Gravitational redshift is the phenomenon where light (or any electromagnetic radiation) loses energy as it climbs out of a gravitational well, shifting toward the red end of the spectrum. Conversely, light falling into a gravitational well gains energy and shifts toward the blue. This is a direct consequence of Einstein's general relativity: gravity affects the flow of time, and a photon's frequency is tied to the ticking of a clock at its location. The effect is tiny on Earth's surface but becomes significant near compact objects like neutron stars or black holes, and it is a key prediction that distinguishes general relativity from Newtonian gravity.
History
The concept was first predicted by Albert Einstein in 1907 and refined in his 1915 general theory of relativity. The first terrestrial measurement was made by Robert Pound and Glen Rebka in 1959 using gamma rays over a 22.5-meter tower at Harvard University.
How it works
In general relativity, a clock deeper in a gravitational potential runs slower than one higher up. A photon emitted from the lower clock has a frequency set by that slower ticking; when it reaches the higher observer, it is compared to their faster clock, so it appears to have a lower frequency — hence redshifted. The energy loss is not a frictional effect but a consequence of the geometry of spacetime: the photon's path through curved spacetime conserves energy in a way that depends on the gravitational potential difference between emission and reception.
In practice
Gravitational redshift is routinely accounted for in Global Positioning System (GPS) satellites, where clocks on orbit run faster than those on the ground by about 38 microseconds per day; corrections are applied to maintain positioning accuracy.
Fun fact
The effect is so precisely measured that atomic clocks on Earth's surface tick at different rates when separated by just one meter of altitude.
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