 In 1959, physicists Robert Pound and Glenn Rebka performed an experiment in the Jefferson Physical Lab at Harvard to demonstrate gravitational redshift. It was based on physicist Rudolf Mossbauer's effect, discovered two years earlier, that involves the emission and absorption of gamma rays from the excited states of iron nucleuses. Here we have an iron atom's nucleus in an excited state. When it falls to a lower energy level, a gamma ray photon carrying the energy is emitted. Once this photon enters a like atom, it will be absorbed, raising the energy level of the encountered atom's nucleus. The problem is that when the gamma ray is ejected, the nucleus recoils. As of energy momentum conservation, the recoiling energy reduces the energy of the gamma ray. The gamma ray is no longer a match for the other nuclei, and it moves right through. There is no absorption. What Mossbauer discovered was that if he embeds the iron atoms in a crystal, the recoil is reduced dramatically, and absorption can be reestablished. Brown-Repka used this Mossbauer effect. They placed an emitter at the bottom of a tower in the laboratory, and installed a detector 22.6 meters above it. No absorption was detected because gravitational redshift changed the frequency of the emitted gamma rays so no energy match existed in the detector. The calculated shift was extremely small, but the Mossbauer effect is sensitive enough to measure it. They adjusted the detector's velocity down until absorption occurred. We get the amount the frequency changed using the well-understood relativistic Doppler redshift equation, just like the Doppler shift in Starlight. These results came in within 1.6% of the value predicted by Einstein's field equations using Schwarzschild's metric. Although this experiment did not produce new results, it showed that gravitational redshift, one of General Relativity's most significant findings, was consistent with all physical conservation laws. This gave the General Theory of Relativity three successes out of three tests.