Master and Langevin equations for electromagnetic dissipation and decoherence of density matrices

We set up a forward - backward path integral for a point particle in a bath of photons to derive a master equation for the density matrix which descri bes electromagnetic dissipation and decoherence. We also derive the associa ted Langevin equation. As an application, we recalculate the, Wigner-Weissk opf formula for the natural line width of an atomic state at zero temperatu re and find, in addition, the temperature broadening caused by the decohere nce term. Our master equation also yields the correct Lamb shift of atomic levels. The two equations may have applications to dilute interstellar gase s or to few-particle systems in cavities.