Gravitational redshift

Gravitational redshift is the decrease in light frequency as light moves to a region of higher gravitational potential. It is a consequence of gravity’s effect on spacetime and on measurements of time.

Gravitational potential: A scalar quantity describing the gravitational potential energy per unit mass at a location. Differences in this quantity determine the frequency shift between emission and observation.
Pound–Rebka experiment: A 1959–1960 experiment that measured gamma-ray frequency shifts over the height of the Jefferson Tower. It directly detected gravitational redshift over a 22.5-meter vertical distance.
General relativity: Einstein’s theory describing gravity as the curvature of spacetime produced by matter and energy. Gravitational redshift is a prediction of its account of gravity and time.
Gravitational time dilation in GPS: The rate difference between GPS satellite clocks and clocks on Earth caused by gravitational potential. The same gravitational clock-rate difference underlies frequency comparisons between GPS signals and ground references.
Gravitational time dilation: The difference in elapsed time measured by clocks at different gravitational potentials. Different clock rates explain why light emitted and received at different heights has different frequencies.
Gravity Probe A: A 1976 space experiment comparing a hydrogen maser clock on a rocket with a ground clock. Its clock comparison tested gravitational redshift over a much larger change in altitude.
Frequency: The number of repetitions of a periodic phenomenon per unit time. Redshift is defined by a change in the frequency measured by an observer.
Gravitational energy: Energy associated with the configuration or interaction of masses in a gravitational field. A photon’s energy as measured by an observer changes with gravitational potential, while local measurements remain consistent.
Equivalence principle: The principle that local effects of gravity are equivalent to those of acceleration. An accelerating rocket predicts a frequency shift that matches the gravitational effect locally.
Solar gravitational redshift: The frequency shift of light escaping the Sun’s gravitational field. Solar spectral lines provide an astronomical setting for measuring the effect.

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