The Pound-Rebka experiment was an experiment by Robert Pound, and Glen Rebka that was done in 1960. In his general theory of relativity, Albert Einstein predicted that gravitation would influence photons the same way it did massive particles: Photons gain energy when traveling toward a gravitational source (the Earth). The experiment measures the change of frequency of light moving in a gravitational field. In this case, the effect was a blue shift, the energy increased.