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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.

For a photon in the flat-background scalar-force theory of gravity, $e^{\Phi }{p}^i$ is constant along its path. In a static field, energies measured by stationary laboratories therefore satisfy the displayed ratio. At first order, $\Delta \nu /\nu =-\Delta \Phi$ in units $c=1$, matching the weak-field Pound-Rebka experiment. This agreement does not imply light bending: a momentum change parallel to the original null tangent changes its scale, not its direction.