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 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