The inhomogeneous electromagnetic wave equation for the electric field can be written in terms of the electric Hertz Potential, π π e , in the Lorenz gauge as ∇ ∇ 2 π π e − − 1 c 2 ∂ ∂ 2 π π e ∂ ∂ t 2 = − − P ϵ ϵ 0 .