where a τ τ , e , m , and μ μ are polarization four-vector, charge, mass, and magnetic moment, u τ τ is four-velocity of electron (in a system of units in which c = 1 ), a τ τ a τ τ = − − u τ τ u τ τ = − − 1 , u τ τ a τ τ = 0 , and F τ τ σ σ is electromagnetic field-strength tensor. Using equations of motion,