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A nucleus bound in a solid can emit or absorb a photon without exciting lattice vibrations, transferring momentum to the solid as a whole. The resulting recoil energy is negligible compared with the isolated-nucleus recoil, making very narrow resonant emission and absorption lines observable. The Pound-Rebka experiment uses this effect to resolve the small frequency shift from gravitational redshift.
The striking feature is the combination of a large photon energy and a tiny natural linewidth: an atomic nucleus can act as an extraordinarily selective energy filter. The Mössbauer effect supplies a narrow shared emission line and absorption line even when nuclear recoil would separate the corresponding free-nucleus lines. A slow mechanical motion can then tune their overlap through the Doppler effect. The Pound-Rebka experiment is an application to general relativity; the discovery experiment below concerns the recoil-free process itself.