Time dilation is the "slowing down" of a clock as determined by an observer who is in relative motion with respect to that clock. In special relativity, an observer in inertial (i.e., nonaccelerating) motion has a well-defined means of determining what events occur simultaneously with a given event. A second inertial observer, who is in relative motion with respect to the first, however, will disagree with him regarding which events are simultaneous with that given event. (Neither observer is wrong in his determination; rather, their disagreement merely reflects the fact that simultaneity is an observer-dependent notion in special relativity.) A notion of simultaneity is required in order to make a comparison of the rates of clocks carried by the two observers. If the first observer's notion of simultaneity is used, it is found that the second observer's clock runs slower than his by a factor of {sqroot (1 - v2/c2)}, where v is the relative velocity of the observers and c equals 300,000 km (186,000 miles) per second--i.e., the speed of light. Similarly, using the second observer's notion of simultaneity, it is found that the first observer's clock runs slower by the same factor. Thus each inertial observer determines that all clocks in motion relative to him run slower than his own clock.

A closely related phenomenon predicted by special relativity is the so-called clock paradox, or twin paradox. Suppose an observer carrying a clock departs on a rocket ship from an inertial observer at a certain time and then rejoins him at a later time. In accordance with the time-dilation effect, the elapsed time on the clock of the noninertial observer will be smaller than that of the inertial observer--i.e., the noninertial observer will have aged less than the inertial observer when they rejoin.

The time-dilation effect predicted by special relativity has been accurately confirmed by observations of the increased lifetime of unstable elementary particles traveling at nearly the speed of light. The clock-paradox effect also has been substantiated by experiments comparing the elapsed time of an atomic clock on Earth with that of an atomic clock flown in an airplane. The latter experiments, furthermore, have confirmed a gravitational contribution to time dilation, as predicted by the theory of general relativity.

Excerpt from the Encyclopedia Britannica without permission.