# TIME LOOPS[1]

TIME LOOPS

[PAUL DAVIES:] 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 — 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 calledThe 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!