The Phase of the Scattering Matrix
  Vacuum polarization in external fields is treated by way of calculating - exactly and then perturbatively - the phase of the quantum scattering matrix in the Shale-Stinespring approach to field theory. The link between the Shale-Stinespring method and the Epstein-Glaser renormalization procedure is highlighted. 
Jos\'e M. Gracia-Bond\'ia (Univ. Complutense de Madrid)