Pound–Rebka experiment

The Pound–Rebka experiment is a well known experiment to test Albert Einstein's theory of general relativity. It was proposed by Robert Pound and his graduate student Glen A. Rebka Jr. in 1959, and was the last of the classical tests of general relativity to be verified (in the same year). It is a gravitational redshift experiment, which measures the redshift of light moving in a gravitational field, or, equivalently, a test of the general relativity prediction that clocks should run at different rates at different places in a gravitational field. It is considered to be the experiment that ushered in an era of precision tests of general relativity.

Consider an electron bound to an atom in an excited state. As the electron undergoes a transition from the excited state to a lower energy state it will emit a photon with a frequency corresponding to the difference in energy between the excited state and the lower energy state. The reverse process will also occur: if the electron is in the lower energy state then it can undergo a transition to the excited state by absorbing a photon at the resonant frequency for this transition. In practice the photon frequency is not required to be at exactly the resonant frequency, but must be in a narrow range of frequencies centred on the resonant frequency: a photon with a frequency outside this region cannot excite the electron to the excited state.

Now consider two copies of this electron-atom system, one in the excited state (the emitter), the other in the lower energy state (the receiver). If the two systems are stationary relative to one another and the space between them is flat (i.e. we neglect gravitational fields) then the photon emitted by the emitter can be absorbed by the electron in the receiver. However, if the two systems are in a gravitational field then the photon may undergo gravitational redshift as it travels from the first system to the second, causing the photon frequency observed by the receiver to be different to the frequency observed by the emitter when it was originally emitted. Another possible source of redshift is the Doppler effect: if the two systems are not stationary relative to one another then the photon frequency will be modified by the relative speed between them.

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