The relativistic Maxwell's equations can be written in covariant form as ◻ ◻ A μ μ = d e f ∂ ∂ β β ∂ ∂ β β A μ μ = d e f A μ μ , β β β β = − − μ μ 0 J μ μ SI ◻ ◻ A μ μ = d e f ∂ ∂ β β ∂ ∂ β β A μ μ = d e f A μ μ , β β β β = − − 4 π π c J μ μ cgs where ◻ ◻ = ∂ ∂ β β ∂ ∂ β β = ∇ ∇ 2 − − 1 c 2 ∂ ∂ 2 ∂ ∂ t 2 is the d'Alembert operator, J μ μ = ( c ρ ρ , J ) is the four-current, ∂ ∂ ∂ ∂ x a = d e f ∂ ∂ a = d e f , a = d e f ( ∂ ∂ / ∂ ∂ c t , ∇ ∇ ) is the 4-gradient, and A μ μ = ( φ φ / c , A ) SI A μ μ = ( φ φ , A ) cgs is the electromagnetic four-potential with the Lorenz gauge condition ∂ ∂ μ μ A μ μ = 0 .