The four Maxwell's equations in a vacuum with charge ρ and current J sources reduce to two equations, Gauss's law for electricity is: ∇ ∇ 2 φ φ + ∂ ∂ ∂ ∂ t ( ∇ ∇ ⋅ ⋅ A ) = − − 1 ε ε 0 ρ ρ , where ∇ ∇ 2 here is the Laplacian applied on scalar functions, and the Ampère-Maxwell law is: ∇ ∇ 2 A − − 1 c 2 ∂ ∂ 2 A ∂ ∂ t 2 − − ∇ ∇ ( 1 c 2 ∂ ∂ φ φ ∂ ∂ t + ∇ ∇ ⋅ ⋅ A ) = − − μ μ 0 J where ∇ ∇ 2 here is the vector Laplacian applied on vector fields. The source terms are now much simpler, but the wave terms are less obvious. Since the potentials are not unique, but have gauge freedom, these equations can be simplified by gauge fixing. A common choice is the Lorenz gauge condition : 1 c 2 ∂ ∂ φ φ ∂ ∂ t + ∇ ∇ ⋅ ⋅ A = 0