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.
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The experiment compares gamma-ray emission and resonant absorption at different heights in Earth's field, using the Mössbauer effect to resolve the small gravitational redshift. Its weak-field result is the frequency shift for an upward ray over height . Any metric theory with the corresponding static lapse predicts this leading effect; the test alone does not distinguish general relativity from the weak-field prediction of Nordstrom theory of gravitation.