EVOLUTION MODES OF THE VACUUM WIGNER FUNCTION IN STRONG-FIELD QED

The physical content of a non-covariant quantum-field theoretic transp ort formalism for strong-field QED is addressed within an integral for mulation of the evolution equations which highlights the relation to a coustic and Zitterbewegung modes of the field-free limit. Through the computation within this framework of the electric susceptibility of th e vacuum at the one-loop level, it is established that the off-shell b ehaviour of the electron-positron field is correctly incorporated. The Zitterbewegung mode is seen to be solely responsible at the one-loop level for vacuum polarization. Implications for the full renormalisati on of the theory are taken up.