In his Special Theory of Relativity, Einstein stated that Newton's idea that space, mass, and time were absolute and unchanging was wrong when put to extreme tests. Because the speed of light is constant under any circumstance -- that is, for moving and stationary observers alike -- Einstein reasoned that space, mass, and time become variable.

That time in the moving reference frame passes more slowly than time in the stationary reference frame is a concept known as time dilation. This means that a clock in motion with respect to a stationary observer is perceived by the observer to run more slowly than a stationary clock. The closer the moving clock's velocity to the speed of light, the more its seconds appear to get stretched out. It's not the mechanics of the clock that get stretched out; it's that time itself gets dilated. Applying this to a biological clock, then, a person traveling on a rocket ship at great speed -- like the twin who travels through space in the "Time Traveler" activity on this Web site -- would age more slowly relative to an observer on Earth.

Though it seems far-fetched, time dilation is a real phenomenon, proven by experiment. In 1971, scientists tested Einstein's theory on time dilation using atomic clocks, which are capable of measuring time to the billionth of a second. One clock was set to record time on the ground, while another traveled around the world at 600 miles per hour on board a jet. Though both clocks showed the same time at the start of the experiment, the jet's on-board clock was behind by a few billionths of a second at the completion. Just as Einstein would have predicted, time had passed more slowly for the clock traveling at a fast speed. Projecting these results over a much greater speed -- like c, the speed of light -- the time difference would be much more exaggerated.