulate the human race against uncertainty or error. That is something science cannot do. But the other side of the coin is that we ought to be embracing new problem situations as good; we have to accept that bad things will happen, but we ought to expect to solve them in turn. Because the only alternative is to stick with the bad things that we have and then we might as well be dead.
In explaining the world, do you think science and philosophy are compatible?
Can they interact? Absolutely. In fact science and philosophy have both gone through a bad period in the 20th century, philosophically speaking. Many blind alleys were explored, many steps for the worse were taken, not in the predictive part of science but in the explanatory part, and in philosophy generally. I think that in the last years of the 20th century people began to realise this and do what is necessary to cure philosophy of these ills. I think it is now basically taken for granted once again that philosophy is about understanding things, questioning things and that logic makes sense and that theories have to be coherent. There are genuine philosophical problems, not just word games; there are such things as solutions even though they are very hard to come by, though perhaps in line with my earlier comment we should really rename the solutions ‘misconceptions’ just so that we understand what they really are. We have a set of misconceptions and we are trying to move to a better set of misconceptions. Scientists ironically do drag their feet, there is still a lot of positivism, a lot of instrumentalism, a lot of not taking philosophy seriously, but things are going in the right direction.
Professor Deutsch, thank you very much. It has been a pleasure talking with you.
• You can find out more about quantum computation at the excellent website of the Centre for Quantum Computation: www.qubit.org
Filiz Peach is working on a PhD on Existentialist perspectives on death. She lives in London.