  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 describes
electromagnetic dissipation and decoherence. As an application, we recalculate
the Weisskopf-Wigner formula for the natural line width of an atomic state at
zero temperature and find, in addition, the temperature broadening caused by
the decoherence term.
