  We investigate strong field vacuum effects using a phase space approach based
on the Wigner formalism. We calculate the Wigner function in a strong
null-field background exactly, using lightfront field theory. The Wigner
function exhibits the distinct features of strong field QED: in particular we
identify the effective mass in a laser pulse, compare it to the well-known mass
shift in a periodic plane wave and identify signals of multi-photon absorption
and emission. Finally, we show how to extend our results to describe vacuum
pair production in colliding laser pulses.
