One of the biggest fascinations to people is the question of whether or not we have the capabilities to travel backwards or forwards in time. This fascination has been expressed through countless works in both literature and cinema. But the question remains: Do the laws of physics allow for travel to a time other than one's own? And if they did, what would be the results? In fact, time travel is permitted by the laws of physics in certain circumstances. Theoretical phenomena such as wormholes, rotating cylinders, and cosmic strings all provide possible means of traveling through time.

First off, it is important to discuss the concept of time itself. For example, why

In order for wormholes to exist certain circumstances must be met. Most importantly it must be assumed that Einstein's theory of general relativity is true and that space-time is curved. In this curved space-time, objects with positive mass-energy density produce positive curves in space-time like a sphere, while those with negative mass-energy density produce negative curves like a saddle. In order for a wormhole to exist negative curvature is required which means negative mass-energy density. Unfortunately all bodies in space have positive density relative to space which is considered zero. Fortunately what is usually considered the void of space actually contains virtual particles. If one were to reduce the number of these particles in a particular area relative to the space around it, it would produce a negative curvature. One possible manner to accomplish this result is by the Casimir effect. The Casimir effect works by placing two parallel metal plates a short distance apart in space. Due to the fact that the virtual particles have specific wavelengths only certain particles will remain between the plates. Those with wavelengths that do not fit a whole number of times in the distance between the plates will not remain in the area. Thus the area between the plates will have a negative mass energy density since there is less between the plates than in the space surrounding it.5

Probably the most widely accepted theory for time travel is the possibility of traveling through wormholes. A wormhole is like a tunnel in space-time creating a shortcut between two points that would not otherwise be connected. One idea concerning wormholes says that since you are arriving at your destination before the light from the place you left does, you are traveling faster than light. You are therefore traveling into the past as explained above.3 Now imagine that one end of the wormhole is moving at a velocity near the speed of light while the other end is stationary. For the end that is moving, time goes by slower than the other end according to relativistic effects. Therefore the two ends would essentially be in different times. One could then travel to the future or the past by this wormhole. The only restriction is that an individual could not travel to a time before the wormhole was created.4

Of course when discussing time travel, one cannot help but think of the many paradoxes that can arise. Some of the more notable paradoxes can fall into two general categories. These would be paradoxes related to the origin of something and disrupting one's own existence. For disrupting one's own existence there is the famous grandfather paradox. This paradox is the question of what would happen if you went back in time and killed your grandfather while he was still a young man. Then supposedly you wouldn't have been born, so you couldn't kill him, but then you were born and so you could kill him, etc. Relating to the origin of something, if a time traveler were to come from the future, and give us the secret to constructing a time machine, which would then be the machine the traveler will use to travel back in time to give the secret, where did the machine originally come from? These paradoxes have no easy immediate answers and many physicists use them as justification that time travel is not possible.12<

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