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Conversation : UNIVERSE

TIME LOOPS

A Talk With Paul Davies [11.1.00]

"As 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 yes, these entities, these wormholes, or some other type of gravitational system do at least in principle exist, and in principle one could visit the past, and we have to find some way of avoiding the paradox. Maybe the way is to give up free will. Maybe that's an illusion. Maybe we can't go back and change the past freely."

Inroduction

The theoretical physicist Paul Davies works in the fields of cosmology, gravitation, and quantum field theory, with particular emphasis on black holes and the origin of the universe. A prolific and influential popularizer of physics, he has written more than a dozen books. 

In recent years he has pursued an antireductionist agenda, making the case for moving both physics and biology onto "the synthetic path," recognizing the importance of the organizational and qualitative features of complex systems. He advocates a meeting of the minds between physicists and biologists, noting that complicated systems, whether biological or cosmological, are more than just the accretion of their parts but operate with their own internal laws and logic.

– JB

PAUL DAVIES is an internationally acclaimed physicist, writer and broadcaster, now based in South Australia. Professor Davies is the author of some twenty books, including Other Worlds, God and the New Physics, The Edge of Infinity, The Mind of God, The Cosmic Blueprint, Are We Alone? About Time and The Fifth Miracle: The Search for the Origin of Life.

He is the recipient of a Glaxo Science Writers' Fellowship, an Advance Australia Award and a Eureka prize for his contributions to Australian science, and in 1995 he won the prestigious Templeton Prize for his work on the deeper meaning of science. The Mind of God won the 1992 Eureka book prize and was also shortlisted for the Rhone-Poulenc Science Book Prize, as was About Time in 1996. Davies has just been awarded the Kelvin Medal by the UK Institute of Physics for his success in bringing science to the wider public.

Paul Davies Edge Bio Page

REALITY CLUB: Joseph Traub, Julian Barbour, Lee Smolin, Gregory Benford

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!

Reality Club Discussion

Lee Smolin
Physicist, Perimeter Institute; Author, Time Reborn

I have great respect and affection for Paul Davies, and often find that I agree with his take on things. But in this case I find myself in disagreement. As it happens Paul and I were just at a conference in the Vatican where we discussed these questions, so this is a continuation of a friendly discussion. Paul finds himself undecided on the possibility of time travel, my view is that the evidence we have from both classical general relativity and quantum theories of gravity is that time travel is not possible in any realistic theory of space and time. The evidence for this conclusion comes independently from classical general relativity, statistical physics and quantum theories of gravity. I would like to briefly describe some of this evidence and then raise the question of why, given its evident unlikelihood, there is so much interest in the possibility of time travel.

Classical general relativity contains a theory of a dynamical system which describes how the geometry of space evolves deterministically given the state of the universe at one time. The topology of space cannot change under this deterministic evolution. A wormhole represents a change in the topology of space, and so one cannot be created in the evolution described by Einstein's equations. If there are wormholes they have been there for all time (which is very unlikely given that the universe itself has evolved from a much different earlier state) or they have been created by quantum processes.

It is true that there are closed timelike loops in certain classical solutions of Einstein's equations, and these cannot then be described in the language of a system evolving in time deterministically from initial data. But these are very special solutions and there is good evidence that the existence of closed timelike loops is an artificial consequence of the imposition of certain symmetries. Among these symmetries is one which requires that these universes never change in time. The real universe has no such symmetry. Further, the evidence is that any small deviation from the exact symmetries turn the solutions with closed time like loops into solutions without them, but with singularities. Even if such an exactly symmetric solution were to exist in nature the symmetry, and hence the existence of a closed timelike loop, would be destroyed by any attempt to send any matter or information around the loop.

One might be inspired to thinking about time travel by contemplating the arguments for the elimination of time as a fundamental concept raised by Julian Barbour. However, as I think Julian has indicated in his response to Paul's piece, that view of quantum gravity does not seem to have a place for time travel as it is rooted in a three dimensional configuration space that does not allow topology to change. Neither is there any support for time travel coming from other approaches to quantum cosmology and gravity that rely on the notion of a spacetime history. Some of these approaches (such as those that come from loop quantum gravity, which are generically called spin foam models) provide descriptions of the world on the Planck scale that are consistent with everything we know and make no artificial assumptions such as symmetry. So far as I know, time travel is impossible in all these models.

There are several reasons for the failure of these detailed studies of classical and quantum theories of gravity to find a realistic possibility for time travel. Some of these were discussed in a previous Edge piece with Stuart Kauffman ("A Possible Solution For The Problem Of Time In Quantum Cosmology"), others of which are discussed in a paper I will shortly post on the archive gr-qc, to be found at xxx.lanl.gov. I believe that a fair summary of the evidence is that the possibility of time travel rests on an incorrect interpretation of general relativity. According to this interpretation, which is sometimes called the "block universe", time is really no different from space and the whole universe in some sense exists "at once", so that a history is just a path in an already existing world. From this point of view, why can't a timelike path make a loop as easily as a spacelike loop?

The answer, I think, is that this view is derived from the study of vastly over-simplified models, rather than the real theory. When one gets to a detailed, realistic description, rather than a model, the complexity of the world (and here I am thinking of the spacetime geometry and not life....) makes it impossible for any reasonable sized part of the universe to ever return to the same state. The reason is that the list of things one would have to know to make even a small part of the universe return to a previous state are vastly (in Dan Dennet's sense) larger than could ever be could be coded into any present state of any subset of the universe. Another way to say this is that the interruption in the web of causal processes that at any moment are in progress, which would be caused by the sudden appearance of a person, or even a camera, from the future, is so vast that it is impossible to imagine engineeering the insertion of the bit of the future into the present. What would be required is vastly more complex than transplanting a brain into a new body.

My own view is that when we are done making the quantum theory of gravity the block universe idea will be as dead as Ptolemy and we will have a view of time in which the future has a very different status than the past and the notion of time travel will be logically impossible. This is described in the papers I mentioned and in a forthcoming book. But one does not need to agree with my view to come to the conclusion that time travel is very unlikely; there is sufficient evidence for this already in what we know about general relativity and quantum theory, so long as one considers their application to the real universe, rather than vastly oversimplified models.

Now, having said this, why is time travel such a popular subject, among both experts and laypeople? One possibility is that it represents another in a genre of ideas that may be called the technological transcendent fantasies. Among these are the idea of achieving immortality by "uploading our brains into computer software", which was recently discussed by Jaron Lanier in his "halfesto". These ideas have three things in common: 1) they take over the logical form of a religious promise of transcendence (as has been pointed out by Margaret Wertheim in her recent book), 2) they promise an escape from mortality and from the general situation of being an animal making a living in a biological world. 3) The more they are investigated in detail the less plausible they look.

I am not sure of the proper response to these technological transcendent fantasies, given that arguing from the evidence produced by research so far seems insufficient. One might ask, as did a certain Italian scientist, "Before you upload yourself into software (or step into your time machine) will you give me the phone number of your girlfriend?" But sitting in the Vatican discussing these things I was struck that transcendent fantasies that promise escape from our time bound mortal existences may be just the thing to fuel the work necessary to establish powerful institutions that perpetuate both the fantasies and the people who teach them. Perhaps in a thousand years priests from the Church of Time Travel and the Church of AI will meet in the Vatican with their Jesuit brothers to discuss the present status of their still unfulfilled hopes. Meanwhile the rest of us will still be transcending time and perpetuating and improving the human race the old fashion way.

Gregory Benford
Emeritus Professor of Physics and Astronomy, UC-Irvine; Novelist, Shipstar

I enjoyed Paul Davies' remarks on time — a problem foregrounded in physics since Einstein's General Relativity, which apparently allows truly paradoxical events. His kind reference to my novel Timescape, published 20 years ago (so even its future in Cambridge, UK lies now in the past, as did my 1963 California) highlights the interplay between literary imagination and hard science in the last century. The deepening interest in other dimensions that began in the late 19th century foreshadowed the higher dimensionality of 20th C. physics.

Application of quantum mechanical ideas to the time problem led me to suggest in the concluding dramas of Timescape that time loops did not connect to the same universes, so that quite truly one could not go home again. I was aware of the quantum loop problem, and encouraged when my scheme came to be used by David Deutsch in his quantum-logical formulation. His adroit calculations showed that these ideas are compatible with quantum mechanics as we know it, but as far as I know have not engendered any further experiment or tests.

So: the difficulty is how to assign scientific value to such ideas. How can they be falsified? This problem occurs also in the meta-universe models for perpetual inflation of Linde and others. Descriptions that are perhaps more aesthetically satisfying but lead to no new experimental tests cannot advance the argument very far. I would be pleased if Paul Davies could suggest a direction we might pursue to make such ideas more satisfyingly concrete. Measurement should lead.

Joseph Traub
Professor of Computer Science, Columbia University; coauthor, Complexity and Information

Two recent Edge postings, "Time Loops: a Talk with Paul Davies" and Phil Anderson's response to Jaron Lanier's "One-Half a Manifesto" are rich in stimulating ideas. However, when they touch on quantum computation, they both make statements that I question.

I'll start with Paul. He talks about a vast chemical decision tree and asks "How did nature find such a weird state (life) amid the the oceans of junk? The answer could be quantum computation. Quantum computation would enable one to search enormous databases with extraordinary efficiency"

What is known about searching large databases on a quantum computer? Grover(1996) discovered a quantum algorithm for finding a single item in an unsorted database with N items in time proportional to the square root of N. The same problem takes time N to solve on a classical computer. For example, if there are 10 to the 100 items in the database (thats 1 followed by 100 zeros) it would take time 10 to the 100 on a classical computer and time about 1 to the 50 (thats 1 followed by 50 zeros) on a quantum computer. I would not call that "searching enormous databases with extraordinary efficiency".

Speaking technically, Grover's algorithm show quantum search is only polynomially faster than classical search. Part of my research is solving continuous problems on a quantum computer and here there are problems which can be solved exponentially faster on a quantum computer.

Grover (1997) shows that one can find an item in one step on a quantum computer, provided that the query is sufficiently complicated, but i don't think that this theoretical result would help Paul, since we wouldn't know how to frame the query.

I'll turn to Phil's remarks. He says "Why does the quantum computer do new things? Why is complexity theory such a poor quide to the real world of problems?"

I infer from the juxtaposition of these two sentences that Phil believes that computational complexity theory is incorrect for quantum computation. I surmise that Phil is thinking of complexity theory for a classical computer. But complexity theory always depends on the model of computation which states what operations are permitted and how much they cost. Think of it as the rules of the game. The model of computation for a quantum computer is ofcourse different than for a classical computer. Thus Phil's two sentences form a non sequitor.

Julian Barbour
Theoretical physicist; Author, The End of Time

In his comments on my book The End of Time and my suggestion that time does not exist, I feel that Paul Davies does not really confront the points I am making, but instead demolishes a straw man, justifying thereby the conclusion: "To say that it [time] doesn't exist at all is nonsensical." I welcome the invitation to respond. 

Paul says that my position 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." If Paul has read my book (he does not say explicitly that he has), I have to feel disappointed, since I obviously failed to make myself clear. I do not think the superstring analogy is correct as a characterization of my position. Paul must have misunderstood me if he came away with such an impression. I am not arguing for the representation of time by something more fundamental but still real (as the analogy with superstrings would suggest) but for its total disappearance. My analogy would be with Ptolemaic epicycles, which were not supplanted by deeper-lying and more elegant 'supercycles' but simply swept away.

As Copernicus and Galileo showed, the senses proclaim that the earth rests and the epicycles run, but inferences must be drawn with care. To quote Copernicus, one "should not attribute to the heavens what is in the observer". All the revolutions in physics, starting with terrestrial mobility, have shown that well-supported first inferences can be wrong. Stand back from the hurly-burly of the jet-setting world. Time could go the way of the epicycles.

The universe exhibits extraordinary order (very low entropy), one consequence of which is our sense of time. All statistical arguments based on current dynamics indicate that our state is exceptional. In the standard account, it makes possible, among much else, the formation of natural records like fossils, our memories and, above all, brain function. I argue that this special state could be misleading us about time as much as the epicycles misled Ptolemy. However, I am not arguing that mental flux is the sole reason we believe in time.

There are three sources of information about time: 1) mental flux; 2) records (both natural and man-made) redolent of a lawful process in time; 3) the structure of our best dynamical theories: general relativity and quantum mechanics. What does general relativity, which has a far more subtle treatment of time than quantum mechanics, tell us? Here we must consider its orgin and subsequent elaboration.

It is usually said that general relativity was created almost in its entirety by Einstein. This is not quite true. What he did do practically single handedly was realize that his highly original heuristic ideas could be implemented through already known mathematics. The mathematics itself he took over more or less ready made. He almost 'bought it off the shelf'. Somewhat surprisingly (as the late Chandrasekhar used to emphasize), Einstein made little attempt to understand the full implications of his theory. Its deep dynamical structure only emerged after his death in the work of Dirac and Arnowitt, Deser and Misner and in a little known paper by Baierlein, Sharp and Wheeler (1962). This last is the nub of my case: It shows that general relativity is as timeless as Newtonian dynamics would be if you considered only the paths bodies follow (for example, the planetary ellipses in the solar system) and not their speeds as well. History in the Einsteinian universe is not a path traversed in time at some speed but simply a path. This is a complete elimination of time.

This may seem to conflict with statements like Paul's "We know that time is real at one level because it can be manipulated - stretched and shrunk". However, in strict fact we never see time, only the readings of clocks. There is no contradiction between the elasticity of clock readings and history as a timeless path. The clocks are part of the landscape through which the path of history passes. They are milestones.

At the practical level Paul is right. So was Ptolemy. Virtually all the movements we make are predicated on the assumption that the earth rests (pilots, astronauts, and gunners are among the few people who have to worry about the earth's rotation). The change I envision is not at the practical (classical) level at which Paul dismisses my proposal. It is not even in the notion of history as a path, which still belongs to the domain of classical physics, though it is a step. The big one comes at the quantum level.

The case is as yet far from conclusive but cannot at all be dismissed out of hand: the universe may not only be static and timeless but even without a path that one might call history. There are just Nows, individual instants like locations in a landscape. Quantum mechanics simply makes some more likely to be experienced than others. This falls out as a distinct possibility from one perfectly respectable approach to the unification of quantum mechanics with the inner timeless structure of general relativity.

In my book, I address the consequences of timelessness and make suggestions how our experience of time, motion and records can be explained. I suggest how the first two sources of temporal evidence listed above arise from timelessness. Since that is more speculative, I will not go into it here. But timelessness in a far more radical sense than Paul sketches is a real possibility.

Incidentally, since Paul was mainly commenting on the possibility of time travel (at the practical classical level), I may mention that the work of Dirac and Arnowitt, Deser, and Misner might kill it. For the dynamical Hamiltonian form in which they recast general relativity is more restrictive than its original spacetime formulation and does not allow the more bizarre solutions of Einstein's theory with closed time loops and the seemingly paradoxical possibility of time travel. Most relativists and particle physicists are committed to the spacetime picture and tend to dismiss the Hamiltonian approach as not being fundamental. But if physics in its entirety is considered the Hamiltonian approach is just as worthy a candidate for being fundamental as the spacetime approach. Indeed, it could well be that the main reason why we do not yet have a theory of quantum gravity is because theoretical physicists have not yet been able to "call" the contest between these two great principles. At the present milestone in the path of history (November 13th 2000), it is rather like the presidential election.

Responding to a different comment in the same Edge number, could Philip Anderson please give details of the book in which "it is shown that the Alexandrians had reached a level of scientific sophistication by 150 BC which was close to that of 17th century England; for instance, that much of Newton's Principia borrowed ideas from Greek texts"? I find it a somewhat surprising claim even though it is probably true that Newtonian science would have been impossible without Greek antecedents.

Last, on my recent trip to the United States I made a linguistic discovery that might make a small contribution to eliminating trans-Atlantic misunderstandings. It concerns the use of 'quite'. In British English, the 'quite' in "He is quite wrong", enhances 'wrong' (as in American English). But in "Her talk was quite good," the 'quite' implies "somewhat, to some extent" (Concise Oxford Dictionary). According to Webster's (which I take to reflect American usage), the weakest implication of 'quite' is "to a considerable extent". I finally started to suspect some mismatch after repeated emails from an American colleague in which 'quite' was used about some ideas I had put to him in a manner that left me quite disappointed (sometimes in the British sense, sometimes even in the American sense). Shortly before calling on him in Maryland, it occurred to me what might be the cause, and I questioned a passing jogger. "Oh he certainly means 'a lot'" was the reassuring response. Our meeting went quite well (American usage). 

Links:
[1] https://www.edge.org/conversation/paul_davies-time-loops