 Pound–Rebka experiment

The Pound-Rebka experiment measured gravitational redshift using the 14.4-keV gamma ray transition of the isotope iron-57. The Mössbauer effect supplies recoil-free emission and resonant absorption in solids. Robert Vivian Pound and Glen Rebka separated a source and an iron absorber vertically by 74 feet, about 22.6 metres. A helium-filled path reduced absorption by air; detectors behind the absorber counted transmitted gamma rays.^[ref]

Moving the source supplied a calibrated Doppler shift. Interchanging source and absorber reversed the gravitational contribution while exposing fixed source–absorber offsets; monitoring temperature controlled a separate shift from thermal motion. The measured reversal agreed with the prediction to about 10 percent.^[ref]

For an upward ray through height $H$ in a weak gravitational field, $\Delta \nu /\nu =-gH/c^2$. At this height its magnitude is about $2.5\times 10^{-15}$. The leading gravitational redshift also follows from Nordstrom theory of gravitation; this experiment alone does not distinguish that prediction from general relativity.

The Pound–Rebka experiment, conducted by physicists Robert Pound and Glen A. Rebka in 1959 at Harvard University, was a pivotal experiment designed to test the predictions of Einstein's theory of general relativity, specifically the gravitational redshift effect. In essence, the gravitational redshift effect posits that light emitted from a source in a gravitational field (such as the Earth) will lose energy as it climbs out of that field.