Mechanical momentum in nonequilibrium quantum electrodynamics
The reformulation of field theory in which self-energy processes are no longer present [Annals of Physics, {\bf311} (2004), 314.], [ Progr. Theor. Phys., {\bf 109} (2003), 881.], [Trends in Statistical Physics {\bf 3} (2000), 115.] provides an adequate tool to transform Swinger-Dyson equations into a kinetic description outside any approximation scheme. Usual approaches in quantum electrodynamics (QED) are unable to cope with the mechanical momentum of the electron and replace it by the canonical momentum. The use of that unphysical momentum is responsible for the divergences that are removed by the renormalization procedure in the $S$-matrix theory. The connection between distribution functions in terms of the canonical and those in terms of the mechanical momentum is now provided by a dressing operator [Annals of Physics, {\bf314} (2004), 10] that allows the elimination of the above divergences, as the first steps are illustrated here.