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. 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. 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.

Background

Equivalence principle argument predicting gravitational red- and blueshift

In the decade preceding Einstein's publication of the definitive version of his theory of general relativity, he anticipated several of the results of his final theory with heuristic arguments. One of these concerned the light in a gravitational field. To show that the equivalence principle implies that light is Doppler-shifted in a gravitational field, Einstein considered a light source

S

2

{\displaystyle S_{2}}

separated along the z-axis by a distance

h

{\displaystyle h}

above a receiver

S

1

{\displaystyle S_{1}}

in a homogeneous gravitational field having a force per unit mass of 1

g

.

{\displaystyle g.}

A continuous beam of electromagnetic energy with frequency

f

2

{\displaystyle f_{2}}

is emitted by

S

2

{\displaystyle S_{2}}

towards

S

1

.