- Variation in the apparent rate at which time passes as measured by clocks moving with relative velocities which are a significant fraction of the velocity of light. Time Dilation is a consequence of the Special Theory of Relativity and its precise size can be obtained from the Lorentz Transformations, which itemise the effects of special relativity at velocities close to that of light. The effect of time dilation is to make clocks travelling at high velocities appear to run slow as seen by a stationary observer. Thus a space traveller making a long journey at velocities close to that of light could come back still young to find that her or his friends had long since died of old age.
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