Clock Hypothesis of Relativity Theory, Maximal Acceleration, and M\"ossbauer Spectroscopy

Results obtained several years ago using the high-resolution 93.3 keV M\"ossbauer resonance in $^{67}$ZnO and $\beta'$-brass have been reanalyzed with the notion that the clock hypothesis of Special Relativity Theory is not sufficient, but that a maximal acceleration $a_m$ exists and that an acceleration $a$ contributes to the temperature dependence of the center shift by a term $\pm(1/2)(a/a_m)^2$. The significance of the sign of this term is discussed in detail. For both substances a lower limit of $a_m>1.5\cdot10^{21}m/s^2$ is inferred which is more than two orders of magnitude larger than the value $a_m=1\cdot10^{19}m/s^2$ suggested by $^{57}$Fe rotor experiments.