Soft Photons and the Classical Limit

Abstract

The first two orders of the low-frequency limit of a bremsstrahlung amplitude are determined by the general principles of quantum field theory. The leading order is in obvious correspondence with the classical limit. We present a conceptual experiment which shows that the next-order term is also determined by the classical limit. In addition, a generating functional is constructed that provides the two leading terms of the bremsstrahlung amplitude for the emission and absorption of an arbritary number of soft photons. This generating functional is used to indicate a classical correspondence to all powers of the electrical charge.