Method of Characteristics and Causality of Field Propagation

It is pointed out that the method of characteristics gives only a necessary condition (not a sufficient one) for causality of field propagation. It is shown that the usual theory of vector fields with anomalous magnetic moment coupling to an external magnetic field is noncausal, contrary to what has been believed, as one of its modes of propagation turns out to be tachyonic, with the velocity of light as minimum velocity.