|MadSci Network: Physics|
the gravitational field shifts the frequency of the light. If you send a light up from the surface of earth, the light will be shifted to the red end of the spectrum. A similar experiment has been performed in 1959. The experiment was carried out at Harvard in some tower comparing the frequencies of photons emitted by iron sources. One source was located at the top one at the bottom of the tower, 22 m apart. Due to the Mossbauer effect in iron (Fe57) very small differences in frequency can be observed. So the emitted photon, say from the top, was not reabsorbed by the iron at the bottom. Therefore the light must have been shifted. For more information click e.g. here or here.
I hope this helps,
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