To boldly go...
by Ian Simmons
[ people - april 02 ]
Paul Davies is visiting professor at Imperial College, London, and adjunct professor at the University of Queensland. His research interests are in the fields of black holes, cosmology and quantum gravity. He has written 25 books. The latest explains the mind-bending physics that makes time travel possible.
Ian Simmons: Your new book is called How to build a time machine. Why you think it is scientifically possible to construct one? What do we know about the universe which suggests time travel is possible?
Paul Davies: Ever since Albert Einstein showed with his theory of relativity that time can be warped by motion or gravitation, the possibility was there for time travel. Travel into the future was clearly predicted already in Einstein's 1905 paper, and has since been verified by experiment, but visiting the past remained an open question for some decades. Eventually, some rather artificial solutions to Einstein's gravitational field equations were discovered that describe travel into the past, but it is only in the last 15 years that a more plausible way to do it was found.
The key to two-way time travel is the wormhole in space, which acts rather like a stargate: if you leap through you come out in another part of space. What was discovered by Kip Thorne and his colleagues at the California Institute of Technology is that you could also come out in the past (or future). So whether or not time travel is feasible hinges on whether wormholes are really possible. At the present state of understanding, it is really a case of there being no known reason why a wormhole time machine is impossible. That means we have to take the idea seriously, with all the consequences for causality and the nature of reality that flow from it.
IS: It may be scientifically possible to build a time machine, but it is clearly not technically feasible to do so today. What technologies would we need to develop to make time travel a reality?
PD: Travel into the future is straightforward; it merely requires the time traveller to move very fast. So far, only microsecond time trips are possible -- far too small for anyone to notice. But significant time warps would occur if a propulsion system were developed to boost a spacecraft to near the speed of light. There are many proposals for this (for example, antimatter drives), but they remain well beyond current technology.
Going back in time is far trickier. It is possible that wormholes exist in outer space, and could be commandered for use as time machines. But that would require cosmic engineering on the scale of a super-civilization. In my book I outline how to build a wormhole time machine in the lab, based on the notion of probing space on an ultra-microscopic scale, where we expect a seething ferment of short-lived wormholes to be created by quantum processes. The key to inflating a wormhole is to inject negative energy, which has an antigravity effect. Using antigravity to inflate space has been the standard expalantion for the Big Bang origin of the universe for 20 years, so the principle is uncontentious. But creating a state of matter with enough negative energy in a controlled manner and injecting it into the wormhole look to be formidible challenges. I suggest a way to trigger the process using lasers, but there may be other ways. My hope is that we will find a much easier method to create the wormhole. This could come about if our understanding of gravity requires modification on a small scale of distance, as suggested by recent theories. It may then be possible to reconfigure space using no more than the next generation of subatomic particle accelerators.
IS: The orthodox explanation for the impossibility of time travel is that it would create so many hideous paradoxes that the universe must be constructed to prevent it actually taking place. How do you see us getting round that?
PD: The paradoxes can be avoided in two ways. First, if all causal loops are self-consistent, visiting the past may be weird, but not paradoxical. The time traveller who goes back and murders his mother before he is born obviously creates an inconsistent history, but a time traveller who saves the life of a little girl who subsequently becomes his mother does not produce a paradoxical time loop. So some actions and histories are permitted within causal loops. What is clearly not allowed is travel into the past combined with unfettered free will.
Another resolution of the paradoxes comes from invoking the many-universes interpretation of quantum mechanics. According to this theory reality consists of a stupendous array of parallel universes, many of them almost identical to ours. In daily life any given observer is restricted to just one universe, which is why we don't notice the others around us. But time travel would enable an observer to slip between very similar parallel worlds and back again. So the time traveller could murder his mother in the past of a parallel universe and return to his own future to find her still alive. In spite of its bizarre nature, the many-universes theory is very popular among theroretical physicists as being the best explanation for quantum uncertainty.
IS: It seems like the most feasible form of time machine will not allow Dr Who-style jaunting to and fro throughout the past and future, but would create a semi-permanent bridge between two periods, say 10 years apart, starting at the time the machine is built and reaching forward only. What use do you think such a machine would be put to?
PD: A wormhole time machine cannot be used to go back to a time before the wormhole is constructed. That would explain why we don't encounter time tourists from the future in 2001. So the type of time machine that science seems to permit would not let us witness important historical events, or observe the dinosaurs. But it would enable someone to jump ahead in time and come back again to tell us the future. It may even act like a genuine crystal ball: you could peer through it and see the future.
IS: How would the existence of a time machine affect the culture which created it?
PD: Obviously life would never be the same again. If you know the future, then all decisions, however trivial, are coloured by your knowledge of what lies in store. In fact, the very notion of decision becomes redundant. Free-will would also cease to be a valid concept. The overwhelming impression we have that "now" is the one true reality, whilst past and future are unreal, would be replaced with the idea of past and future as equally real -- just as Einstein insisted is the case nearly a century ago. However, deducing it from physics is one thing: living your life with that assumption is quite another. We cannot even begin to guess what life would be life with past and future intermingled, even slightly.
IS: Time-travel has been a fiction staple for over 100 years. Which stories have come close to getting a feel for what time travel might actually be like, should one a time machine be built?
PD: My favourite time travel story is Timescape by Gregory Benford. However, the story is restricted to signalling back in time, not travelling. From the point of view of the paradoxes, it amounts to the same thing.
IS: If we could travel in time the way people do in fiction, when and where would you go?
PD: I would choose two periods. The first, about 40,000 years ago, coincides with the sudden emergence of culture -- language, religion, art. The second is the transition from hunter-gatherer tribes to the first settled communities, which occurred in the Near East about 10,000 years ago, at the end of the last Ice Age. These changes in human culture were absolutely pivotal, yet we know very little about them.
IS: In Britain, and increasingly elsewhere, our ability to introduce radical new technologies is being curtailed by public fear of the consequences, not always justified. If this kind of pressure mounts, it is likely that something like a time machine would never be built, even if it were practical to do so. Of the current crop of scares (phone masts, GM food and so on), are there any that you - as a scientist - feel are justified? are there less well-known problems that you feel would be a better focus of public concern?
PD: There seems to be very little relationship between public fear and actual risk. For example, there was near-hysteria about the tiny probability that Mir would crash onto houses, but almost total apathy about the much greater risk of destruction due to asteroid or comet impacts with the Earth. Likewise, people fret about rare diseases like CJD, anthrax or meningitis, whilst remaining blasé about mega-killers like malaria. More distinction needs to be made between the need for prudent precaution and outright banning. I think fears of cloning are overdone, but more attention needs to be given to the risks of releasing genetically-modified organisms into the environment.
IS: What does the scientific community need to do to address people's concerns so that problems can be resolved sensibly and a degree of trust established between the various parties?
PD: Clear comparisons need to be given, so that single standards apply. For example, in assessing the risk of, say, flying, it needs to be pointed out that it is far safer than driving the same distance. If we are told that the expectation of death from the accidental realease of nuclear waste from an underground storage facility is x per year, whereas the risk of premature death from a dam burst, or global warming, or breathing toxic waste from coal-fired power stations, is y, then people can make a rational choice about the method of power generation. I believe, incidentally, that x is much less than y. But few members of the public would have any idea of that. The "better-the-devil-you-know" principle is a very deep-rooted part of human psychology, though it is far from rational, and should not be the basis of strategic decision-making.
IS: A century ago, we thought we had a pretty good picture of how the universe worked, with just a few loose ends to tie up in relation to the way light acted. Clearing up those loose ends opened up the massive vistas of 20th century science and changed physics completely. Do you see any corners of physics which have the potential to revolutionise our 21st century view of the universe in a similar way?
PD: Indeed I do. The most obvious is the difficulty of merging quantum physics with gravitation. There is much theoretical activity in this field at the moment. As a matter of fact, time machines are relevant to that, because they cast light on what such a theory might be like. Conversely, we will only be sure about the possibility or otherwise of time travel when we have such a theory at our disposal.
IS: After the time machine, I guess the next most longed for piece of sci-fi tech must be the transporter beam. What are the odds on that being feasible?
PD: It is already practicable to clone and transport a single quantum particle like a photon, so we undersand the science well enough to see what we are up against with a transporter. But doing it for a macroscopic body looks about as hard as building a wormhole time machine. Both remain firmly in the realm of Star Trek, but tantalising because they do seem to be possible in principle!
