Two solutions of these equations for the same current configuration differ by a solution of the vacuum wave equation ∂ ∂ μ μ ∂ ∂ μ μ A ν ν = 0. In this form it is clear that the components of the potential separately satisfy the Klein–Gordon equation, and hence that the Lorenz gauge condition allows transversely, longitudinally, and "time-like" polarized waves in the four-potential. The transverse polarizations correspond to classical radiation, i.e., transversely polarized waves in the field strength. To suppress the "unphysical" longitudinal and time-like polarization states, which are not observed in experiments at classical distance scales, one must also employ auxiliary constraints known as Ward identities. Classically, these identities are equivalent to the continuity equation ∂ ∂ μ μ j μ μ = 0.