The vector potential in the Coulomb gauge is described by ∇ ∇ 2 A − − 1 c 2 ∂ ∂ 2 A ∂ ∂ t 2 = − − 4 π π c J t where the transverse current J t is the solenoidal current (see Helmholtz decomposition) generated by a second particle. The divergence of the transverse current is zero.