We consider here the exchange of an arbitrary number of photons between two fermions at high energies, and an exponentiation, similar to that in the Glauber approximation and the droplet model, is obtained. When this result is applied to the processes of electron-electron scattering, Delbrück scattering, and photon-photon scattering, the scattering amplitude in each case can again be cast in the impact-factor representation. Although some modifications from the lowest-order result are necessary to account for the multiphoton effects, the impact factors themselves are not changed at all. The limitation of this exponentiation is discussed in connection with more complicated exchanges.

• Received 14 May 1969

DOI:https://doi.org/10.1103/PhysRev.186.1611