David Deutsch

This interview appeared in Philosophy Now 30 December 2000

David Deutsch is a distinguished quantum physicist and a member of the Centre for QuantumComputation at the Clarendon Laboratory, Oxford University. He hasreceived the Paul Dirac Prize and Medal from the Institute of Physics for‘outstanding contributions to theoretical physics’. He recently talkedwith Filiz Peach about his work and hopes.

David Deutsch’s book The Fabric of Reality offers a startling newworldview which combines quantum physics, evolution, epistemology andcomputation. He also deals with quantum computation, a new field ofphysics in which he has been a pioneer. His explanation of the nature ofthe universe in terms of quantum physics is inspiring andthought-provoking. However, his favoured interpretation of quantum theoryin terms of there being many parallel universes (or a ‘multiverse’ as hecalls it) is not widely accepted in the scientific community, or at leastnot yet. But it may well be part of a new unifying theory of the universein the 21st century. The Fabric of Reality is a clearly-written book,intelligible even for those of us who are not scientists. It wasshortlisted for the 1997 Los Angeles Times Book Prize, and the 1998Rhône-Poulenc Prize for Science Books.

Professor Deutsch, could you please tell our readers why you becameinterested in quantum physics?

I am interested in anything that is fundamental. Quantum physics and theGeneral Theory of Relativity are the two most fundamental theories thatphysics has. They are the theories within which other theories areformulated; 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 theoryin curved space-time – a topic that is on the boundary between quantumtheory and the General Theory of Relativity. It was hoped – it still ishoped – that one day these two theories will be unified. Logically, theyare in deep conflict with each other, and this conflict is not within thereach of present day experiments to resolve. We know that a unificationisn’t going to be easy. That unified theory would be called quantumgravity. The reason for studying quantum field theory in curved space-timewas that it was hoped that when we understood that well, it would provide aclue to quantum gravity. We did eventually understand it well, and it didnot provide a clue to quantum gravity. But it did convince me that quantumtheory is at present the deeper of the two, and also, for the moment at anyrate, provides more promising lines of research. Peter Medawar said oncethat science is the art of the soluble. You cannot necessarily solve themost profound problems right away. You have to go for the most profoundsoluble problem. And in that respect, I thought that quantum theory wasthe more promising.

So you now believe that quantum mechanics will provide a unifying theory ofthe universe?

‘Provide’ is not quite the right word. Quantum theory will be a pathway, acomponent of some future more unifying theory which will involve amongother things the General Theory of Relativity. But also I think it willinvolve areas which are now not even considered part of physics. Certainareas of epistemology, certain parts of philosophy and mathematics, and thetheory of evolution will also be part of the new unifying theory, of whichwe 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 notthink that there are any. Although there are still open areas of researchwithin quantum mechanics I do not think that they are fundamental mysteriesprovided that one adopts the many-worlds interpretation of quantum physics. There are mysteries in physics, principally the unificationof quantum theory with General Relativity. We really have only clues atthe moment, and I would be rash to predict that this would be solved in thenext 20 years, although this is one of those areas where the solution couldcome at any time. And then there would be a frantic rush to work out itsmeaning. 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 seemslikely to me that the 400 year old consensus in science that human beingsare insignificant in the fundamental scheme of things in the universe hasto break down. It is not that we know what the true role of humans is. Itis that the arguments that humans don’t have a fundamental role in thescheme of things, which used to seem so self-evidently true, have allfallen away. I mean, it is no longer true that human beings arenecessarily destined to have a negligible effect on physical events,because there is the possibility that humans will spread and colonize thegalaxy. If they do, they will necessarily have to affect its physicalconstitution in some ways. It is no longer true that the fundamentalquantities of nature – forces, energies, pressures – are independent ofanything that humans do, because the creation of knowledge (or ‘adaptation’or ‘evolution’ and so on) now has to be understood as one of thefundamental processes in nature; that is, they are fundamental in the sensethat one needs to understand them in order to understand the universe in afundamental way. So, in this and other ways, ‘human’ quantities – humanconsiderations, human affairs and so on – are fundamental after all. Butwe do not yet understand the details of how they fit in with the morefamiliar 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. Ithink it is principally Karl Popper, and to a lesser extent Jacob Bronowski(through The Ascent of Man) and William Godwin, who is a very underrated18th century philosopher, with a broader, more integrated and moresophisticated perspective that, say, Locke or Hume. He is underratedbecause he made serious mistakes too. For instance, he completelymisunderstood economics and that led him to advocate a sort of communisticlifestyle. Yet many of his political ideas are actually spot on, and verymodern.
As far as the scientists go, one can divide them into two categories, thatis scientists who personally influenced me, and those whose work influencedmy work. The ones who personally influenced me were Dennis Sciama, thecosmologist and astrophysicist who sadly died last year, and John Wheeler.Both had the very rare attribute of being able to choose and nurtureexcellent students. Sciama, for example, was the supervisor of MartinRees, Stephen Hawking and, in all, over a dozen of the foremost physicistsand cosmologists in Britain. And the same is true in America with JohnWheeler. The third person I should mention is Bryce de Witt, who I workedunder when I was in Texas as a student. He was the one who introduced meto Everett’s many-worlds interpretation of quantum mechanics, and to thewider implications of quantum field theory, and it was because of his takeon both the formalism and interpretation of quantum mechanics that I gotinterested in quantum computers.

In the context of the current interest in human consciousness how do yousee the relationship between the material explanation of the human beingand consciousness? How does consciousness fit into the quantum world?

First of all, I do not believe in the supernatural, so I take it forgranted that consciousness has a material explanation. I also do notbelieve in insoluble problems, therefore I believe that this explanation isaccessible in principle to reason, and that one day we will understandconsciousness just as we today understand what life is, whereas once thiswas a deep mystery.

Are you saying that human consciousness can be reduced to neural activitiesin the brain?

No, no. ‘Reduction’ to an underlying level is just one possible mode ofexplanation. For instance, although we know that living processes, at thereductionist level are nothing more than physical and chemical processes,we also know that their explanation cannot be made at that underlyinglevel. That is, although the physics of life is not different from thephysics of anything else, the explanation of life requires a substantivenew theory, namely the theory of evolution. That is the kind ofrelationship, I think, that consciousness has with physics; the explanationof consciousness again needs a different mode of explanation, except thatfor consciousness it has not yet been invented, that is the problem. I amcompletely unsatisfied with modes of explanation such as Daniel Dennett’swhich try to say that the problem is already solved. In general I thinkthat it is rare for a situation to exist where a lot of people think thereis a problem and in fact it is already solved. In the case ofconsciousness I think that there are genuine problems, for instance theproblem of what are qualia (such as the subjective experience of seeingred). This is clearly unsolved and Dennett’s proposals don’t solve it.

In your book, The Fabric of Reality, you are challenging the singleuniverse conception of reality. In Chapter II, you clearly explain quantumtheory which tells us about the behaviour of microscopic particles. Youalso explain the ‘single particle interference’ experiment and argue thatthere are intangible shadow particles, and then that there are paralleluniverses each of which is similar to the tangible one. This is adifficult step for many of us. Could you please clarify how you proceedfrom intangible particles to many universes (or multiverse as you call it)?

Let’s start with the microscopic world, because it is only at themicroscopic level that we have direct evidence of parallel universes. Thefirst stage in the argument is to note that the behaviour of particles inthe single slit experiment reveals there are processes going on that we donot see but which we can detect because of their interference effects onthings that we do see. The second step is to note that the complexity ofthis unseen part of the microscopic world is much greater than that whichwe do see. And the strongest illustration of that is in quantumcomputation where we can tell that a moderate-sized quantum computer couldperform computations of enormous complexity, greater complexity than theentire visible universe with all the atoms that we see, all taking placewithin a quantum computer consisting of just a few hundred atoms. So thereis a lot more in reality than what we can see. What we can see is a tinypart 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 eventhose parts are far more complicated than the whole of what we see. Theonly remaining intermediate step is to see that quantum mechanics, as wealready have it, describes these other parts of reality, the parts that wedon’t see, just as much as the parts we do see. It also describes theinteraction of the two, and when we analyse the structure of the unseenpart we see that to a very good approximation, it consists of many copiesof the part that we can see. It is not that there is a monolithic ‘otheruniverse’ which is very complicated and has different rules or whatever.The unseen part behaves very like the seen part, except that there are manycopies.
It is rather like the discovery of other planets or other galaxies. Havingpreviously known only the Milky Way, we did not just find that there arevast numbers of stars out there, far more than in the Milky Way. There aremore galaxies out there than there are stars in the Milky Way. We alsofound that most of the stars outside the Milky Way are actually arranged inother little Milky Ways themselves. And that is exactly what happens withparallel 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 arrangedin groups that resemble the seen part. Within one of these groups, whichwe call a parallel universe, the particles all can interact with eachother, even though they barely interact with particles in other universes.They interact in much the same way as the ones in our seen universeinteract with each other. That is the justification for calling themuniverses. The justification for calling them parallel is that they hardlyinteract with each other, like parallel lines that do not cross. That isan approximation, because interference phenomena do make them interactslightly. So, that is the sequence of arguments that leads from theparallelism, which by the way is much less controversial at the microscopiclevel than the macroscopic level, right up to parallel universes.Philosophically, I would like to add to that that it simply does not makesense to say that there are parallel copies of all particles thatparticipate in microscopic interactions, but that there are not parallelcopies of macroscopic ones. It is like saying that someone is going todouble the number of pennies in a bank account without doubling the numberof Pounds.

But couldn’t this interference phenomenon be due to a yet unknown law ofphysics within this universe?

Well, there are very sweeping theorems that tell us that no single-universeexplanation can account for quantum phenomena in the same way that the fullquantum theory does. Quantum theory explains all these phenomena to thelimits of present day experiment perfectly, and it is, according to somemeasures anyway, the best corroborated theory in the history of science.And there are no rival theories known except slight variants of quantumtheory itself. We know that an alternative explanation could not be madealong single-universe lines, unless perhaps it is a completely new kind oftheory. So, the answer is ‘no’.

A few years ago, BBC Horizon did a documentary on time travel in which youexplained the parallel universes theory and suggested that there was ‘hardevidence’ for it. Well, it is a controversial theory and is accepted onlyby a minority of physicists, as you yourself acknowledge in your book. Whydo you think there is such a strong reaction to this theory in thescientific community? And how do you reply to their criticism?

I must confess that I am at a loss to understand this sociologicalphenomenon, the phenomenon of the slowness with which the many universesinterpretation has been accepted over the years. I am aware of certainprocesses and events that have contributed to it. For instance Niels Bohr,who was the inventor of the Copenhagen interpretation, had a very profoundinfluence over a generation of physicists and one must remember thatphysics was a much smaller field in those days. So, the influence of asingle person, especially such a powerful personality as Niels Bohr, couldmake 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 makecertain philosophical moves of the form "we must not ask such and such aquestion." 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 notunderstanding the theory fully. Another thing is that quantum theoryhappened to arise in the heyday of the logical positivists. Manyphysicists – perplexed by the prevailing interpretations of quantum physics– realised that they could do their day-to-day job without ever addressingthat issue, and then along came a philosophy which said that thisday-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 manyphysicists took it and it is a very popular view within physics even tothis day. Nobody will laugh at you if, in reply to the question "are therereally parallel universes or not?", you answer "that is a meaninglessquestion; all that matters is the shapes of the traces in the bubblechamber, that is all that actually exists." Whereas philosophers haveslowly realised that that is absurd, physicists still adopt it as a wayout. It is certainly no more than ten percent, or probably fewer, ofphysicists talking many universes language. But it is heartening that theones who do tend to be the ones working in fields where that question issignificant, which are quantum cosmology and quantum theory of computation.By no means all, even in those fields, but those are the strongholds ofthe many-worlds interpretation. Those also tend to be the physicists whohave thought most about that issue. But why it has taken so long, whythere 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 thecounter-intuitive character of the quantum world, it must be a verychallenging project.

It is a very hard technical task, and the science is in its infancy. I amnot involved in any of the experimental work, except as a spectator. Iwork only on the theory. I can only say that I am extremely impressed bythe power of the experimental techniques that are now available. Thesepeople routinely manipulate individual atoms and individual photons, andengineer interactions between them and measure them with extraordinaryprecision, and they are very optimistic about the possibility of buildingworking quantum computers. At the moment the most powerful quantumcomputer in the world probably has 3 or 4 qubits. One would probably needseveral 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 aquantum computer that can actually perform useful quantum computations, weare decades away. But there are many intermediate objectives of greattheoretical and philosophical interest which will happen before that.

Could you perhaps tell us how a quantum computer can contribute to ourunderstanding 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 willcontribute in two separate ways. One is that the theory of quantumcomputation appears to be a very elegant and powerful way of looking atquantum mechanics in general, and quantum mechanics in general is arguablythe deepest theory in physics along with General Relativity. Expressingthe theories of physics in the language and notation of quantum computationmakes 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 tounderstand the many universes theory. Before quantum computation theprototype experiments which would demonstrate the existence of paralleluniverses were things like the two-slit experiment where the number ofuniverses 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 thecomplexity of what is happening is very high so that philosophically, itbecomes an unavoidable obligation to try to explain it. It is not just acorrection to something else; it is the overwhelmingly dominant effect. Itis not just crude; the outcome is a complex and subtle function of how theexperiment is set up, and of what happens in the hidden parts of themultiverse. One can then take those results and as with any othercomputation one can put them into a further quantum computation and thesecond one will work only if the first one produced all the right resultsin all the universes. It really cries out for explanation rather thansimply prediction. This will have philosophical implications in the longrun, just in the way that the existence of Newton’s laws profoundlyaffected 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 atall coloured a great deal of philosophical discussions subsequently. Thatwill happen with quantum computers I am sure.
In our everyday lives, that is still an open question, because that ratherdepends on how feasible it is to build quantum computers and how cheap theywill be when we do build them. It also depends on, theoretically, how manyuseful types of quantum algorithm are invented. The only general-purposeuseful algorithm so far is Grover’s algorithm, which is a search algorithm.If quantum computers can be built economically then they will have animpact because of Grover’s algorithm. Search is a component of almostevery computer program because searching through a list of possibilities iswhat you do in every case where there is not a clever mathematicalalgorithm to get what you want. An obvious example is chess playing [see correction below – DD]; thereis no formula for the best chess move given a certain position. All you dois search through all the possibilities of how the given position cancontinue. And the fastest known algorithms are simply search algorithms.They take one move after another and just search down to whatever depththey can in a given time.

Can an ordinary computer do the same job?

Yes, ordinary computers can perform searches; the best existing chesscomputers are ordinary computers which do normal searches. But Grover’salgorithm does searching much faster than any classical algorithm could do.It is a feature of classical searching that if you are searching through npossibilities, the time taken is proportional to n – that is, basically itis n times the time taken to look at one possibility. Quantum computingusing Grover’s algorithm uses the square root of n steps, so it needs onlythe 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 itanother way, in the time a classical computer can perform a thousand searchsteps, a quantum computer can perform a million. In the time the classicalone can perform a million, a quantum computer can perform a trillion. Yousoon get to the region where the classical computer is outclassed even ifthe quantum computer is slower in terms of the actual steps. I think theexisting computers perform hundreds of millions of search steps per secondand to play a chess move takes a few seconds. So, a quantum computer doingthe same kind of thing would be able to perform some trillions of timesmore analyses and therefore would completely outclass Deep Blue, the bestexisting chess machine. But it is not just chess, it is any problem whereone has to search through possible solutions: cryptography, design, whereyou are trying different wing shapes for an aeroplane or whatever.Anywhere where there is not a formula to the answer. Probably most ofcomputer time that is currently devoted to solving problems, is devoted tosearching of some kind or other.

In your research do you get support from your colleagues or is there ageneral scepticism around?

I would say that I am sceptical myself about, for instance, the speed ofprogress that we can expect in quantum theory and experiments. I amsceptical but optimistic at the same time. As regards the subject ofquantum computers, it is generally regarded as an exciting growth area.The Centre for Quantum Computation has been formed at the ClarendonLaboratory in Oxford and it is attracting world class researchers and theyseem to get some outstanding research students too. We are making aremarkable progress and the field is regarded by the physics community atlarge as very promising. Of course, we cannot predict the future growth ofknowledge, as Popper would say. So, we do not know that our progress inthe 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 thefield has now grown so much and become so complex that I cannot follow itall. There are whole areas, for instance, in the mathematical theory ofquantum computers, quantum complexity theory, where I just do not knowenough to follow the latest research in detail. So I have to pick andchoose. For many years I was in the fortunate position of being in a very,very new field, and everybody knew everybody. Everybody understoodeveryone else’s research. That is no longer the case. It is just too bigand too diverse. We do still have, though, the atmosphere of camaraderiewhere we all help each other that we had originally. I think what tends tohappen when fields get big is that competition and rivalry set in, andpeople tend to hide their results from each other. So far, that is nothappening in our field and it is wonderful.

In view of scientific developments in areas like biochemistry, DNAresearch, genetic engineering, information technology, are you optimisticabout the 21st century? Or do you see a dark side?

Oh yes. I am optimistic about tecnological progress, but there is bound tobe a dark side. There are bound to be many horrible unintendedconsequences of new knowledge. That always happens. I think rationalism,the whole philosophical stance of advocating reason and progress would domuch better to glorify problems than theories. It is problems that areinherently wonderful; solutions are merely useful. And the fact thatsolutions always create new problems is not, on balance, a drawback buttheir most useful attribute. Science ought to be regarded as a transitionfrom one problem situation to the next. The theory – the means by which wemake a transition – is secondary. It is the problem that is primary. Infact, I even sometimes say, only half jokingly, that theories ought to berenamed ‘misconceptions’, and that progress consists of moving from onemisconception to a preferable misconception. That is, from a misconceptionthat contains a great deal of falsehood to one that contains lessfalsehood. Then perhaps we would not be tempted to hubris when we make agreat discovery. Also the public would not gain the mistaken impressionthat science claims to know everything and to solve everything and toinsulate the human race against uncertainty or error. That is somethingscience cannot do. But the other side of the coin is that we ought to beembracing new problem situations as good; we have to accept that bad thingswill happen, but we ought to expect to solve them in turn. Because theonly alternative is to stick with the bad things that we have and then wemight as well be dead.

In explaining the world, do you think science and philosophy arecompatible? Can they interact?

Absolutely. In fact science and philosophy have both gone through a badperiod in the 20th century, philosophically speaking. Many blind alleyswere explored, many steps for the worse were taken, not in the predictivepart 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 realisethis and do what is necessary to cure philosophy of these ills. I think itis now basically taken for granted once again that philosophy is aboutunderstanding things, questioning things and that logic makes sense andthat theories have to be coherent. There are genuine philosophicalproblems, not just word games; there are such things as solutions eventhough they are very hard to come by, though perhaps in line with myearlier comment we should really rename the solutions ‘misconceptions’ justso that we understand what they really are. We have a set ofmisconceptions and we are trying to move to a better set of misconceptions.Scientists ironically do drag their feet, there is still a lot ofpositivism, a lot of instrumentalism, a lot of not taking philosophyseriously, but things are going in the right direction.

Professor Deutsch, thank you very much. It has been a pleasure talkingwith you.

You can find out more about quantum computation at the excellent website of the Centre for Quantum Computation: http://www.qubit.org/

Filiz Peach is working on a PhD on Existentialist perspectives on death.She lives in London.

Copright © 2001 by David Deutsch and Philosophy Now

* This was a bad example. Scott Aaronson at UC Berkeley has since drawn my attention to somecomments by Richard Cleve (quant-ph/9906111) pointing out thatchess and chess-like games (with a fixed number of choices per move,especially if this number is small) are not very suitable for speedup byGrover searching. At best one would expect a speedup by a moderate, fixedfactor. This does not rule out quantum chess-playing algorithms altogether, justalgorithms based on Grover-accelerated brute-force searching. But there isno special reason to expect better quantum chess algorithms to exist.