Pound–Rebka experiment

Experiment designed to test Albert Einstein's theory of general relativity
Jefferson laboratory at Harvard University. The experiment occurred in the left "tower". The attic was later extended in 2004.

In the Pound–Rebka experiment, the emitter was placed at the top of a tower with the receiver at the bottom. General relativity predicts that the gravitational field of the Earth will cause a photon emitted downwards (towards the Earth) to be blueshifted (i.e. its frequency will increase) according to the formula:

The frequency shift for the two cases has the same magnitude but opposing signs. When subtracting the results, Pound and Rebka obtained a result twice as big as for the one-way experiment.