A phenomenon called time dilation is the key here. Time passes more slowly the closer you approach the speed of light -- an unbreakable cosmic speed limit. As such, the hands of a clock in a speeding train would move more slowly than those in a stationary clock. The difference would not be humanly noticeable, but when the train pulled back into the station, the two clocks would be off by billionths of a second. If such a train could attain 99.999 percent light speed, only 1 year would pass onboard for every 223 years back at the train station.
But speed isn't the only factor that affects time. On a much smaller scale, mass also influences time. Time slows down the closer you are to the center of a massive object.
"Time runs a little bit faster in space than it does down on Earth," Davies says. "It runs a little faster on the roof than it does in the basement, and that's a measurable effect."
A clock aboard an orbiting satellite experiences time dilation due to both the speed of its orbit and its greater distance from the center of Earth's gravity.
"Both gravity and speed can give you a means of jumping ahead," Davies says. "So in principle, if you had enough money, you could get to the year 3000 in as short a time as you like -- one year, one month, whatever it takes. It is only a question of money and engineering."
Forward, not back?
Time travel into the future is an established and fundamental aspect of Albert Einstein's theory of relativity. Scientists have tested and retested this in both experimental and practical settings. But what about time travel in the opposite direction?
"Going back in to the past is a whole different kettle of fish." Davies says. "There's nothing in Einstein's theory, which is the best theory that we have about the nature of time, which precludes it. There's nothing in even his general theory of relativity, published in 1915, which precludes travel back into the past, but many scientists are deeply uneasy about it because of all the well-known paradoxes that it unleashes."
For instance, imagine going back in time and killing your own mother. Then she'd never give birth to you, and just how would you have been able to travel back in time to commit matricide in the first place?
Wormholes as spacetime shortcuts
Davies surmises that, given our current understanding of the nature of time and physics, time travel into the past simply isn't possible. But the universe is full of mysteries, and one of them -- the hypothetical wormhole -- might just permit such a journey.
"This is a little bit like a tunnel or shortcut between two distant points," Davies says, "So for example, if I had a wormhole here in my hotel room and I jumped through it I wouldn't come out on Pennsylvania Avenue, I'd maybe come out near the other side of the galaxy."
Scientists have theorized that such a shortcut through time and space could be turned into a time machine.
"If a worm hole could exist and could be traversable, then it would provide a means of going back in time," Davies says. "So it all hinges on whether stable wormholes are a reality or if there's some aspect of physics -- not relativity, because there's nothing wrong from that point of view -- but some other aspect of physics might intercede and prevent the wormhole from forming. That's an open question."
World-famous theoretical physicist Stephen Hawking has proposed that wormholes occurring at a quantum level could theoretically provide a foothold for time travel, but University of California at Santa Barbara physicist Andrew Cleland urges caution on that front.
"I'm an experimentalist, and physics is ultimately an experimental science," Cleland says. "Any predictions that are made based on mathematics or on philosophical or intellectual speculation have to pass the test of experiment, and I am certainly not aware of any experiment that demonstrated the possibility of traveling backward in time."
Cause and effect
Cleland also points out that the fundamental principle of causality stands in the way of travel into the past. The entire universe, as we understand it, is beholden to this rule.
"Something occurs first and the outcome of that occurrence happens afterward," Cleland says, "and there has never to my knowledge been an experiment that came out different from that. I am not aware of any experimental tests of quantum mechanics that have shown any violation of causality, in spite of the fact that many experiments could reveal such a violation."
Still, in the same way that time dilation isn't flashy enough to seem like time travel into the future, the public often overlooks a very common means of "traveling" into the past.
"In a sense, astronomers are always traveling backward in time, but it is in a way that most people are not so excited about," Cleland says. "When we measure the cosmic microwave background, we're looking back more than 10 billion years in time. That's how long it took for the light to reach us."
A number of questions about time travel remain unanswered. Will time tourists from the future ever show up to help us out? We'll just have to wait and see. But if they come here using a wormhole time machine, we'll have to build one first. After all, you couldn't cross a bridge if only one side had been completed, right?
"Theoretically, it would take more than 100 years to create a 100-years' time difference between the two ends of a wormhole," Davies says, "so there's no way that our descendants could come back and tell us we're wrong about this."
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