"With the help of W Furry, Weisskopf ... showed that the inclusion of positrons in the electron self-energy calculation reduced the degree of divergence to ln(1/r0), where r0 again represents a minimum cut-off distance. Thus positrons were a partial, but not sufficient, help. Another infinite quantity occurring in quantum electrodynamics arises as a result of the production of virtual electron-positron pairs by a photon. Like the electron self-energy, this vacuum polarization divergence depends on the logarithm of a cut-off parameter. Despite the infinite nature of vacuum polarization, it was possible to calculate its effect on the Coulomb interaction, for instance in a hydrogen atom, by comparing the interaction at large and shorter distances."