Time dilation
Einstein's relativity predicts that moving clocks run slow and clocks deeper in gravity run slow, effects measured with atomic clocks.
Einstein's theories of relativity say that time does not pass at the same rate for everyone. In special relativity, a clock moving relative to you ticks more slowly than one at rest beside you. In general relativity, a clock deeper in a gravitational field, nearer a large mass, ticks more slowly than one higher up. Both effects are called time dilation.
At everyday speeds and heights the differences are minute, but atomic clocks can measure them. In October 1971 the physicists Joseph Hafele and Richard Keating flew four caesium-beam clocks around the world on ordinary airliners, first eastward and then westward, and compared them with clocks at the US Naval Observatory. The eastbound clocks lost about 59 billionths of a second and the westbound clocks gained about 273 billionths. Both results agreed with relativity's predictions within the experimental uncertainty, once the effect of speed and the effect of height, which makes clocks at altitude run fast, were added together.
This is not just theory. Satellite navigation systems such as GPS rely on precise clocks moving fast and high above Earth, and both relativistic effects are routinely built into their calculations; without the corrections, positions would soon drift badly.
A common misconception is that time dilation is a fault in the clocks. It applies to every process equally, including the ageing of living things.
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