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In the theory of special relativity, the slowing down of a clock as perceived by an observer in relative motion with respect to that clock. Time dilation becomes noticeable only at speeds approaching that of light and has been accurately confirmed by the apparent increased lifetime of unstable subatomic particles traveling at nearly the speed of light and by the precise timing of atomic clocks carried on airplanes. See alsoLorentz-FitzGerald contraction.
This entry comes from Encyclopædia Britannica Concise. For the full entry on time dilation, visit Britannica.com.