The prospect of time travel has, for long, caught the imagination of many. Stephen Cauchi attempts to find out how realistic it really is.
Stephen Hawking, the famously disabled British physicist whose book, A Brief History of Time, brought cosmology to the masses, turned 60 last year. Scientists marked the occasion by gathering at Cambridge University to discuss the vexing issue of time travel.
"Most physicists view time travel as being problematic, if not downright repugnant," says New Zealand relativity expert Professor Matt Visser, citing the well-known paradox of a time traveller killing his grandmother in the cradle. But he concedes there is no mathematical proof that rules out a time traveller changing the past. "Is chronology protected? Despite a decade's work, we don't know for certain."
It is easy to write off time travel as Dr Who-style science fiction. But at least one detailed model for a time machine has forced physicists such as Hawking to try and find some "chronology protection conjecture" that would ban time travel into the past. As the laws of physics permit cause-and-effect mix ups such as the grandmother paradox, there is something about our understanding of the universe that is lacking.
Furthermore, time travel into the future, in small amounts, is already commonplace. Einstein's special and general theories of relativity, written in 1905 and 1916 respectively, showed that superfast speeds or strong gravitational fields could warp time like a bowling ball on a rubber sheet. The higher the speed, or the stronger the gravity, the greater the warp (known technically as dilation).
Orbiting satellites, for example, travel at about five kilometres per second. This is enough to cause their clocks to run fractionally slower than those on Earth, and over time the difference builds up. "These clocks undergo time dilation because they're moving through space and they're at a different point in the gravitational field," says Dr Hugh Luckock, a senior lecturer in mathematics at Sydney University. "When they're designing these satellite global positioning systems, they have to allow for time dilation or they'd get totally the wrong results."
Time travel, as portrayed in the movies, is an extreme version of this. In The Time Machine, the hero takes only minutes to travel years into the future. He sees clocks in the outside world move super-quickly. But if outside observers were able to watch the clock on his control panel, they would see it move super-slowly. Eventually, the clocks are decades apart.
"We know that going into the future as far as you like is allowed by the theory of relativity," says Paul Davies, popular science author and professor of natural philosophy at Macquarie University. "It's something that depends on money and not on physics."
For that reason, theoretical physicists are focused, instead, on unravelling the mysteries of travelling backwards in time, not forwards. While allowed by Einstein's theories, it is extremely contentious. Travelling forwards in time merely requires that one travel very fast. Travelling backwards in time involves exotic and dubious feats of engineering.
Davies outlined the most popular backwards-in-time model, a man-made variant of a black hole called a wormhole, in his 2001 book How to Build a Time Machine. Black holes are formed from large stars that have burnt out and then imploded. Making a time machine out of them requires building a pair of linked black holes, known as a wormhole. The wormhole would be, literally, a portal into the past.
American physicist Kip Thorne in the mid-1980s first examined how this might be done, and Davies' book outlines how 21st-century technology might build one.
Firstly, a minuscule wormhole would be created in a particle accelerator, a synchrotron-like structure such as that at CERN in Switzerland or Brookhaven National Laboratory in New York. Secondly, the wormhole would be inflated and maintained using as-yet-undeveloped "exotic matter" such as antigravity. Thirdly, one mouth of the wormhole would be whirled around in the particle accelerator at close to the speed of light for a decade to establish a time dilation difference between the two wormhole mouths. Bring the two mouths back together, and - presto! - one back-in-time machine.
This model has certain limitations: the earliest one could travel back to would be to when the wormhole, and its "exit" mouth, was built. This rules out travel to, say, World War II, and explains why tourists from the future haven't arrived. However, such a machine faces spectacular engineering and philosophical problems. "My money would be on the fact that it probably couldn't be built, but I couldn't say that for sure and I don't think anyone else could say it for sure either," says Luckock.
"No-one can say the physics says it's not possible," says Dr Leo Brewin, a senior lecturer in mathematics at Monash University. "What is problematic is the sense of scale . . . the energy required to construct wormholes is enormous. It's hard to see how one could go about making them."
Professor Ray Volkas, a researcher of theoretical particles at Melbourne University, said the engineering challenges of building wormholes are "extremely considerable". "Einstein's general relativity can allow this wormhole-type situation (so) it's worth thinking about . . . it's still an open question as to whether this sort of gizmo is possible."
Great though the engineering obstacles are, they are dwarfed by the philosophical objections, such as the grandmother paradox.
Even if only signals can be sent back in time, similar problems result. Suppose the time machine is hooked up to an explosive device placed next to it, which can destroy the time machine if the right radio code is sent. The code is sent through the machine at 3am, aimed at 2am - one hour before. If the device explodes, and the machine is destroyed at 2am, how could the code have been sent at 3am? The results are nonsensical. Yet relativity theory, which has remained foolproof for nearly 80 years, allows for back-in-time travel. No proof has been devised which rules it out, although scientists in experimental fields such as superstrings or quantum gravity may find one.
"Stephen Hawking has a thing called the Chronology Protection Conjecture, which basically says such things shall not happen because we don't know how to make sense of them," says Brewin.
"You take the physics and cut it back and say even though the equations and the mathematics say those things are allowed," as humans we reject those sorts of solutions as nonsensical.
The most popular way round the paradoxes is the "many universes" interpretation. This holds that the universe - as suggested in the Gwyneth Paltrow film Sliding Doors - is consistently splitting off into a myriad of alternative universes. Travelling back in time and killing your grandmother poses no paradox under this scenario.
At the exact moment you arrive back in time, the universe splits off into many universes: the one in which we know your grandmother lived, and others in which she may or may not have.
The time traveller, by going back in time, is condemned to enter one of these parallel universes and is forever unable to meddle with, or return to, his "home" universe.
Not everyone buys the many-universes theory, including Brewin. "It seems to be too complicated that at every stage you and I make a decision, the universe cleaves off into something different to keep everything consistent . . . I feel uncomfortable with that.
"What I prefer is the alternative, which is that in any time travel, whatever changes you make will not have a consequence back in the future." In other words, time travel into the past may be permitted, as long as a paradox is not created.
No matter how you look at it, travelling back in time is inherently unsatisfactory. But until a Chronology Protection Conjecture arrives that makes the world safe for historians, it is a field that theoretical physicists will continue to pursue. "It's not serious because there's a bunch of us who really think we're going to build a time machine," Davies says.
"It's serious because we feel it's important to know what is the causal structure of spacetime."
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