
pmid: 9959036
A gravitational null red-shift experiment has been carried out using the M\"ossbauer effect to compare the red-shift between two states of $^{57}\mathrm{Fe}$ in different chemical environments. An upper bound of 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ has been found for the relative difference in red-shift, in agreement with the principle of equivalence.
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