When H.G. Wells published his epochal novel "The Time Machine" in 1895, time travel was outlawed by the laws of physics. But that was Newtonian physics, and everything changed 10 years later with Einstein's theory of relativity.

That theory -- which ushered in the age of E=mc2 and set the speed of light, 186,000 miles per second, as the cosmic speed limit -- allows for time travel to the future, physicists say. Here's how:

One consequence of Einstein's theory is that a clock in motion will always appear to run slowly compared with one at rest (and since all motion is relative the clock at rest will appear to go slowly from the vantage of the one moving).

This leads to the famous "twin paradox" in which one twin is rocketed at high speed across the galaxy and back home. Even at a velocity close to the speed of light, the journey would take tens of thousands of years from the vantage point of Earth, but because of his high relative motion the astronaut would age more slowly than he or she would than on Earth, and would return home only a few years older. His twin would be long dead.

In effect the astronaut would have traveled into the future, said Dr. J. Richard Gott, a Princeton astrophysicist.