energy due to the recoil of the nucleus. (The recoil is to conserve momentum.) When the nucleus is in a crystal, the whole crystal takes up the recoil. It moves very little because of its large mass, and the energy loss by the gamma-ray is small. This means that the gamma-ray energy is well defined.

If the gamma-ray is emitted by a nucleus in one crystal, it can be absorbed by a nucleus in an identical crystal, as long as there is no wavelength shift while the photon is in motion. A group of physicists tried an arrangement in which the gamma-rays were emitted in the basement and absorbed on the roof. The small gravitational redshift was enough for the gamma-rays to arrive at the roof with the wrong wavelength to be absorbed. The gamma-rays could be blueshifted back to the right wavelength by moving the crystal on the roof towards that in the basement. By seeing what Doppler shift is necessary to offset the gravitational redshift, the size of the gravitational redshift can be measured. The result agrees with the theoretical prediction.

A phenomenon related to the gravitational redshift is gravitational time dilation. All oscillators or clocks run slower in a strong gravitational field than they do in a weaker field. If we have two clocks at r1 and r2, the times they keep will be related by the same expression as the gravitational redshift. That is t2 tl

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