Earliest criticisms of assumed P and T symmetries

The origins of the 1950 paper by Purcell and Ramsey are discussed. In that publication the authors pointed out that the then universal belief in parity invariance for nuclear particles and nuclear forces was based on assumptions and that there was no evidence to justify them in the cases of nuclear particles and forces. As a test of the parity assumption they proposed a sensitive search for a neutron electric dipole moment but none was found in the subsequent experiment. In 1957, when Yang, Lee, and Wu et al. found there was a strong violation of parity conservation in the weak nuclear force, a number of leading theorists pointed out that there still could be no electric dipole moment because of time reversal symmetry. Ramsey and Jackson, then independently emphasized that time reversal invariance, like parity at an earlier period, was merely an assumed symmetry with there being little direct evidence for the assumption in the case of strong and weak interactions. These early publications, however, were mostly ignored until the experimental discovery of CP non-conservation by Christenson et al. in 1964