What experiment first measured gravitational redshift using gamma rays on Earth?

The Pound–Rebka experiment first measured gravitational redshift using gamma rays on Earth. It tested the prediction that light loses energy, and therefore shifts toward longer wavelengths, when moving upward through Earth’s gravitational field.

Robert Pound and his graduate student Glen Rebka performed the experiment in 1959 at Harvard University’s Jefferson Tower. They placed an iron-57 gamma-ray source and absorber 22.5 metres apart vertically. The experiment used the Mössbauer effect, which produces extremely narrow gamma-ray lines, allowing the tiny frequency shift to be detected.

The measured shift agreed with Einstein’s equivalence principle and general relativity to roughly one percent. The result was remarkable because the height difference was small and the predicted change was minuscule. Pound and Joseph Snider later improved the measurement in 1964–1965. The Hafele–Keating experiment tested relativistic time dilation with airborne atomic clocks, while Michelson–Morley tested the effects of Earth’s motion through the supposed ether. “Pound–Snider” refers to the later improved experiment, so the original experiment’s canonical name is Pound–Rebka.