String Theory For Dummies
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In physics, time travel is closely linked to Einstein’s theory of relativity, which allows motion in space to actually alter the flow of time. This effect is known as time dilation and was one of the earliest predictions of relativity. This sort of time travel is completely allowed by the known laws of physics, but it allows only travel into the future, not into the past.

Time dilation and black hole event horizons, both of which are explained below, provide intriguing ways of extending human life, and in science fiction they’ve long provided the means for allowing humans to live long enough to travel from star to star.

Time dilation: Sometimes even the best watches run slow

The most evident case of time acting oddly in relativity, and one that has been experimentally verified, is the concept of time dilation under special relativity. Time dilation is the idea that as you move through space, time itself is measured differently for the moving object than the unmoving object. For motion that is near the speed of light, this effect is noticeable and allows a way to travel into the future faster than we normally do.

One experiment that confirms this strange behavior is based on unstable particles, pions and muons. Physicists know how quickly the particles would decay if they were sitting still, but when they bombard Earth in the form of cosmic rays, they’re moving very quickly. Their decay rates don’t match the predictions, but if you apply special relativity and consider the time from the particle’s point of view, the time comes out as expected.

In fact, time dilation is confirmed by a number of experiments. In the Hafele-Keating experiments of 1971, atomic clocks (which are very precise) were flown on airplanes traveling in opposite directions. The time differences shown on the clocks, as a result of their relative motion, precisely matched the predictions from relativity. Also, global positioning system (GPS) satellites have to compensate for this time dilation to function properly. So time dilation is on very solid scientific ground.

Time dilation leads to one popular form of time travel. If you were to get into a spaceship that traveled very quickly away from Earth, time inside the ship would slow down in comparison to that on Earth. You could do a flyby of a nearby star and return to Earth at nearly the speed of light, and a few years would pass on Earth while possibly only a few weeks or months would pass for you, depending on how fast you were going and how far away the star was.

The biggest problem with this is how to accelerate a ship up to those speeds. Scientists and science fiction authors have made various proposals for such devices, but all are well outside the range of what we could feasibly build today or in the foreseeable future.

As you accelerate an object to high speeds, its mass also increases, which means it takes more and more energy to keep accelerating it. This formula of mass increase is similar to the formula that describes time dilation, so this makes it fundamentally difficult to get significant levels of time dilation.

Black hole event horizons: An extra-slow version of slow motion

One other case where time slows down, this time in general relativity, involves black holes. Recall that a black hole bends space-time itself, to the point where even light can’t escape. This bending of space-time means that as you approach a black hole, time will slow down for you relative to the outside world.

If you were approaching the black hole and your best friend Dean was far away watching (and could somehow watch “instantly,” without worrying about the time lag from light speed), Dean would see you approach the black hole, slow down and eventually come to rest to hover outside of it. Through the window of your spaceship, Dean would see you sitting absolutely still.

You, on the other hand, would not notice anything in particular — at least until the intense gravity of the black hole killed you. But until then, it certainly wouldn’t “feel” like time was moving differently. You’d have no idea that as you glide past the black hole’s event horizon (which you possibly wouldn’t even notice), thousands of years were passing outside of the black hole.

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Andrew Zimmerman Jones received his physics degree and graduated with honors from Wabash College, where he earned the Harold Q. Fuller Prize in Physics. He is the Physics Guide for the New York Times' Web site. Daniel Robbins received his PhD in physics from the University of Chicago and currently studies string theory and its implications at Texas A&M University.

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