By Paul Sussman for CNN
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(CNN) -- Bruce Willis did it, with the aid of a large plastic tube and a set of bellows, in "Twelve Monkeys;" Michael J. Fox did it -- three times -- in the "Back to the Future" series; the crew of the Starship Enterprise can't stop doing it (at least 14 times in television episodes alone); while as the "Terminator" Arnold Schwarzenegger also gave it a go, albeit stark naked.
Time travel, and all the various paradoxes and temporal anomalies associated with it, have long been a staple of the world of fiction.
From H.G. Wells' seminal 1895 novel "The Time Machine," to the equally thrilling if slightly less influential "Beastmaster ll: Through the Portal of Time," the idea of human beings transporting themselves backwards into the distant past, or forwards in the far future, is one that never fails to capture the popular imagination.
While it has long been achievable with the artistic licence of novel-writing or movie-making -- one of the earliest time travel stories, Samuel Madden's "Memoirs of the Twentieth Century", was written as far back as 1733 -- the idea of time travel as a practical reality has generally been dismissed as impossible.
Over the last 25 years, however, with some starling advances in the field of quantum physics, an increasing number of serious scientific thinkers have come round to the view that not only is time travel theoretically feasible but also, in some circumstances, practically so as well.
"In theory it is absolutely possible," says Neil Johnson, a physics professor at Oxford University. "It might seem to go against common sense, but one of the beauties of Einstein's Theory of Relativity is that it allows for situations that are against common sense."
Dr. Cliff Pickover, a leading US science commentator and author of numerous books on the subject (including "Time: A Traveler's Guide"), agrees.
"We know for certain that time travel is possible," he told CNN. "For instance, scientists have demonstrated that objects traveling at high speeds age more slowly than stationary objects, so if I traveled on a high-speed rocket into outer space and returned moving close to the speed of light, I could travel thousands of years into the Earth's future.
"Time travel into the past may be more difficult, but there are still numerous ways in which time machines for past travel might be built that do not seem to violate any known laws of physics."
Hairy wormholes and quantum foam
The science of time travel is a fiendishly complex, at times mind-bending and counter-intuitive area of study, one with an idiom -- Gott Loops, Tipler Cylinders, Hairy Wormholes, Quantum Foam -- that smacks less of serious scholarship and more of some surreal Monty Python sketch poking fun at academics.
The starting point for the whole subject, as with so much of contemporary theoretical physics, is the early 20th Century work of Albert Einstein, and in particular the studies of light, time, motion and gravity contained in his Special and General Theories of Relativity (the term "relativity" was actually coined by a contemporary of Einstein's, German physicist Max Planck).
Among other arguments, Einstein posited two revolutionary conjectures, both of which, a century later, remain fundamental to this branch of physics.
Firstly he suggested that not only does the speed of light -- which he designated with the letter c -- remain constant irrespective of the observer's point of view, but that the closer a person comes to that speed, the more time slows down for them relative to someone who remains stationary.
"What Einstein showed, and what science has subsequently proved, is that time can look different depending on who is doing the measuring and where they are doing the measuring from," explains Valerie Jamieson of New Scientist Magazine.
"Someone traveling at high velocity will have a very different relationship with time to someone who is stationary."
Secondly, Einstein argued that space and time do not exist separately, but are rather inextricably bound together in a four-dimensional arrangement known as "spacetime" (with time being a fourth dimension alongside length, width and height).
His equations visualized this phenomenon as being essentially smooth, like a taut rubber sheet.
While heavy objects, such as our Sun, will create dips in this four dimensional fabric -- rather like a billiard ball sitting in the middle of the sheet -- Einstein also postulated the existence of infinitely dense objects ("black holes" as they were later named) capable of warping spacetime to the extent that all the normal laws of physics break down.
He called this warping a "singularity" and theorized that rather than simply bending the imaginary rubber sheet, such singularities might actually tear right the way through it into another time and place.
In other words, just as one can navigate through space, so in certain circumstances one might also be able to do so through time (although Einstein himself always dismissed the idea of backwards time travel as impossible).
The grandfather paradox
Einstein's equations provide the theoretical lynchpins for time travel. But how close are we to actually achieving it in practice?
The answer very much depends on how one defines the concept.
Most of us have experienced a very simple form of time travel -- whenever you fly in a plane you are party to a small time shift in relation to someone on the ground.
Such a shift, however, involves only a matter of nanoseconds, measurable on an atomic clock but otherwise so miniscule as to be unnoticeable.
Time travel as it is popularly imagined -- someone sitting in a high-tech machine that propels them with a bang and a flash of light into the past or future -- remains a far more difficult proposition, and one that is unlikely to be realized any time soon.
For one thing the energy required to achieve the near-light-speed velocities on which time travel depends is way beyond anything humanity currently possesses.
Even if we were able to generate the necessary energy the forces involved would be so enormous as to vaporize the unfortunate time traveler before they'd actually gone anywhere.
"The idea of a Back-To-The-Future-type DeLorean zooming off into the past or future is a very long way off," says Neil Johnson.
"It's basically the equivalent of strapping an atomic bomb to yourself. The person driving would be totally destroyed."
In addition time travel, especially into the past, involves certain paradoxes that for physicists such as Stephen Hawking make such an endeavor a non-starter.
The "Grandfather Paradox" for instance -- first described by French science fiction writer René Barjavel in 1943 -- postulates someone traveling back in time and killing their own grandfather, thereby insuring they would never have been conceived in the first place and would thus not exist to travel back in time (Hawking's "chronology protection conjecture" uses this paradox to undermine the whole idea of time travel).
"The consensus opinion, certainly at the moment, is that time travel, as we traditionally think of it, is an impossibility," says Valerie Jamieson.
"I'd love to rewind to last Friday night and change what happened but I don't think I ever will."
Superstrings and the Tenth Dimension
If the problems are substantial, especially at our current level of technological development, there are nonetheless many serious thinkers who maintain that time travel will, one day, become a reality.
"Through history physicists have found that if a phenomenon is not expressly forbidden -- and time travel isn't -- then it often occurs," says Cliff Pickover.
"Today designs for time machines are proliferating in top science laboratories. In the next few hundred years our heirs will explore space and time to degrees we cannot currently fathom."
So how exactly can time travel be made to work? There are many different avenues currently being explored.
One, for instance, suggests that the universe contains many more dimensions than the four of which we are currently aware. Hard as it is to imagine, by accessing these higher dimensions it might be possible to move through time in a way that would be impossible in a limited-dimension universe.
"A lot of the thinking in theoretical physics is that there are actually 10 dimensions," explains Valerie Jamieson, "And that if we can find a way of breaking out into this 10-dimensional world we can use it to shortcut through time into the past or the future."
Another, contentious area of research concerns "string theory." This holds that the Universe is made up of vibrating, sub-atomic "superstrings" that exert a gravitational pull so strong they are capable of warping spacetime.
Although their existence is only speculative, some physicists, notably Princeton University's J. Richard Gott, have argued that by harnessing the gravitational force of such superstrings it might be possible to achieve sufficient velocity to travel backwards in time.
Finally, a lot of thinking has focused on so-called "wormholes."
First posited in the 1920's by German mathematician Hermann Weyl -- although the name was actually coined in 1957 by U.S. physicist John Wheeler -- these are an expansion of Einstein's black hole equations: tunnels in spacetime through which one can, theoretically, move from one point in time to another.
The possibility of using these to create a workable time machine has most notably been championed by the great American physicist Kip Thorne, who in a prestigious 1988 article in the Physical Review Letters provided a detailed analysis of how wormholes might offer a realistic means of accessing the past and the future.
While acknowledging that it would be many years before our civilization was able to generate sufficient energy to power a wormhole time machine, Thorne nonetheless confidently concluded: "From a single wormhole an arbitrarily advanced civilization can construct a machine for backward time travel."
And what of the possibility of time travel in our own lifetimes?
Only the most optimistic -- and some would argue deluded -- commentators maintain that at present human beings traveling through time is anything other than a cherished aspiration.
That said, some of the most interesting, and boundary-pushing time travel research currently being conducted concerns the possibility not of humans being propelled into the past or future, but rather sub-atomic, or quantum particles.
"I have a feeling that while we can't do it with people, we might be able to make time travel work with particles, and thus, by extension with information," says Neil Johnson.
"This side of things could happen quite soon, possibly in the next 50 years."
Valerie Jamieson agrees. "The best we can hope for at the moment is to see the effects of time travel on a sub-atomic level," she told CNN.
"Particles being accelerated up to near light speed and as a result doing strange things with time."
Work with neutrinos, for instance -- "the most tiny quantity of reality ever imagined by a human being," as their discoverer Frederick Reines described them -- has revealed some extraordinary time-warping properties.
"If you picture spacetime as a rubber sheet," explains Neil Johnson, "At a sub-atomic level that sheet is not smooth but extremely rough and frothy, full of gaps."
It might, he argues, be possible to push very small particles through those gaps, and hence back and forth in time.
"The extraordinary, counter-intuitive thing about some of these particles is that they lead a sort of double life -- they seem to be able to exist in different places at the same time.
"It's way beyond our imaginations -- Einstein described quantum physics as "spookiness" -- but in essence we might be able to accelerate a particle into the future or the past while that same particle stays anchored in the present.
"Quantum teleportation seems to have been achieved -- passing information from one place to another without it physically moving.
"On that basis it should be possible send simple binary messages forward or back in time as well."
Even if time travel as H.G. Wells envisaged it is a long way off, it would nonetheless appear that we are standing on the threshold of some truly epoch-making developments.
As Clifford Pickover puts it: "Before long we are going to be creating new melodies in the music of time. There are infinite harmonies to be explored."
Physicists agree that, in theory at least, time travel is possible
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