Time dilation

Time dilation is a difference in elapsed time between clocks caused by relative motion or differences in gravitational potential. It is a measurable consequence of special and general relativity.

Lorentz factor: A factor relating measurements between inertial frames moving at a constant relative velocity. It quantifies the motion-dependent time dilation predicted by special relativity.
Hafele–Keating experiment: A 1971 experiment that compared atomic clocks flown around Earth with clocks kept on the ground. Its airborne clocks registered elapsed-time differences from both motion and Earth's gravity.
Twin paradox: A relativity scenario in which twins following different spacetime paths reunite at different ages. It shows how different travel histories produce unequal elapsed times without a contradiction.
Relativistic geodesy: The use of precise clock comparisons to measure Earth's gravitational potential and height. Clock-rate differences provide a way to map elevation and the geoid.
Proper time: The time measured by a clock along its own path through spacetime. It distinguishes a clock's experienced elapsed time from coordinate-time comparisons.
Pound–Rebka experiment: A 1959 experiment measuring the gravitational frequency shift of gamma rays over a short height. The measured shift tested the gravitational clock-rate difference across Earth's field.
Length contraction: The shortening of an object's measured length along its direction of motion relative to an observer. It is another frame-dependent relativistic effect, but concerns lengths rather than clock readings.
Atomic clock: A clock whose timing is based on stable frequencies of atomic transitions. Atomic clocks supply the precision needed to observe and correct relativistic time shifts.
Spacetime interval: A quantity combining spatial and temporal separations that remains invariant between inertial frames. Its geometry explains why observers assign different time intervals to separated events.
GPS: A satellite navigation system that determines positions using timed radio signals. Satellite clocks require relativistic rate corrections to maintain accurate positioning.

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