Royal Astronomical Society

Robert V. Pound of Harvard University and Glen A. Rebka, Jr., of Yale University have been awarded the Royal Astronomical Society's Eddington Medal for 1965. They were honored for their series of experiments which confirmed Einstein's prediction that gravitational potential would shift the apparent frequency of electromagnetic radiation. Pound and Rebka took advantage of Mössbauer's (then recent) discovery that atoms in certain crystals would emit gamma rays virtually without recoil, thereby providing a beam of very sharply denned energy. Since the frequency spread of such a beam was less than the predicted gravitational shift over a practicable distance of fall, Pound and Rebka were able to measure the frequency shift sustained by a beam from $Fe57$ as it fell 74 feet in the interior of a tower in Harvard's Jefferson Physical Laboratory. Their initial report in Physical Review Letters for April 1, 1960, indicated measurements averaging 105 percent of Einstein's predicted value with an experimental uncertainty of ten percent. Subsequent work by Pound and J. L. Snider has refined the result to 99.7 percent of the predicted value with an uncertainty of less than one percent.