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Time dilation arises from Einstein's special theory of relativity (1905), which is built on two postulates: the laws of physics are the same in all inertial frames, and the speed of light in a vacuum is constant for all observers regardless of their motion. From these postulates it follows mathematically that if a clock moves relative to an observer, the observer measures it to tick more slowly. The Lorentz factor γ = 1/√(1−v²/c²) quantifies the effect. At everyday speeds the effect is negligible; at 87% of the speed of light a clock runs at half speed. The effect has been confirmed experimentally many times. The most direct test was the Hafele–Keating experiment (1971), in which caesium atomic clocks flown around the world in aircraft were compared with identical clocks on the ground — the results matched relativistic predictions. GPS satellites must have their clocks corrected for both special-relativistic (velocity) and general-relativistic (gravity) time dilation to maintain metre-level accuracy.
answered by Omniscientia Team · 178 words · 18 Mar 2026