T violation The discussion of why a nonzero EDM for the neutron violates P and T and thus CP is not strictly true for a spin 1 2 particle the only axis of quantisation and thus the only well defined direction is proportional to its spin Hence we have to define the EDM d to be along that axis the term entering the Hamiltonian is d E which is T odd and P odd Acting with T reverses the neutron s spin and thus d acting with P reverses the direction of E hence giving rise to the CP violation I m not convinced by the magnetic moment picture painted and I may well update the page after consulting with my more learned colleagues 129 67 119 242