In the Pound–Rebka experiment of 1959, which effect on gamma rays was measured in Earth’s gravitational field?

The 1959 Pound–Rebka experiment measured gravitational redshift in gamma rays.

Robert Pound and Glen Rebka placed a gamma-ray source and absorber at different heights in the Jefferson Tower at Harvard University. General relativity predicts that light loses energy as it moves upward through a gravitational field, producing a tiny shift toward lower frequency, or redshift.

The experiment used the Mössbauer effect, which allows gamma rays to be emitted and absorbed with extremely small recoil. Pound and Rebka used a moving source to apply a controlled Doppler shift and identify the gravitational change in the gamma-ray frequency. Their measured result closely matched Einstein’s prediction.

The effect was extremely small because the height difference was only about 22.6 metres. It was not ordinary Doppler redshift, which comes from relative motion, and it was not Compton scattering, which changes photon wavelength through collisions with electrons.