Time travel is not just the stuff of science fiction, argues physicist Paul Davies, but to make it real we need a 'theory of everything'

In 1905, just ten years after the appearance of H. G. Wells's The Time Machine , Albert Einstein published the first part of his theory of relativity in which he predicted that rapid motion warps time. For an object moving at near the speed of light, he argued, time would get seriously out of step relative to Earth time. Einstein's prediction has been tested using atomic clocks flown in aircraft and spacecraft. Although these vehicles move much slower than light, the associated time warp - measured in nanoseconds - is readily detectable.

By slowing time, high-speed motion permits travel into the future. You could reach Earth year 2020 in six months by zooming in a space ship to a nearby star and back at 99.97 per cent of the speed of light (just under 300,000km per second). If you left your twin sister behind, you would find on your return to Earth that she was nearly 18 years older than you.

Einstein went on to predict that gravity also warps time: it runs a bit faster at the top of a building than at the bottom, for example. This too has been verified with experiments. Really big time-warping effects require enormous gravitational fields. On the surface of a neutron star - an imploded stellar remnant - time is slowed by about 30 per cent. Take up residence there and events on Earth would whiz by like a fast-forward video.

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Travel into the future is therefore possible in theory, but going back in time is far more contentious. On its own, Einstein's theory of relativity does not forbid visiting the past. Indeed, in 1948, the mathematician Kurt Godel found a solution to Einstein's gravitational field equations that described travel into the past. But Godel's solution was dismissed as a mathematical curiosity because it assumed that the entire universe is rotating, which it is not.

Several other scenarios for going back in time were investigated over the following decades - all unrealistic. Then, in the late 1980s, a more plausible idea for a time machine was proposed, based on the concept of the "wormhole". A wormhole is a hypothetical tunnel linking two distant points in space to form a shortcut. It resembles a black hole in being a region of intense gravity, but, whereas a black hole offers a one-way journey to nowhere for any astronaut foolish enough to fall in, a wormhole has an exit as well as an entrance. You could jump through one and come out in a different part of the universe.

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Wormholes were studied in the late 1980s by a group of physicists led by Kip Thorne at the California Institute of Technology, largely for amusement. The scientists soon discovered that, if a wormhole existed, it could be adapted to make a time machine by holding the entrance still and whirling the exit around at high speed. An astronaut jumping through the wormhole in one direction would come out in the future. Going the other way, he would leap into the past. By traversing the wormhole and speeding home across ordinary space, he could get back before he left.

Although there is not a shred of astronomical evidence for the existence of wormholes, nobody has proved they are impossible. It would not be easy to make one: the intense gravity of the wormhole would quickly turn it into a black hole unless it were shored up by a form of anti-gravity. One way to do this would be to produce a field with negative energy and, hence, negative mass. Though such a physical state is exotic, it is known to be possible. For instance, laser beams can create tiny regions of negative energy that are gravitationally repulsive. If enough negative energy could be concentrated in the throat of the wormhole, it could in principle remain open to permit transit by an object.

Ignoring the daunting practicalities of making a wormhole time machine, it is fascinating to speculate on what would happen if we possessed such a gateway to the past. It seems that all sorts of disturbing paradoxes would be unleashed. For instance, what would happen if you went back in time and murdered your mother as a child? Then you would never have been born. How then could you carry out the murder? Or suppose you go forward in time to witness a friend's accidental death and then return and take action to prevent the accident?

Fiction writers are familiar with this type of temporal conundrum. Most of them fudge the issue. But physicists have given it a great deal of thought. It is, after all, the purpose of science to provide a coherent and rational account of reality. If physical theories lead to genuine paradox, they must be abandoned. Do time-travel paradoxes imply that Einstein's theory of relativity is wrong?

Not necessarily. There is nothing inherently paradoxical about causal loops, so long as they are self-consistent. For example, there is no problem about going back in time to rescue from drowning a girl who subsequently becomes your mother. Paradox looms only when the time traveller is permitted unfettered free will and deliberately creates causal confusion by trying to change history.

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One way to have your cake and eat it - time travel with free will - is to invoke the many-universes theory. The idea here is that the physical world we observe is just one among a vast assemblage of alternative realities. The other universes cannot be reached by journeying through space; they exist in parallel to our space and time. In daily life, these other universes remain almost completely disconnected from ours, but time travel would create a significant overlap, allowing a doughty temponaut to slip through the interstices of spacetime and emerge in a parallel reality. Murdering mother in a parallel world would not affect the future of the world from which the time traveller came. He could return to find his mother still alive and well.

Despite its bizarre overtones, the parallel-universes theory is accepted by many distinguished physicists as the most plausible interpretation of quantum mechanics - the physical laws that apply in the atomic realm. But others believe that far from rescuing time travel from paradox, quantum effects would serve to stymie the very attempt to create a portal to the past.

The issue hinges on the central tenet of quantum physics, called Heisenberg's uncertainty principle. This states, roughly speaking, that all physical quantities are intrinsically a bit uncertain and can undergo rapid spontaneous fluctuations. For example, energy can suddenly appear from nowhere in empty space so long as it fades away again quickly. By briefly "borrowing" energy out of the blue, a subatomic particle can, for example, leap out of a trap - a process that underlies the phenomenon of alpha radioactivity.

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The Heisenberg principle is a rule for payback on the energy loan: the shorter the loan, the more the energy on offer. Quantum energy uncertainty is utterly insignificant in everyday human affairs, where time intervals are measured in seconds or more. Only in the frenetic domain of atoms and molecules are the fluctuations important. But this restriction would vanish near a time machine, because it would be possible to borrow the energy and then go back in time and repay it instantly. Heisenberg's uncertainty principle would set no limit on the amount of energy that could surge out of nowhere. On the face of it, it seems that runaway quantum energy would wreck the wormhole's innards and block off the past.

However, there is no consensus over whether quantum effects, or some other physical processes, will always intervene to prevent travel backwards in time. Physicists are still struggling to understand the interplay of gravity and quantum physics, and a complete theory remains elusive. Without it, nobody knows whether quantum fluctuations would be amplified by a wormhole to the point where its gravitational field is seriously distorted and its intriguing temporal properties compromised.

Though it came out of science fiction, the study of time travel has a serious side to it too. By challenging the foundations of physical theory, it is helping frame the much sought-after "theory of everything", in which gravitation would be united with the other forces of nature in an all-embracing mathematical scheme.

An answer would be effortlessly forthcoming if time travellers from the future were to visit us and show us the theory of everything, or lend us a time machine. Then does not their absence prove that time travel will never happen? Not so. It turns out that a wormhole cannot be used to go back to a period before the time machine is created. So the absence of time tourists from the future is no surprise. Those anxious to know whether comprehensive time travel is truly possible will just have to wait and see.

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Paul Davies is adjunct professor of physics at the University of Queensland and a visiting professor at Imperial College, London. His book How to Build a Time Machine is published this week by Allen Lane: Penguin Press, ?9.99.