Relativity and time

Einstein's theory of relativity describes time not as absolute but as depending on the observer. Special relativity (1905) shows that fast-moving clocks run slower; general relativity (1915) that clocks in a stronger gravitational field run slower.

These effects are small but real and measurable with atomic clocks, even over height differences of a few centimetres. Precise timekeeping and satellite navigation are impossible without relativistic corrections.