A 22-metre tower at Harvard proved gravity itself slows down light

In 1959, Robert Pound and Glen Rebka Jr. measured how gamma rays falling just 22.5 metres down a tower at Harvard's Jefferson Laboratory gained a minuscule, precisely predicted amount of energy from Earth's gravitational field. It was the first laboratory test — rather than an astronomical one — of Einstein's prediction of gravitational redshift from general relativity, and it matched theory to within about 1%.

— Robert V. Pound and Glen A. Rebka Jr., Apparent Weight of Photons — Physical Review Letters, 1960