Time Travel

Typography
What is time? One of the earliest concepts proposed is that is simply the fourth dimension, another direction. Humans travel daily one way into the future. Science fiction writers have long been obsessed with the subject from the legendary "The Time Machine" by HG Wells to the the Doctor Who series about the last time lord and his adventures. From summer blockbusters to sensational science headlines, modern culture is constantly inundated with tales of time travel. But when you boil down the physics involved, is it possible to travel through time?

What is time?

One of the earliest concepts proposed is that is simply the fourth dimension, another direction. Humans travel daily one way into the future.

Science fiction writers have long been obsessed with the subject from the legendary "The Time Machine" by HG Wells to the the Doctor Who series about the last time lord and his adventures. From summer blockbusters to sensational science headlines, modern culture is constantly inundated with tales of time travel. But when you boil down the physics involved, is it possible to travel through time?

!ADVERTISEMENT!

Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced. There is also how humans perceive time as fast (interesting) or slow (dull). All are only way into the future.

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and it's debatable whether they should be called time travel. However. time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

A phenomenon called time dilation is the physics known key. 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 on board for every 223 years back at the train station. For that human on board it will be like leaving in 2010 and arriving in 2233.

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 (theoretical physicist and cosmologist Paul 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. The effect is small but is there.

"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."

So 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? To go back to the year from 1000 from 2010?

There is nothing forbidden in physics for backwards time travel. However, it does lead to endless paradox. The classic one is a man going back in time and killing his own father before he has been born. If that happens, then that person could not go back in time and kill his father...or can he?

There are many unproven theories of time. For example there are alternate time lines. In one time line you kill your grandfather and lived on. In another you never did. Another theory is time cannot change and a paradox will be resisted (hence time travel cannot exist as the simplest solution). One last theory is that time does change and simply incorporates what ever change the time traveler made so that time did not change, it was always that way.

In that way backwards time travel has not been proven or disproved. The alternate time lines cannot be found at least as of yet.

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.

The fundamental principle of causality does certainly stand in the way of travel into the past. The entire universe, as we understand it, is beholden to this rule. there is an order to the way world exists so that a result can not precede a cause. This seems a major stumbling block to backwards time travel.

For further information: http://news.discovery.com/space/is-time-travel-possible.html or http://en.wikipedia.org/wiki/Time_travel