Here c = 1 ε ε 0 μ μ 0 is the vacuum velocity of light, and ◻ ◻ is the d'Alembertian operator with the (+ − − −) metric signature. These equations are not only valid under vacuum conditions, but also in polarized media, if ρ ρ and J → → are source density and circulation density, respectively, of the electromagnetic induction fields E → → and B → → calculated as usual from φ φ and A → → by the equations E = − − ∇ ∇ φ φ − − ∂ ∂ A ∂ ∂ t B = ∇ ∇ × × A