The Dirac spinor is the bispinor u ( p → → ) in the plane-wave ansatz ψ ψ ( x ) = u ( p → → ) e − − i p ⋅ ⋅ x of the free Dirac equation for a spinor with mass m , ( i ℏ ℏ γ γ μ μ ∂ ∂ μ μ − − m c ) ψ ψ ( x ) = 0 which, in natural units becomes ( i γ γ μ μ ∂ ∂ μ μ − − m ) ψ ψ ( x ) = 0 and with Feynman slash notation may be written ( i ∂ ∂ / − − m ) ψ ψ ( x ) = 0