The concept of a time machine typically conjures up images of an implausible plot device used in a few too many science-fiction storylines. But according to Albert Einstein's general theory of relativity, which explains how gravity operates in the universe, real-life time travel isn't just a vague fantasy.
Traveling forward in time is an uncontroversial possibility, according to Einstein's theory. In fact, physicists have been able to send tiny particles called muons, which are similar to electrons, forward in time by manipulating the gravity around them. That's not to say the technology for sending humans 100 years into the future will be available anytime soon, though.
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Time travel to the past, however, is even less understood. Still, astrophysicist Eric W. Davis, of the EarthTech International Institute for Advanced Studies at Austin, argues that it's possible. All you need, he says, is a wormhole, which is a theoretical passageway through space-time that is predicted by relativity.
"You can go into the future or into the past using traversable wormholes," Davis told LiveScience.
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Wormholes have never been proven to exist, and if they are ever found, they are likely to be so tiny that a person couldn't fit inside, never mind a spaceship.
Even so, Davis' paper, published in July in the American Institute of Aeronautics and Astronautics' journal, addresses time machines and the possibility that a wormhole could become, or be used as, a means for traveling backward in time.
Both general relativity theory and quantum theory appear to offer several possibilities for traveling along what physicists call a "closed, timelike curve," or a path that cuts through time and space -- essentially, a time machine.
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In fact, Davis said, scientists' current understanding of the laws of physics "are infested with time machines whereby there are numerous space-time geometry solutions that exhibit time travel and/or have the properties of time machines."
A wormhole would allow a ship, for instance, to travel from one point to another faster than the speed of light -- sort of. That's because the ship would arrive at its destination sooner than a beam of light would, by taking a shortcut through space-time via the wormhole. That way, the vehicle doesn't actually break the rule of the so-called universal speed limit -- the speed of light -- because the ship never actually travels at a speed faster than light.
Theoretically, a wormhole could be used to cut not just through space, but through time as well.
"Time machines are unavoidable in our physical dimensional space-time," David wrote in his paper. "Traversable wormholes imply time machines, and (the prediction of wormholes) spawned a number of follow-on research efforts on time machines."
However, Davis added, turning a wormhole into a time machine won't be easy. "It would take a Herculean effort to turn a wormhole into a time machine. It's going to be tough enough to pull off a wormhole," he told LiveScience.
That's because once a wormhole is created, one or both ends of it would need to be accelerated through time to the desired position, according to general relativity theory.