Retarded potential

Summary

The retarded potentials are the electric potential and the magnetic vector potential produced by given charge and current densities. Each source point contributes with the delay that light needs to travel to the field point. They are the solution of the inhomogeneous electromagnetic wave equations in the Lorenz gauge that is built from the past of the sources only. They express causality in classical electrodynamics: a change in the sources is felt at distance $d$ only after the time $d / c$.