Equations of Motion for Spinning Particles in External\\Electromagnetic and Gravitational Fields
Karl Yee, Myron Bander
TL;DR
The equations of motion for the position and spin of a classical particle coupled to an external electromagnetic and gravitational potential are derived from an action principle and in general the spin is not Fermi-Walker transported nor does the position follow a geodesic.
Abstract
The equations of motion for the position and spin of a classical particle coupled to an external electromagnetic and gravitational potential are derived from an action principle. The constraints insuring a correct number of independent spin components are automatically satisfied. In general the spin is not Fermi-Walker transported nor does the position follow a geodesic, although the deviations are small for most situations.