In investigating the consequences of James Clerk Maxwell's equations of electromagnetics, Albert Einstein developed a theory that shows how light waves are observed to behave in different frames of reference. One of the consequences of Maxwell's equations -- and a foundation of special relativity -- is that the speed of a light wave is constant: 300,000 km/sec, no matter what reference frame it is observed in. Einstein built on Maxwell's equations and established that time is not unchangeable, as nearly everyone preceding him had thought.

Einstein later developed a general theory of relativity that explains gravity and the geometry of the universe. In both of his relativity theories, he established that time and space are bound together in a four-dimensional "space-time" continuum. Space-time consists of the three coordinates of space -- up-down, left-right, and backward-forward -- plus one of time, and events can only be analyzed and understood in a system in which space and time are linked in fascinating ways.

These theories show that time travel to the future is, in a certain sense, possible. Time passes at different rates in two separate reference frames that are moving with respect to one another. In principle, it is possible for two people to travel in separate directions and return to the same place with more time having passed for one of them than for the other. Whether or not this constitutes travel into the future, however, is a matter of interpretation. In addition, atomic clocks have been used to prove that the warping effect of gravity changes the way time is measured, which is a tenet of general relativity. In order to get significant effects from either of these relativistic effects, however, very fast speeds or strong gravitational fields are required.

But since Einstein's conclusions -- and, indeed, even long before then -- people have considered how one might travel through time, and the implications of such travel. Contrary to popular conceptions, most theories of time travel do not rely on time machines. Instead, proponents suggest that time travel will likely be done by way of entirely speculative natural phenomena -- things like rotating black holes, wormholes, and cosmic strings -- that will transport us instantly forward or backward from one point in time to another.