  (Talk presented at the 7th Marcel Grossmann Meeting on General Relativity,
Stanford, CA, July 24-30, 1994) We study the semi-classical limit of the
solution of the Dirac equation in a background electromagnetic/gravitational
plane wave. We show that the exact solution corresponding to an asymptotically
fixed incoming momentum satisfies constraints consistent with the classical
notion of a spinning particle. In order to further analyze the motion of a
spinning particle in this external inhomogeneous field one has to consider
wave-packet superpositions of these exact solutions. We are currently
investigating the existence of a classical theory of a phenomenological spin
tensor which reproduces our quantum-mechanical results.
