The Weight of Light

Harvard Univ. & Physics in Perspective 2, 224 (2000)br>The highs and lows. Robert Pound, stationed at the top of a tower in a Harvard physics building (top), communicated by phone with Glen Rebka in the basement during calibrations for their experiment. The team verified Einstein's prediction that gravity can change light's frequency.

Pound and Rebka placed an emitter at the top of a tower in the Jefferson Physical Laboratory at Harvard and installed a detector 74 feet below. By measuring the detection rate as they jiggled the emitter up and down slightly, the researchers could find the velocity difference between source and detector that compensated for the gravitationally induced change of frequency.

By reversing their experimental set-up to also measure the frequency shift of gamma rays going up the tower, Pound and Rebka could eliminate several sources of experimental error. The difference between the up and down measurements--a frequency change of only a few parts in 1015--represented the pure gravitational effect and matched Einstein's prediction to 10 percent accuracy. By 1964 they had improved the agreement to within one percent [1].