Shortly after the 1957 discovery that mirror symmetry (conservation of parity P) is violated in the weak interactions, Edward Purcell and Norman Ramsey devised an experiment to look for a nonzero electric dipole moment (EDM) in the neutron—which P conservation would have forbidden. They found none, but they were able to set an upper limit of 5 × 10−20e·cm on its magnitude—impressively small with the available technology.
That null result would have come as a relief to theorists of the day. Though P conservation had been overthrown, it was believed that the combination CP was still a good symmetry operation (C being the replacement of particles by their antiparticles). That is, particles were presumed always to behave like their antiparticles viewed in a mirror. And CP conservation itself forbids an EDM for any elementary particle.
Seven years later, however, CP conservation was found to be...
