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David Deutsch: Moving to a better set of misconceptions...‏




This interview appeared in Philosophy Now 30 December 2000

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 world view 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 short listed 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. 

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 self-evidently 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 fundamental 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 under rated18th century philosopher, 
with a broader, more integrated 
and more sophisticated perspective than, 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 life style. 

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 JohnWheeler. 

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 
many-worlds 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 interacts lightly. 

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 single-universe 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 counter-intuitive 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  (Year 2000)  
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 a tall 
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. 
[see the following correction by David Deutsch:
* This was a bad example. Scott Aaronson at UC Berkeley 
has since drawn my attention to some comments by Richard Cleve (quant-ph/9906111) 
pointing out that chess and chess-like games (with a fixed number of choices per move,
especially if this number is small) are not very suitable for speed up by Grover searching. 
At best one would expect a speed up by a moderate, fixed factor. 
This does not rule out quantum chess-playing algorithms altogether, 
just algorithms based on Grover-accelerated brute-force searching. 
But there is no special reason to expect better quantum chess algorithms to exist].

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 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 technological 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: 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

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