Time travel is more than mere fantasy. Some scientists believe it is theoretically possible. Physicist Paul Davies reports
In a modest museum beside Waterloo Bridge in London stands an old-fashioned police box. For a generation of TV viewers, this is no ordinary phone booth. It is Dr Who's Tardis. The BBC's Dr Who series, which featured an intrepid Time Lord gallivanting about the universe, brought the idea of time travel to millions of homes.
But it was science-fiction writer H. G. Wells who blazed the trail with The Time Machine, published in 1895. Hollywood made a screen version in 1960 and followed up with slick productions of yarns such as Back to the Future. The latest time-struck writer is Michael Crichton, whose blockbuster Timeline hits cinema screens early next year.
Travelling through time makes great fiction, but could it be done?
Wells's classic was written a decade before Albert Einstein published his theory of relativity, which is still the party line among scientists on matters temporal. Before Einstein, the official view was that time is the same for everybody everywhere: "flowing equably without relation to anything external", was the way Isaac Newton put it.
Einstein demolished this commonsense myth with his bizarre notion of flexitime. According to the theory of relativity, your time and my time may not be the same. They can creep out of sync if, for example, we move differently. Fly from London to Rome and back, and you return slightly displaced, temporally speaking, from your stay-at-home neighbours.
The effect is known as time dilation and it is not just a theory. In the 1970s, American physicists flew atomic clocks around the world to test the prediction and confirmed that the flying clocks lost several billionths of a second, relative to their grounded counterparts. This is not the sort of discrepancy to cause missed appointments, but the size of the time warp precisely confirmed Einstein's 1905 formula, which says that as the speed rises, so does the stretch. Approach the speed of light - a dizzy 300,000km per second - and time slows to a crawl.
Translated into human experience, time warps lead to some weird scenarios. Alice and Bob are twins. In 2001, Alice zooms off in a rocket ship to a nearby star at 99.9 per cent of the speed of light, while Bob stays at home. When Alice returns to Earth in the year 2011, she looks remarkably young compared with Bob. In fact, she has only aged 23 weeks. Alice and Bob are no longer twins, but now differ in age by more than nine and a half years. A form of time travel has taken place: in effect, Alice has leapt nine and a half years into Bob's future.
Our generation of rockets can reach a paltry 0.01 per cent of the speed of light, so nobody notices the differential ageing of astronauts. But subatomic particles whirled around in giant accelerator machines routinely travel close enough to the velocity of light to experience warp factors of 100 or more. Measurements confirm that these speeding particles decay more slowly than those at rest.
Although travel into the future is a fact - jet-setters do it all the time in nanosecond amounts - travel into the past is more problematic. Alice cannot get back to 2001 by reversing her trajectory; high speed can only propel her further into the future. But motion is just one way of warping time; another is gravity. In 1915, Einstein generalised his theory of relativity to include gravity and predicted that it slows time. Again, experiments proved him right. Physicists have sent atomic clocks into space and confirmed that time runs a little bit faster up there than down here. In fact, scientists can measure time so accurately these days that it is possible to detect the gravitational time warp between the bottom and top of a building.
In the case of Earth's gravity, the effects are too tiny for humans to get excited about, but there are some objects in the universe whose gravity is so enormous that time gets seriously distorted. At the surface of a neutron star, for example, clocks might tick at half the rate they would on Earth.
At the surface of a black hole, conditions are more extreme, for time effectively stands still, relative to Earth time. Fall into one, and you enter a region of space and time that lies beyond eternity. That is why entering a black hole is a one-way journey to nowhere. You can never escape from the interior, for to do so you would have to come out before you fell in, which is absurd. But herein lies the key to visiting the past. If a black hole can be modified to provide an exit, then travel backwards in time might be achievable.
A hole in space with two ends is called a wormhole, an idea originally mooted 40 years ago by the American physicist John Archibald Wheeler. He suggested that in some circumstances two points in space could be connected in two distinct ways: across ordinary space, and through a narrow tunnel. The situation is analogous to drilling a hole through the Earth from London to Sydney to create a shortcut, only with a wormhole the tunnel is not made through anything; it is made of space itself.
Wormholes remain conjecture, but the idea was propelled to prominence by the film Contact, based on the novel by the late Carl Sagan. In the story, the heroine is dropped into a sort of giant kitchen mixer in Japan and emerges a few minutes later near the centre of the galaxy, having traversed a wormhole of unspecified construction. When Sagan wrote the story, he asked his friend Kip Thorne, a theoretical physicist at the California Institute of Technology, to check whether his fictional wormhole idea could work. Thorne discovered that a wormhole would not just provide a convenient means of travelling through space, it could also be used to travel through time - forwards and backwards.
To visit the past, the traveller first plunges into one mouth of the hole, traverses the "throat", and emerges from the remote end. He must then zoom back home the conventional way - across "normal" space - at close to the speed of light. If this itinerary is followed, the traveller will arrive home before he left. So by completing a closed loop in space that threads through the wormhole, the traveller also executes a loop in time.
Even if wormholes could exist, they still may not provide the gateway to the past. As Dr Who fans are aware, travelling through time is fraught with paradoxes. What happens to the time-traveller who seeks out his mother as a young girl and murders her? In that case, the traveller would never have been born, so could not go back to commit the crime.
Such paradoxes suggest there is something deeply unphysical about journeying back in time. For this reason, many physicists reject the idea out of hand. Stephen Hawking, for example, once proposed a "chronology protection hypothesis" that would forbid anyone from visiting the past and changing it.
Others have adopted a more sanguine attitude. The Oxford physicist David Deutsch has proposed a way to resolve the paradoxes at a stroke by appealing to the fashionable notion of parallel universes. Suppose, conjectures Deutsch, that there is not one reality, but a multiplicity of similar worlds. Each parallel world has its own space, time, matter and human beings. When a temponaut visits the past and murders his mother, he changes one of the parallel worlds, but not his own. So when he goes "back to the future" he finds his mother alive and well in his own world, even though she is dead in the parallel one. Logically, Deutsch's speculation is sound, but it hinges on the existence of a vast assemblage of alternative universes. Not all physicists are convinced such universes exist.
Travel backwards in time remains a possibility, although it may not be easy. Making a wormhole would involve an engineering project of literally astronomical proportions. Unusually, the cost is one thing that would not present an obstacle. Knowing the state of the stock market in his own era, the first time-traveller could pop back 100 years or so and make some sure-fire investments, then return to the future and collect the profits to pay for the trip.
Paul Davies is visiting professor at Imperial College, London, and author of About Time, published by Penguin. He will lecture on "Time travel: fact or fiction" at The Royal Society in September.