Gravitational redshift
Photons climbing out of a gravitational potential lose energy and frequency: Δν/ν = gh/c² for a tower of height h on Earth's surface; equivalently, clocks at lower potential tick slower than clocks at higher potential. A direct consequence of the equivalence principle, derivable without the field equations.
Definition
Gravitational redshift is the energy-and-frequency loss suffered by a photon as it climbs out of a gravitational potential well. For a tower of height h on Earth's surface, a photon emitted at the bottom and received at the top is observed at a frequency reduced by Δν/ν = gh/c² — equivalently, clocks at lower gravitational potential tick slower than clocks at higher potential by the same fractional amount. The effect is a direct consequence of the equivalence principle alone — the same kinematic argument that says a uniformly accelerated rocket would observe identical Doppler shifts on internal photon-exchange experiments — and so is derivable without invoking the full field equations of general relativity. The Pound-Rebka experiment in 1960 measured Δν/ν ≈ 2.46 × 10⁻¹⁵ in a 22.5-meter tower at Harvard's Jefferson Physical Laboratory using the Mössbauer effect, the first laboratory test of GR. The full topic page treats the derivation, the Pound-Rebka apparatus, and the cosmological-redshift connection in detail.