Gravitational time dilation

Gravitational time dilation is the difference in elapsed time between locations at different gravitational potentials. A clock deeper in a gravitational field accumulates less time than one farther away.

Gravitational potential: A scalar measure of gravitational energy per unit mass at a location. Differences in gravitational potential determine the leading-order rate difference between nearby clocks.
Global Positioning System: A satellite-based system that determines position and time using signals from orbiting atomic clocks. GPS accounts for both gravitational and motion-related clock shifts to keep positions accurate.
Special-relativistic time dilation: The difference in elapsed time between observers moving at different velocities in flat spacetime. Satellite clocks also experience this kinematic effect, distinct from their gravitational rate difference.
Gravitational time dilation in the early universe: The relation between local clock rates and gravitational potentials in cosmological models of the early universe. Large-scale gravitational structure complicates comparisons between cosmic time and local clock readings.
Gravitational redshift: The change in a signal’s frequency as it travels through a gravitational field. A photon’s frequency shift provides an observable counterpart to gravitational clock-rate differences.
Atomic clock: A clock that uses an atomic transition frequency as its timekeeping reference. High precision lets atomic clocks detect gravitational rate differences across modest height changes.
Newtonian gravity: A theory describing gravity as a force between masses in absolute space and time. It predicts gravitational potential but lacks the relativistic change in clock rates.
Geoid: An equipotential surface approximating mean sea level and extended beneath continents. Clocks at the same geopotential rate define a practical reference surface for precise timekeeping.
Schwarzschild metric: A solution to Einstein’s field equations describing spacetime outside a spherical, nonrotating mass. Its time component gives the clock-rate variation outside an idealized spherical body.
Pound–Rebka experiment: A 1959–1960 experiment that measured gravitational redshift in gamma rays traveling up a tower. It tested gravitational clock-rate differences through the frequency shift of emitted photons.

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