Traveling in Time

The second law of thermodynamics, which says that entropy gen- erally increases over time, is what gives time a direction in which to flow. But some people have wondered whether it is possible to travel through time in any direction or any speed they wish.

In a sense, everyone travels in time—slowly and in the forward direction. The question is whether the journey can in some way be altered. The answer is yes, thanks to Einstein’s ideas.

As mentioned earlier, Einstein said that time slows down at high speeds. Physicists have tested this idea, and the results sup-

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port the theory. This means it is possible to get time to slow down by hopping aboard a fast spaceship. The spaceship must be able to travel at a significant fraction of the speed of light or the effect will be too small to notice. Although such speeds are not possible at present, they may become reachable in the future. With such a ship, astronauts could go to places that are so far away it would take hundreds of years to reach them. But the hundreds of years would be “Earth time”—because time slows down for the astronauts, they would not experience this much time, nor would they age for that long. In “ship time,” they might spend only a few years. This concept has been the basis of many science fiction stories and movies, such as Planet of the Apes (the original movie was released in 1968 and starred Charlton Heston). The astronauts in Planet of the Apes make a voyage that to them lasts only a few months, but 2,000 years pass on Earth. The science in this science fiction movie is based on real physics.

Future spacecraft are shown here flying in formation. This drawing depicts a possible mission to locate Earthlike planets orbiting other stars. (NASA)

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What about traveling in the other direction, into the past? This may not so easy. The whole idea seems absurd, at least at first. The past has already occurred and is long gone, so it would seem impossible for anyone to go to a place that no longer exists. But as far as physics is concerned, time is relative. What is past for one person is not necessarily past for the universe as a whole. Maybe traveling into the past is possible, though it is certainly difficult to conceive.

One possible method of traveling backward in time makes use of another one of Einstein’s ideas. Einstein said not only does time slow down at high speeds, but it also slows down because of gravity. Once again Einstein was right, because physicists have made measurements with precise atomic clocks and determined that they run slower as gravity increases.

In most situations, time’s interaction with gravity is not significant. But in one case, involving strange and mysterious objects, there are some interesting possibilities. One of these objects is called a black hole. Physicists believe black holes form when matter becomes so dense that the force of gravitation squeezes it down into

a point called a singularity. Matter is normally strong enough to withstand gravitational forces, except when a large amount of matter, such as the amount of matter in a large star (much bigger than the Sun), is compressed into a small space. While the star is shining, there are energetic reactions taking place inside to generate enough force to prevent the star’s matter from getting pulled inward due to gravitation. But when the reactions stop—and they will, for all stars shine from energy released in nuclear reactions but eventually run out of fuel—gravitational forces cause the star to collapse. The star gets denser, which increases the force of gravitation. Soon gravitation becomes too strong for matter to withstand it. The physics of what happens next is not entirely clear, but apparently the matter gets squeezed into a point of practically infinite density.

Black holes get their name because their gravity is strong enough to prevent light from escaping (and so they appear totally black). They are “holes” because they are places where a star was once located but now is gone. Black holes cannot be seen, but they can be detected because their gravitational force remains.

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Astronomers believe they have found several objects in the galaxy that may be black holes, including one in the center of the galaxy with a mass a million times greater than the Sun.

Gravity is so strong near a black hole’s singularity that time appears to come to a halt to any outside observers. In the singularity itself, time does not really have any well-understood meaning (nor do any of the other basic concepts in physics). But some physicists believe under certain circumstances singularities may provide a tunnel to other places—and perhaps other times. These ideas are speculative, meaning they have not been confirmed by experiment. The physics is extremely complicated, and no one knows for sure what is happening inside a black hole, and more to the point, no one knows how to survive the journey to find out. Gravitational forces would be so strong that except under special circumstances they would tear apart anyone who tried.

Another possible backward time-travel method involves an object called a wormhole (also known as an Einstein-Rosen bridge). A wormhole is a tunnel through space and time. No one knows if wormholes exist, but some of Einstein’s theories seem to allow the possibility. In theory, wormholes are caused by the pres- ence of large masses such as black holes, and the idea is that these masses link together, forming a bridge. The bridge would be like a shortcut, and a traveler could pass quickly through vast distances and, perhaps, time as well.

Although some physicists believe backward time travel may

be possible, there are more concerns than just physics. Traveling backward in time can create a paradox—an impossible situation. For instance, in the 1985 movie Back to the Future, a young man goes back in time hoping to meet his parents when they were his age. But the young man finds himself in trouble when he nearly becomes responsible for preventing his parents from falling in love with each other—which would have disastrous consequences for him, their future son!

The paradoxes created by traveling backward in time, such as

a person preventing his or her own birth, suggests these journeys are not possible. It is difficult to understand how a time traveler could make a change that affects the future. Even more difficult

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is how time travelers could affect their own future to the extent of preventing themselves from ever existing!

Perhaps time travel into the past is impossible after all. This would explain the absence of any known visitors from the future— if backward time travel becomes possible in the future, one would think there would be people dropping in from the future, since the present time—today, for example—is their past. At present there is no well-understood law of physics that precludes backward time travel, and some serious thought, as mentioned above, has gone into considering the possibility. Yet the potential paradoxes indi- cate there is probably a law of physics to prevent such travel. If so, finding this law is important because it may reveal much more than is presently known about the nature of time.

Traveling in Time