A wave equation for massive particles of arbitrary spin
A wave equation is given which, in the force-free case, describes a particle of unique spin and mass and which remains consistent when interactions are included. The equation is simple in the sense that it involves only matrices which satisfy the Pauli commutation rules. This simplicity is achieved at the expense of extending the particle field vector from one of 2j + 1 components to one of 4j. However, the extra 2j — 1 components are just what are needed to remove the inconsistency of the interacting system. In the force-free case these additional components vanish and the resulting equations are equivalent to the usual ones of Dirac, Fierz and Pauli. One the other hand, for the interacting system, the extra components do not vanish. The second order propagation equations are deduced in the case of external electromagnetic and gravitational fields.