DAVIES: I happen to be
reading Michael Crichton's latest book,
Timeline, one of a
succession of books and movies that have come out over the last
few years exploring the idea of time travel — it's not a new
idea, it goes back a hundred years to H. G. Wells, probably even
before that. The basic idea of a time machine, already captured
in Wells's original story, is that it's possible to travel in
time in much the same way that you can travel in space.
It's easy to imagine building such a
machine, throwing a lever and propelling yourself into the
future or back into the past. Wouldn't that be fun! Wells
already recognized the paradoxes that would occur if it's
possible to travel backwards in time, although he didn't address
them especially well. Traveling forward in time doesn't involve
any sort of
paradox, however, so long as the time traveller
can't go back again to his original time.
Remarkably, Wells's story was written about ten years before the
publication of Einstein's special theory of relativity was
published. Special relativity showed that time is elastic,
flexible. It isn't simply there — the same for everybody, as
Newton supposed. There's your time and my time, and they can
differ depending on how we move.
If I jump in a rocket ship and head
off at nearly the speed of light to a nearby star and come back
again ten earth years later, I may have aged only, say, one
year. This is called the twins effect, because if I left my twin
brother at home, when I returned we would no longer be the same
age. He would be ten years older, and I only one year older. In
effect, I will have time-travelled nine years into his future.
Bizarre though this time-stretching effect seems, we know it's
true. In fact, you can even measure it using the motion of
aircraft.
If I fly to London from New York, for example, then I will lose
a few billionths of a second relative to you, staying here on
the ground. That's a measurable effect, using atomic clocks. It
has been tested. So we know that time travel is possible, but
I'm talking here about travel into the future. It's easy; it's
been done. You just have to move fast enough to get a
significant effect. Since in daily life our speeds are much less
than that of light, we don't notice anything weird going on with
time. But the effect is definitely real.
Travel into the past is much more problematic, though. The
significant thing is, our best understanding of the nature of
time, which comes from Einstein's general theory of relativity,
leaves open the possibility of travel into the past. It doesn't
say you can't do it, there's no known law within the theory of
relativity to forbid it. But finding a plausible scenario to
actually travel into the past is not an easy thing.
The first person to come up with a proposal was Kurt Gödel, the
Austrian-born logician and mathematician, who worked at
Princeton's Institute for Advanced Study alongside Einstein in
the 1940s. Gödel discovered that if the universe were rotating
it would then be possible for an object to travel in a certain
closed loop in space and come back to its starting point before
it left! In other words a person could travel around a loop in
space — and discover that it is also a loop in time. It has to
be said that Gödel's scenario is highly unrealistic; there is
good evidence that the universe as a whole is not rotating, but
the very fact that the general theory of relativity does not
forbid travel into the past is deeply unsettling. It certainly
unsettled Einstein.
The main reason concerns the causal
paradoxes it unleashes. For example, imagine visiting the past
by going on a journey through space and returning yesterday, and
then, assuming you still had freewill, doing something yesterday
that would prevent you from leaving in the first place (for
example, blowing up the time machine).
If you never left, then you wouldn't
have travelled back in time to make the change. But if you
didn't make the change, nothing would prevent you from embarking
on the journey. Either way, you get contradictory nonsense.
Because science is rational, it must always yield a consistent
picture of reality, so these sort of causal paradoxes strike at
the very heart of the scientific understanding of nature.
Time travel paradoxes are very familiar to authors of science
fiction. The question is, what are we physicists to make of
them? Do they imply that time travel is simply not on, or that
reality is subtler than we suppose? This is where opinions start
to differ. Some physicists, most notably David Deutsch, think
the way out of this is to assume that there are multiple
realities, so that when you travel back into the past, the world
you change is not the same one that you left, but a parallel
imitation.
This topic is often cast in the parable of the grandmother
paradox: you go back 50 years and kill your grandmother,
ensuring that you were never born in the first place. One way
around it is that if you go to a parallel world, and kill your
parallel grandmother, you can return to your own time to find
Granny still alive and well. That's a possible resolution. There
isn't any consensus on it. Perhaps the existence of parallel
realities is a worse prospect than that of causal loop
paradoxes.
Some people feel that the problems of travel into the past are
so great that there must be something in nature to prevent it
actually happening. For a while Stephen Hawking flirted with
this position, and formulated what he termed the chronology
protection hypothesis. It implied that although the laws of
physics would seem to allow travel backwards in time, in every
practical case something would intervene to prevent it
happening. Nature would always outmaneuver attempts to change
the past. But we don't know, this is still an open question.
Today, most of the research in this field is being done finding
more plausible ways to travel backwards in time. Gödel's idea of
the rotating universe is just one scenario; there are others.
The most popular is the wormhole in space, which is a little bit
like a black hole but different. Wormholes were made famous by
Jodie Foster, who fell into one in the film "Contact." This
movie was based on
Carl Sagan's
book of the same name. In the
movie what happens is that this wormhole is manufactured
according to a prescription sent to earth by alien beings in a
radio message.
Jodie Foster gets dropped into what
looks like a gigantic kitchen mixer, and 18 minutes later
emerges at a different part of the galaxy. The wormhole in
effect connects two distant points in space so as to form a
shortcut. It's a little bit like drilling a hole from New York
to Sydney. If you wanted to go see the Olympics the quick way
would be to plunge through the hole, rather than fly the long
way around the earth's surface.
Einstein's theory of relativity
tells us that space is curved by gravity, so imagine that it was
warped in such a way that it connected earth with the center of
the galaxy through a tube or a tunnel that might only be a few
kilometers long — who knows?
The point is that if a wormhole is possible, it can be adapted
for use as a time machine, as shown by Kip Thorne at Caltech,
and his colleagues, and now the subject of an international
cottage industry in research papers. To travel in time, what you
do is this. You first plunge through the wormhole and exit at
the remote end, then you zoom back home again through ordinary
space at nearly the speed of light. If the circumstances are
right, you can get back before you leave.
Wormholes are a marginal and very speculative idea, but from
what we understand of the nature of gravity when combined with
quantum physics, it looks like yes, in principle, such an entity
would be possible. As a practical matter, however, I have to say
that it would be a very expensive proposition. To make one,
probably you would need to capture something like a
black hole,
and then adapt its interior to create a wormhole. We're talking
about cosmic-scale engineering here; I don't think any of my
professional colleagues regard this as terribly credible. But
that's not the issue. The point is that if it is in principle
possible for a wormhole to exist, if it could either be
engineered or delivered to us ready-made by Mother Nature, then
it opens up the possibility of paradoxical time loops.
By providing an insight into the nature of reality, and the
nature of the physical universe, this whole area is really
fascinating. I've thought a lot about it over the years, and I'm
still undecided as to whether nature could never permit such a
crazy thing, or whether wormholes, or some other type of
gravitational system, might be possible so that in principle one
could visit the past. If so, we must find some way of avoiding
the paradoxes, maybe by giving up freewill. In daily life we
imagine that we are free to do most of what we want, but if you
find yourself in a causal loop, you might discover that you just
can't do anything that is going to change the world in a manner
that is inconsistent with the future you've come from.
There's a famous story, I think originating with Richard
Feynman, about the time traveler who goes back in time and, in
an adaptation of the grandmother-killing scenario, decides to
shoot his younger self to see what would happen. He takes a
rifle with him, seeks out his younger self and raises the rifle
to shoot through the heart. But his aim isn't very good, it's a
little bit wobbly, so he hits his younger self in the shoulder
instead, merely wounding him. The reason his aim isn't so good
is because he's got this shoulder wound from an earlier shooting
incident! So you see, it's possible to conceive of temporal
loops of that sort without encountering a paradox.
If you look at the way science fiction writers deal with this —
well, most of them just fudge the whole issue. Then some of them
have the time traveler go back in time, and change the past
stepping on a beetle perhaps, or shooting Adolf Hitler — and
then when they return to their own time, they find everything
has changed. Well that's simply inconsistent if there is only
one world, one reality. That's no way out at all. It may make a
good story but it doesn't make sense. So this is a subject that
goes right to the heart of physics, and right to the heart of
the nature of reality. I think it's a terrific topic.
EDGE: I am aware that the work of physicists influence science
fiction writers, but is it a two-way street?
DAVIES: Oh yes, there's no doubt about that. For a start, a lot
of young people get into doing science through reading science
fiction. I remember a postdoc colleague of mine who reckoned he
got into physics from reading "Superman" comics. 'I owe a great
debt of gratitude to that guy,' he once remarked. If I think of
my own scientific development, I read a lot of H.G. Wells in my
teens — War of the Worlds, The Time Machine, plus a number of
his books on social and political issues — so they certainly had
an influence on me.
I also read most of John Wyndham's
books— this was in the 50s and early 60s. It's a bit hard to say
whether the science fiction turned me on to the science, or
whether I was already interested in the science and naturally
gravitated to science fiction. I was never a great fan of Isaac
Asimov, but a lot of my scientist friends have been. I prefer
Arthur C. Clarke.
These writers are definitely
inspirational. If you think back to the 60s — for most people
that was an era of rebellion, drugs, Vietnam War protests and so
on. But for me the influences of the 60s were less John Lennon,
more Arthur C. Clarke. Stanley Kubrick's movie 2001 A Space
Odyssey came out in the late 60s when I was a PhD student in
London, and I found it wonderfully confident and inspiring, a
great antidote to the pessimistic dropout culture of the times.
EDGE: How has your own work influenced science fiction writers?
DAVIES: Several times a year I get sent science fiction
manuscripts based upon my work. I just had one last week in
fact, which was actually a time travel story by an Australian
science fiction writer. He wanted to get the physics right. The
best-known science fiction writers who have drawn my work are
Gregory Benford and Margaret Atwood.
Benford came to see me in the early
70's to discuss time travel, and in his Nebula-winning book
Timescape he features me as a character! It's the first time I
appeared in somebody's novel. Atwood's book Cat's Eye has some
element of physics, which she thanks me for. More recently, I
have been helping a film director with a movie about a scientist
who is the target of an obsessional admirer.
Although it is a two-way street, I would probably say that
professional scientists are more influenced by science fiction
than the other way around. You see, a fiction writer can create
a purely imaginary world. It's in the nature of fiction that you
don't have to stick to the rules. People use the term science
fiction as though it refers to a uniform genre, but it's
doesn't. It shades from what we might call hard sci-fi — the
sort of stuff that Michael Crichton might write, which is my
preference — right off into fantasy, fairy stories with
scientific overtones.
Terry Pratchett, who writes humorous
fairy stories with a science basis to them, is a classic example
of the latter. I'm afraid I don't like that sort of stuff
terribly much personally, though Pratchett's Discworld novels
are hugely successful. Anyway, the point is that there's no
obligation for him to stick to the usual laws of nature. In
fact, there's even a book called The Science of Disc World which
invents an imaginary science for Discworld — well and good.
While most science fiction writers
have some understanding of basic science, they aren't studying
very carefully what is going on at the forefront of science.
They may pick up some ideas, but they're mostly not going to
study the detailed technicalities of the science itself. Very
few of them try and get it completely right. Michael Crichton
and Arthur C. Clarke are exceptions.
But I guess the old adage applies:
why let the facts stand in the way of a good story.
EDGE: Let's get back to the science. How and when would time
travel ever manifest itself?
DAVIES: Well I've already mentioned that travel into the future
is a reality — but of course it's trivial — the sort of leaps
into the future you get from traveling in a jet aircraft amounts
to a few billionths of a second, so that's not going to excite
anybody. And the only place where you see very significant
temporal distortions is in particle physics, where the particles
are moving very close to the speed of light. But to most people
they're not very interesting objects, these subatomic particles.
A human being is never going to
travel, in the foreseeable future, at an appreciable fraction of
the speed of light. So we're not talking about an effect that's
of any practical value, or even any curiosity value, it's just
too small for us to notice. But if you could achieve speeds
close to the speed of light, or find another way to travel into
the future, then I guess that would be of great interest because
it would then be possible to make space journeys over many light
years in a human lifetime.
It would be wrong to suppose that if
you wanted to travel to a star a hundred light years away that
the journey's going to take you a hundred years — in your frame
of reference. If you're traveling close to the speed of light,
it might take just ten years. In terms of wanting to get there
within your lifetime, this is a significant effect. But again,
we're talking about something that is so far beyond current
technology; it's pretty fanciful.
When it comes to traveling backwards in time, well, you might
think that if it is achieved at some stage in the future, we're
going to see time travelers coming back to visit us now. This is
an argument that is often used against time travel. Where are
they? Where are these tempanauts? Shouldn't they be popping up
all over New York saying, 'Yeah, time travel is possible, we
invented the time machine in the year 3000, and we're coming
back to tell you about it.'
Now there is a let-out for this
argument in the case of the wormhole time machine. According to
the physics of the wormhole, you can't use it to travel back to
a time before the construction of the wormhole itself. If we
managed to build a wormhole time machine this year, we could put
it in a warehouse and wait ten years and travel back to 2000,
but we couldn't go and see the dinosaurs or anything of that
sort. The only way we could do that is if some aliens made a
wormhole millions of years ago and lent it to us.
So maybe the reason we don't see
time travelers from the future is simply because the only type
of time machine that you can make is one that can't be used
before the manufacture date on the machine. Then we're not going
to see these time tourists. It's anybody's guess as to when such
a machine might be built. But if the wormhole is the only way to
do it, then we're talking about cosmic-scale engineering,
something on the outer fringes of the possible.
If we take a Freeman Dyson view of the future of the universe,
of mankind, or maybe robotic descendants, or some engineered
descendant of human beings, spreading out through the solar
system and eventually through the galaxy, harnessing natural
energy on galactic dimensions, we'd be talking hundreds of
millions of years of development here.
At that stage our descendants might
be capable of manipulating entire stars or black holes, and
creating something like a wormhole, but it's not the sort of
thing that's going to be done in a hundred years or even a
thousand years — unless there's another way of doing it. This is
of course always the excitement in a scientific topic: have we
overlooked something? And given that we know time is elastic,
that time can be manipulated, some way of traveling into the
past seems to be possible.
So is there a much easier method that
we've overlooked?
The great hope for building a time
machine in the foreseeable future is that that is the case, that
something involving maybe weird aspects of quantum physics is
going to do it for us, some other type of physical process that
we haven't yet discovered — but it's going to have to have
gravitation in there somewhere.
EDGE: Maybe it's just that little red pill.
DAVIES: Sorry, but no. Here is where H. G. Wells got it wrong.
His time traveler sat in this machine and then pressed a few
buttons or something and effectively threw the great cosmic
movie into reverse. Everything ran backwards. Then when he got
to where he wanted to go he hit the stop button, just like the
fast rewind on a video player.
But the time travel that I'm talking
about is not like that.
It's not a method of somehow reversing
the arrow of time. It is going off on a journey through space,
in a closed loop, and arriving back at your starting point
before you leave. There is no reversal of the arrow of time, no
putting the great cosmic movie into reverse. Everything around
you continues in a forward direction, so in your local
neighborhood the arrow of time is unchanged. Eggs still break
and don't reassemble themselves.
It's not that you're going backwards
in time, it's that you visit the past. There's a distinction
between going backwards in time, in the sense of reversing
through time, and going to the past, which is what I'm talking
about.
EDGE: How does all this fit in with the views expressed by
Julian Barbour in his book
The End of Time?
DAVIES: Barbour argues that time doesn't really exist, to
express his work somewhat simplistically. Clearly time exists at
the practical level — at the level of gravitation and
engineering and everyday Newtonian mechanics. To say there's no
time is rather like saying there's no matter, on the basis that
ultimately matter is made up of vibrating superstrings or
something, You might be tempted to say about matter, well, it's
not really there at all.
The truth is, matter manifests
itself in our everyday quasi-classical quasi-microscopic world,
and space and time manifest themselves in that world too. I
concede that space and time may not be the ultimate reality. It
could well be that space and time — and we really have to link
them together — are ultimately derived concepts or derived
properties of the world. It could be that ultimate reality is
something more abstract, some sort of pre-space-time, component
out of which space-time is built.
Just like matter, time may be a
secondary or derived concept. But nevertheless, at a
sufficiently large level of size, there is the familiar
space-time we know. You can't wish it away, or define it away
through mathematics — it's something that you can try to
explain. Wood, for instance, is not a primary substance, it's
made up of something else, which in turn is made up of something
else, and so on. But that doesn't mean that wood is unreal. It's
still there. The same goes for time.
We know that time is real at one
level because it can be manipulated stretched and shrunk by
the processes I have been discussing.
Your question is very pertinent though, because before the
theory of relativity, it was fashionable in some quarters, and
maybe it still is, to try to make out that time is somehow
merely a human construct, deriving from our sense of the flux or
flow of events, that it's something to do with the way we
perceive the world as a temporal sequence. I'm not denying that
we perceive time as flux, but time is not solely a human
invention or a human category.
For the physicist, time and space,
along with matter, form part of the equipment that the universe
comes with. Or rather, it's what the universe is made of.
To say that it doesn't exist at all
is nonsensical.
EDGE: You mention aliens. Who are the aliens?
DAVIES: We don't know. We could be totally alone in the
universe; at this particular time it's impossible to say. But we
can speculate that there might be life, even intelligent life,
elsewhere.
EDGE: Could they be our ancestors? Or our God?
DAVIES: Descendants maybe, not ancestors. Well, I guess if it's
possible to travel through time as well as through space, we can
imagine the universe being populated by a single species far
into the future and also backwards into the past, so they could
also be our ancestors too. It wouldn't be necessary to have life
popping up independently in many different places. That would be
a curious twist on the time-travel story. We would go backwards
in time and seed other planets with life at an earlier epoch.
Yes, that's always conceivable.
EDGE: Could it be that the universe is a computational device?
DAVIES: It's interesting to look back through history on this
one. Each age has its pinnacle of technology, and each age uses
that technology as a metaphor for nature, for the universe. In
ancient Greece, the technological marvels were musical
instruments and the ruler and compass. The Greek philosophers
tried to build an entire cosmology from number, harmony,
proportion, form, and so on from mathematics, basically.
Remember the music of the spheres?
The Pythagoreans believed that
nature was a manifestation of rational mathematics. Later on the
pinnacle of technology was the clockwork. Newton wanted a
clockwork universe, the entire universe as a gigantic clockwork
mechanism, with all the parts interlocking and ticking over with
infinite precision. Then in the 19th century along came steam
power, and the universe was then depicted as an enormous heat
engine, or thermodynamic machine, running down toward its heat
death.
Today the computer is the pinnacle
of technology, so it's now fashionable to talk about nature as a
computational process. All of these ways of describing the world
capture to a certain extent the way it is, but I would say that
the universe is a universe, not merely a clockwork or a computer
or whatever.
EDGE: Isn't your heart a pump? Isn't your brain a computer?
Don't you clear your RAM by taking a long run, or getting some
sleep?
DAVIES: The helpful way of thinking about the universe is in
terms of information processing. Just think of the solar system,
of the planets are going around the sun; if we write down the
positions and motions of all the planets today then that can be
considered as some input information for an algorithmic process.
We can let the solar system run and
then measure those quantities again next week; that's the output
information. You could say that the solar system has mapped the
input into the output, which is a computational process. You
could look at the whole of nature like that. What impresses me
is that if you look at the subatomic level, or the quantum
level, what you find is that the information processing power of
nature goes up exponentially.
The information can attach to the
amplitude of the wave function, rather than the probability. It
is much greater because it involves interference effects and
phase information. If you can maintain quantum coherence, the
amount of information you can process is staggeringly bigger
than with classical material objects.
The computers that we have on our desks are classical computers,
they compute using ordinary on-off type switches. The quantum
computer can be in superpositions of on and off states, so if
you have a whole collection of switches then the number of
possible combinations goes up exponentially. If you can keep
quantum conference, you can compute with enormous power. Now why
has nature got that? Why do we live in a universe that has the
capability of processing such a huge amount of information at
the subatomic level?
Of course, that's not a scientific
question, it's a philosophical question. But I've a sneaking
feeling I know the answer, which is that it plays a crucial role
in the origin of life, and possibly in the nature of
consciousness too. I'm less sure about the consciousness.
Life is a clear example of where nature is a computational
process, because the living cell is not some sort of magic
matter, but an information replicating and processing system of
enormous power. If you consider the structure and operation of
the living cell, it is a very particular and peculiar state of
matter, a very odd combination of molecules, which you wouldn't
expect to create if you just shuffle them around at random. How
did nature discover life? How did matter go from a disorganized
jumble of molecules into something so special and so specific as
a living organism?
You can regard this question as a
type of search problem, requiring a search algorithm. Imagine a
network of possible chemical reactions in some primordial
pre-biotic soup. It constitutes a vast decision tree; every time
a chemical reaction occurs there's a new branch on that decision
tree. Over time one is dealing with an almost infinitely complex
tree, with some tiny little twiglets on the tree representing
this very special and peculiar thing we call life. The rest is
chemical junk.
How does nature find such a weird state amid the oceans of junk?
The answer could be quantum computation.
Quantum computation
would enable one to search enormous databases with extraordinary
efficiency. So if nature somehow harnessed the power of
information processing at the subatomic level, it could be that
this is how life began: a quantum search of the chemical
decision tree, with life being 'the winner.' To be sure, that's
a rather speculative hypothesis.
But I come back to this question,
why does nature need all that computational power? Why can't we
live in a universe that just processes information in the
classical way? Maybe the answer is because we couldn't live in
such a universe, because life itself depends on precisely that
enormous computational power. But that's a quasi-religious
statement, that's not a scientific statement.
To finish where we started, I was amused to see that in Timeline
Michael Crichton makes use of the ideas of quantum computation
as a way to travel backward in time. Basically, quantum spacetime foam provides a labyrinth of tiny wormholes through
which (at least in the story) the time traveller's atoms can be
squeezed one by one. This could be a much better method of time
travel than harnessing a single giant wormhole.
So maybe there
is link between life, quantum information processing and time
travel?
That would be something!
