Time is not a physical constant. Motion and gravity effect time by dilating (slowing) it or by expanding its duration. In 1905 Albert Einstein described the effect of motion on time in his special theory of relativity. In 1916 he described the effect of gravity on time in his general theory of relativity.
Time dilation effects due to motion were experimentally observed in the early 1970s. Researchers placed atomic clocks on commercial airliners and observed the expected changes in time as measured by those clocks relative to similar clocks on the ground. In particular, when the planes traveled east, in the direction of Earth’s rotation, the clocks on the airliners were 59 nanoseconds (59 billionths of a second) slow relative to the atomic clocks on the ground. When the airplanes traveled west, the clocks were 273 nanoseconds fast. This discrepancy is caused by the rotation of Earth, which causes an additional time dilation. If the effect of Earth's rotation is removed, the time dilation produced by the motion of the airliners confirms Einstein's theory of how time changes with motion, as the dilation is in agreement with predictions made by the theory.
Time dilation effects due to gravity have been experimentally verified in many ways. For example, time on the Sun's surface runs about two parts in a million slower than on Earth because of the Sun's much higher gravity. In 1968 American physicist Irwin Shapiro confirmed this effect when he showed that radar signals (see Radar Astronomy) and their reflections from planets are delayed when the Sun is near the path of the signals.
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also called Time Dilatation, in the theory of special relativity, 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 Ö(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 bythe 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.