In SI units : [ ( γ γ 1 ) μ μ ( p 1 − − A ~ ~ 1 ) μ μ + m 1 c + S ~ ~ 1 ] Ψ Ψ = 0 , [ ( γ γ 2 ) μ μ ( p 2 − − A ~ ~ 2 ) μ μ + m 2 c + S ~ ~ 2 ] Ψ Ψ = 0. where c is the speed of light and p μ μ = − − i ℏ ℏ ∂ ∂ ∂ ∂ x μ μ