Motion of Spin-1/2 Particles in External Gravitational and   Electromagnetic Fields
  (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. 
Stamatis Vokos