Gravitational Redshift

Gravitational redshift is the shift in the frequency and wavelength of electromagnetic radiation — toward lower energy and longer wavelength — as the radiation climbs out of a gravitational potential well. It is a direct consequence of the equivalence principle and the time dilation of clocks in curved spacetime: light emitted from a region of stronger gravity is observed at a lower frequency by a distant observer, because the clocks at the emission point run slower relative to the observer's clocks. The effect is not a Doppler shift due to relative motion; it is a geometric effect due to the curvature of spacetime itself.

The gravitational redshift was predicted by Einstein in 1911, before the completion of general relativity, and was first measured by Pound and Rebka in 1959 using gamma rays climbing 22.5 meters in the Harvard Jefferson Laboratory tower. The measured shift matched the prediction to within 1%. Modern measurements using atomic clocks and GPS satellites confirm the effect to parts in 10^15, making gravitational redshift one of the most precisely verified predictions of general relativity.

The effect is ubiquitous in astrophysics. Light from the surface of a white dwarf is redshifted by the white dwarf's strong surface gravity; spectral lines from the accretion disk of a black hole are redshifted by the gravitational potential near the event horizon. The extreme redshift at the horizon itself — where the frequency approaches zero as seen by a distant observer — is the reason black holes are 'black': any light emitted from the horizon is infinitely redshifted before reaching infinity.

Gravitational redshift is often taught as a 'test' of general relativity, one of the classic three (along with light bending and perihelion precession). This framing is wrong. Gravitational redshift is not a test of general relativity in the way that a laboratory experiment tests a hypothesis. It is a definitional consequence of the equivalence principle, which is itself a foundational assumption of the theory. If the equivalence principle holds, gravitational redshift must hold. The Pound-Rebka experiment did not test general relativity; it tested the equivalence principle. Confusing these levels of theoretical structure is a common error in physics pedagogy, and it persists because the 'three tests' narrative is more memorable than the 'foundational assumption' narrative. But the memorable narrative is misleading.