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 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 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.
