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 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 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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