Time waits for all mankind
(Filed: 31/01/2004)
Travel to the future and the past has long been a staple of science fiction - and this year sees more Hollywood films and books on the theme. Science Editor Roger Highfield asks could it happen in reality?
Who wouldn't want to travel in time? It is the most romantic of all science-fictional dreams. A time machine would be jolly handy for visiting a long-lost lover, correcting a mortifying mistake or reliving a triumph over and over again. You could put historians out of business. You could jump back to the future and return with sufficient insider know-how to change history, save a life, even rule the world.
Crackpots, mystics and movie scriptwriters adore the infinite paradoxical possibilities. The BBC is to explore them once again, by dusting off the Tardis and regenerating its iconic character Dr Who next year. This year, Hollywood is embracing the theme of time travel. Forthcoming attractions include Primer, about two small-time inventors whose experimental time machine has a disastrous impact on their lives, and The Butterfly Effect, in which the current teen favourite Ashton Kutcher has the mysterious ability to travel back in time to alter aspects of his childhood with unexpected and creepy side-effects. Meanwhile, Audrey Niffenegger's haunting first novel, The Time Traveller's Wife, has become an international bestseller and now has been optioned by New Line and Plan B, the production company of Brad Pitt and Jennifer Aniston.
Niffenegger has rejected traditional time machines and instead plumped for Chrono-Displacement Disorder, a genetic illness of the future. Henry DeTamble, the hero of her tale, is the first to be diagnosed and the possible harbinger of a new species of human. He tends to revisit the most stressful pivotal points of his life, such as the decapitation of his mother as she drives her white 1962 Ford Fairlane. Although the six-year-old Henry time-travels out of danger, he returns endlessly to the grim scene.
Mostly, our hero returns to the side of his sweetheart, Clare Abshire, an "art babe". Pff! Their enduring passion, from Clare's first encounter with Henry as a six-year-old to her last as an octogenarian - long after his death - is at the heart of this book. Sometimes, there are two Henrys. Often, there is none at all. This is most likely what attracted Aniston and Pitt to the celluloid possibilities. But what of any real-life possibilities?
We all have a "subjective" clock in our heads that, with tinkering, could change our perception of time, and a circadian timepiece that harnesses our body rhythms to the rising and setting sun. But the possibilities raised by altering various "clock genes" (described, along with vanishing GM time-travelling rodents, by the novel's resident boffin, Dr David Kendrick) fall far short of the temporal hopes that Niffenegger describes.
But who cares? This is fiction. She uses the disorder as a bold, interesting (sometimes illogical) conceit for encouraging the reader to think about the intimacy of time, how ineffable it is and how it shapes us.
Although scientists cannot imagine how genes could ever make time travel possible, they are all happy to confirm that a "weak" form is very much a reality. "Travel into the future is not only possible, we've done it," declares Paul Davies of the Australian Centre for Astrobiology and the author of How to Build a Time Machine.
Experiments with atomic clocks on aircraft and spacecraft have demonstrated what scientists call time dilation, a consequence of the special theory of relativity, unveiled by Einstein in 1905. If you move almost as fast as light, or if you are exposed to strong gravity near a neutron star or a black hole, time is stretched relative to you and me.
The classic demonstration of this effect involves twins. Imagine you had a twin sister who set off in a spaceship that travelled at near-light speed. Now, if she came back to Earth after what seemed to you to be 20 years, she would think that far less time had passed. You, however, will look, feel and be much older. "This is travel into the future for one twin," said Davies.
Given the speeds typically attained by even the fastest people on Earth, the effects are so modest that they hardly make for a Dr Who-style caper. Richard Gott, a Princeton theorist and the author of Time Travel in Einstein's Universe (who came up with one time-machine design) calculated that the cosmonaut Sergei Avdeyev is 0.02 of a second younger than he would be if he had not ventured repeatedly into space.
We can go faster, of course. If a space ship went very near the speed of light (the cosmic speed limit), it might seem to the astronaut on board that the trip to the heart of our galaxy had taken only a few years.
The same goes for high gravity. An astronaut who managed to navigate into the closest possible orbit around a rapidly spinning black hole - without falling in - could, in a subjectively short period, view an immensely long time span unfold around him. But the downside, according to Stephen Hawking, is that, when he returned home, everyone he knew would be dead and forgotten.
Travel into the past also seems possible: no known law of nature seems to rule it out. But this has triggered deep unease among scientists and philosophers because it allows "time loops", in which events in the future "cause" events in the past that then "cause" their own causes.
They mutter darkly about the grandfather paradox, where you would travel back into the past and kill your grandfather, so that your mother, and therefore you yourself, were never born. In which case, you could not have gone back in time to kill your grandfather . . . and so on. Time travel destroys the all-too-reasonable idea of cause and effect. As Niffenegger's Henry remarks: "Things get kind of circular, when you're me. Cause and effect get muddled."
The science-fiction writer Isaac Asimov created the notion of busy "time police" to keep people from murdering their grandmothers. Others have proposed the Principle of Self-consistency.
Niffenegger makes a nod in this direction because her hero can't take anything with him ("If we time travellers started to move things around in time, pretty soon the world would be a big mess") or prevent harm coming to a little girl ("I wanted to warn her mother and I couldn't"), raising issues about free will. But she does allow him to cart information back and forth to teach his younger self the art of pickpocketing, give stock tips and win million on the lottery. The 13-year-old Clare tells her temporal true love: "You are making me different."
Scientists are still arguing about time travel, even if it was first given serious lab cred decades ago - in 1949 - by the great logician Kurt Godel while considering a rotating universe. Godel used the general theory of relativity (which tells you how to change the shape of a blend of space and time called spacetime and is the descendant of Einstein's special theory) to create temporal loops: the river of time can contain whirlpools and eddy currents. Several later theorists also used general relativity this way to make short cuts through spacetime that allow journeys into the past.
Even the great Einstein, while working with Nathan Rosen in Princeton in the 1930s, discovered that his equations could bridge time, without realising it. Such an "Einstein-Rosen bridge" - which we now call a wormhole - could lead to the possibility of movement through cosmic distances. But he did not seem to appreciate that, by moving one end of the bridge, a wormhole might just as well link two different times as two different places.
However, the wormhole itself does not exist for long. In effect, gravity quickly slams this portal shut. This proved to be a headache when the late astronomer Carl Sagan decided to write a science-fiction novel, Contact.
Sagan wanted to fix this problem to allow his heroine to travel from Earth to a point near the star Vega. In 1985, he approached Kip Thorne at Caltech for help; he in turn enlisted the aid of his students.
Thorne, Michael Morris and Ulvi Yurtsever speculated that with the help of quantum theory - the somewhat bizarre theory that governs the subatomic world in terms of probabilities, not certainties - it might indeed be possible to travel between different places and times. In 1987, they reported that, for a wormhole to be held open, its throat would have to be threaded by some form of exotic matter, or some form of field, that would exert negative pressure or negative energy and have antigravity associated with it.
Other researchers hunting for flaws argued against such theoretical time machines. In 1990, for example, Hawking proposed a Chronology Protection Conjecture, which says that the laws of physics disallow time machines. Three years later, Amos Ori of the Technion in Israel, concluded that the possibility of constructing a time machine from conventional materials could not be ruled out. "There are loopholes in Hawking's arguments," he said.
Hawking describes in his recent book, The Universe in a Nutshell, how he tried to put his Chronology Protection idea on a firm theoretical basis. With Michael Cassidy, Hawking investigated what are called rotating Einstein universes, which admit time loops - time travel - and found that the probability of their having sufficient warping for a time machine to function is almost zero.
The probability that Kip Thorne could go back and kill his grandfather was less than one in 10 with a trillion trillion trillion trillion trillion zeros after it, according to Cassidy. "That's a pretty small probability," said Hawking. "But if you look closely at a picture of Kip, you may see a slight fuzziness around the edges. That corresponds to the faint possibility that some bastard from the future came back and killed his grandfather, so he's not really there."
Tongue in cheek, Hawking adds that there is experimental evidence that time travel doesn't exist: "We have no reliable evidence of visitors from the future. (I'm discounting the conspiracy theory that UFOs are from the future and that the Government knows and is covering it up. Its record of cover-ups is not that good.)"
Perhaps time-travellers are models of discretion. Henry remarks in Niffenegger's book: "If everybody time-travelled, it would get too crowded."
But it seems highly likely that, if time travel were possible, we would have had thrill-seekers from the future converging on Afghanistan years ago to fight over assassinating Osama bin Laden, attempting to prevent the horrors of September 11, saving the life of Diana, Princess of Wales, and so on.
Scientists who defend the right to time travel point out that it is not possible to use these sorts of time machines to venture back to before the machine was built. You can voyage to the future and come back to where you started, but no farther. This may explain why no time travellers from our future have yet visited us.
And there may even be a way around the paradoxes, too, according to David Deutsch of Oxford University. He argues that time travel shifts between different branches of reality, basing his claim on the so-called "many-worlds" formulation of quantum theory. This was first glimpsed by the great quantum pioneer, Erwin Schrodinger, but actually published in 1957 by Hugh Everett III, when wrestling with the problem of what actually happens when an observation is made of something of interest - such as an electron or an atom - with the intention of measuring its position or its speed.
In the traditional brand of quantum mechanics, a mathematical object called a wave function, which contains all possible outcomes of a measurement experiment, "collapses" to give a single real measurement. Everett came up with a more audacious interpretation: the universe is constantly and infinitely splitting, so that no collapse takes place. Every possible outcome of an experimental measurement occurs, each one in a parallel universe.
If one accepts Everett's interpretation, our universe is embedded in an infinitely larger and more complex structure called the multiverse, which as a good approximation can be regarded as an ever-multiplying mass of parallel universes. Every time there is an event at the quantum level - a radioactive atom decaying, for example, or a particle of light impinging on your retina - the universe is supposed to "split" into different universes.
Accordingly, when Niffenegger's Henry DeTamble moves to a different era, he arrives in a different parallel universe. "We do not create a new reality. We just go to an existing reality and make things happen there," says David Deutsch. "And they start happening when we arrive, not when we do the 'paradoxical' thing."
In this way, the "many worlds" interpretation of quantum mechanics allows a time traveller to alter the past without producing problems such as the notorious grandfather paradox.
It remains a matter of taste whether going to a parallel universe counts as true time travel. "If it became possible to receive messages from the future, few would quibble about terminology just because it was the future of a slightly different universe," he says.
Today, Sir Martin Rees, the Astronomer Royal, admits there is "still no consensus about whether travel into the past is impossible in principle or merely because of extreme technical limitations".
The debate probably says more about the deficiencies of present-day physics than the future possibilities of time machines. And, although few scientists admit to studying time travel (they prefer to mutter about studies of "closed time-like curves"), this endeavour is not a waste of time.
According to Kip Thorne, asking these questions will help us understand time itself, which remains mysterious.
Crucially, Einstein's relativity theories are not the whole story. They fail to include quantum mechanics, which allows time travel at an atomic level. Efforts to fuse the two into a grand theory of quantum gravity - the effort that preoccupies Stephen Hawking and many others - have yet to succeed.
Experiments may also become possible. Paul Davies is looking forward to 2007, when a huge new atom smasher, the LHC, may be able to create mini wormholes - and possibilities for time travel at the atomic level.
The controversy over the possibilities of time travel is likely to rumble on for years. Indeed, there are even those who don't believe in time at all.
In his book The End of Time, the independent British physicist Julian Barbour departs radically from the conventional view of time - that it is a primary constituent of the universe, analogous to space - and argues that the universe is timeless. Barbour therefore thinks that time travel is impossible because there is no time in which to travel.
Intriguingly, Niffenegger's hero, Henry, muses on whether we live in a "block cosmos", where past, present and future coexist: this echoes Barbour's timeless universe. Barbour also believes that we already do something much more impressive than using a time machine for a temporal hop: "Through memories and anticipations in our present 'Now' we are in a very real sense also present in 'Nows' in our past and our future."
This chimes with the constant yearning of the book's heroine to be reunited with her beloved Henry, as Niffenegger explores the incredible gravitational tug of human memory.
'The Time Traveller's Wife' by Audrey Niffenegger (Jonathan Cape) is available for £10.99 plus £2.25 p & p. To order, please call Telegraph Books Direct on 0870 155 7222.
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