"... That gravity warps time was verified in 1960 in a terrestrial experiment performed by R. V. Pound and G. A. Rebka of Harvard University. They set out to show that at two different points in a gravitational field, at the top and at the bottom of a tower, for example, time passes at different rates.
Pound and Rebka did not have to look far for a suitable tower. As Pisa has its own tower, so too the Harvard physics dept. A simple calculation using the equivalence principle predicts that two clocks placed at the top and bottom of the tower attached to the Harvard physics building would differ by one sec after 100 million yrs. To detect such a fabulously tiny deviation, Pound and Rebka had to stretch their ingenuity to the limit.
The experimenters used a photon as a clock. We all know that electromagnetic waves oscillate at definite frequencies, a fact enabling us to tune in our favorite programs on radio and television. Thus a photon, oscillating at some definite number of cycles per sec, can serve as a clock. Pound and Rebka beamed photons down the Harvard tower and meticulously measured their frequencies at the top and at the bottom of the tower. If time passes at different rates at the top and at the bottom of the tower, then the frequency of a photon as it travels down the tower, should change slight, as it did. The experiment dramatically confirmed Einstein's theory. Years later, Pound would joke that he learned the true meaning of gravity while lugging the heavy experiment equipment up and down the tower."
p. 89 - Warped Time.
Experiment equipment for Pound-Rebka experiment.