A Talk with Paul Davies
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
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 outmanoeuvre 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 —
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
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
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
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
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
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
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!