Pound–Rebka experiment
The Pound–Rebka experiment monitored frequency shifts in gamma rays as they rose and fell in the gravitational field of the Earth. The experiment tested Albert Einstein's 1907 and 1911 predictions, based on the equivalence principle, that photons would gain energy when descending a gravitational potential, and would lose energy when rising through a gravitational potential.[p 1][p 2] It was proposed by Robert Pound and his graduate student Glen A. Rebka Jr. in 1959,[p 3] and was the last of the classical tests of general relativity to be verified.[p 4] The measurement of gravitational redshift and blueshift by this experiment validated the prediction of the equivalence principle that clocks should be measured as running at different rates in different places of a gravitational field. It is considered to be the experiment that ushered in an era of precision tests of general relativity.
Pound and Rebka's experiment[edit]
Sources of error[edit]
After evaluating various γ-emitters for their study, Pound and Rebka chose to use 57Fe because it does not require cryogenic cooling to exhibit recoil-free emission, has a relatively low internal conversion coefficient[note 2] so that it is relatively free of competing X-ray emissions that would have been difficult to distinguish from the 14.4 keV transition,[note 3] and its parent 57Co has a usable half-life of 272 days.[5]
Pound and Rebka found that a large source of systematic error resulted from temperature variations, which they attributed primarily to a second order relativistic Doppler effect due to lattice vibrations. A mere 1°C difference in temperature between emitter and absorber caused a shift about equal to the predicted effect of gravitational time dilation.[p 7][6]
They also found frequency offsets between the lines of different combinations of source and absorber stemming from the sensitivity of the nuclear transition to an atom's physical and chemical environment.[note 4] They therefore needed to adopt methodology which would allow them to distinguish these offsets from their measurement of gravitational redshift. Extreme care was also needed in sample preparation, otherwise inhomogeneities would limit the sharpness of the lines.[p 4]