Evidence of a Gravitational Redshift
Relativity predicted that time slows down in a gravitational field, an effect known as the gravitational redshift. During the 1960s, scientists at Harvard University utilized a phenomenon called the Mössbauer effect to verify whether clocks actually do slow down in regions of greater gravity. They used the clock-like property of the radioactive nuclei of certain atoms, assuming that these atoms emitted gamma radiation, or number of "ticks", at a precisely periodic frequency. They performed the experiment on the roof and in the basement of a tall building. Measuring the frequencies of gamma ray emission, they found that atoms in the greater gravitational field of the basement, which is closer to the Earth, released radiation on a slightly less frequent period than atoms on the roof. Their findings coincided with Einstein's predictions with an accuracy of more than 1 percent.
Tests of Relativity