Can one of the advanced theoretical physics people here answer this question. Is there a Lagrangian, either classical or quantum, that describes the dynamics of the electromagnetic field together with charged particles, electrons if we need to be specific? From what I see, there is a Lagrangian in which the field variables are varied, with fixed charges and currents, to give Maxwell's equations. A second Lagrangian with particle coordinates varied, for fixed fields, gives the Lorentz force law. And finally a third Lagrangian (see Landau & Lifschitz, "Classical Theory of Fields") tries to meld the two together, but is only valid to order (v/c)^2. I am looking for One Lagrangian that rules them all, one that determines the exact theory of electrodynamics. Gene __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com