Time travel is an idea found in science fiction, philosophy, and physics. Usually it involves someone or something traveling outside the normal flow of time to the distant past or future. Time travel is a popular theme in many books, television shows, and motion pictures. It is also a serious subject studied by scientists.
Time travel happens when there is a difference between the amount of time a journey takes and the normal duration between the journey’s start and finish. For example, lying in bed for an hour would not be considered time traveling to a time one hour in the future. However, if one could spend one minute in a time machine and emerge an hour into the future, that would be time travel. In this case there is a discrepancy between the one-minute journey time and the hour that passed by for others not in the time machine.
The possibility of time travel.
The question for both scientists and creators of science fiction is whether time travel is actually possible. In our everyday lives, time seems to proceed in a set direction. No one travels to the past or takes shortcuts to the distant future. However, the basic laws of physics may permit such behavior. To determine whether time travel is possible, one needs to decide whether the idea is coherent (logically consistent) and whether the laws of nature allow it.
A thought experiment known as the grandfather paradox illustrates that some time-travel stories are incoherent. This paradox involves a hypothetical time machine that allows a person to travel to the past. With such a machine, a person could travel back in time and kill his or her own grandfather as a youth before he ever met the person’s grandmother. This action would result in one of the person’s own parents having never been born, which in turn would result in the time traveler never having been born. If the time traveler were never born, then of course he or she could not have traveled back. Thus, the story is inconsistent, for it claims that the traveler was both born and not born, traveled and not traveled, killed grandfather and didn’t kill grandfather, and so on. For this reason, many scientists think that time travel to the distant past is probably impossible.
However, not all examples of time travel to the past are incoherent. For example, suppose a person time travels back to the age of dinosaurs and is fatally impaled (pierced through) by a Triceratops horn. It may be that the fact that this person died in the distant past is why we experience the present the way it is. Present-day paleontologists may even be puzzled to find the person’s fossilized bones among those of long-extinct dinosaurs. So long as no truths are violated, as one would be if a time traveler killed an ancestor, this example of time travel can be logically consistent.
The theory of special relativity, developed by the German-born physicist Albert Einstein, holds that the amount of time that passes for an object depends on its path through space-time. This entity is a combination of the dimension of time and the three dimensions of space—length, width, and height. Before Einstein developed his theory, physicists thought the duration between any two events is the same for any two objects that experience both events. But in special relativity, the duration depends on how you get to each event. Suppose a person built a spaceship, flew into outer space, and then returned in what he or she experienced as five years. Upon return, the pilot would find that many more years have passed on Earth. For example, it’s possible that 50 years passed on Earth while only 5 years passed for the pilot.
The path-dependence of duration has been rigorously confirmed by scientists many times. If we understand time travel as a discrepancy between a traveler’s journey time and the normal amount of time taken between two events, this relativistic phenomenon counts as time travel. However, this type of time travel isn’t as exciting as that found in fiction. One reason is that, strictly speaking, everyone counts as a time traveler because none of our paths through space-time are exactly the same. We just don’t notice this effect in normal life because the effect is small at the speeds we tend to travel. Another reason is that this sort of time travel only allows one to visit the future with no return. It does not allow anyone to travel to the past.
Closed timelike curves.
Suppose a journey begins and ends close to the same point in time. For example, if a person were able to create a time-traveling device, he or she might go back to visit the moment when he or she was born. A path through space-time would obtain from the birth to entering the time-travel machine and back to the birth. Physicists call such time-travel paths closed timelike curves. The curves are closed (or almost closed) because the path a time traveler takes is closed on itself, like a circle. The path is timelike because it moves through time. A closed time curve allows the exciting sort of time travel associated with stories in fiction. For instance, they may allow for visiting past events such as your birth.
Scientists are not certain if closed timelike curves are actually possible. Einstein’s general theory of relativity allows them. That is, mathematical solutions that allow such paths through time do exist. However, scientists do not know if such solutions are realistic. For example, time travel to the past may require an extraordinary amount of energy that is impossible to harness.
