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=mc˛ 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.
The slowing clock prediction has been confirmed by flying atomic clocks around Earth on jets.
"If you take a plane east around the world you will come back 59 nanoseconds younger than if you had stayed home," Dr. Gott said.
The record holder for this type of travel, he said, is the Russian astronaut Sergei Krikalev, who came back from 748 days orbiting in the Mir space station a full one-fiftieth of a second younger than he would have if he had stayed on the ground.
In his 1905 paper Einstein predicted that because of the rotation speed of Earth, clocks would also run slower at the Equator than the poles, but that turned out to be wrong.
In a recent article in Physics Today, Dr. Alex Harvey of Queens College in New York and Dr. Engelbert Schucking of New York University pointed out that Einstein had not taken account of an effect of general relativity, then 10 years in the future, which says that clocks run slower the more deeply in a gravitational field they sit.
The rotation of Earth causes it to bulge at the Equator, lifting clocks there and making them run slightly faster relative to those at the poles by just enough to compensate for the extra speed.
So the two effects just cancel out, and clocks at the Equator and poles run at the same speed. "It's a deep coincidence," Dr. Gott said.
The two effects could be combined for an even deeper trip into the future by going to Mercury, which is both deep in the gravitational field of the Sun and also zooming around it at high speed. A 30-year stay there, Dr. Gott said, would save 22 seconds of an astronaut's life.
A few seconds might not sound like much of a trip in time, but Dr. Gott points out that astronauts haven't been that far into space, either. The Moon, humanity's most distant destination so far, is only about 1.3 light-seconds away, about like hopping over the Atlantic, he said.
"The astronauts are the Lindberghs of time travel," Dr. Gott said.