Eur. Phys. J. C (2013) 73: 2679
https://doi.org/10.1140/epjc/s10052-013-2679-0

Regular Article - Theoretical Physics

Counting photons in static electric and magnetic fields

¹ Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Via Cintia, 80126, Napoli, Italy
² Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Via Cintia, 80126, Napoli, Italy

^* e-mail: wolfgang.mueck@na.infn.it

Received: 4 November 2013
Revised: 25 November 2013
Published online: 5 December 2013

Abstract

We describe the electromagnetic field by the massless limit of a massive vector field in the presence of a Coulomb gauge fixing term. The gauge fixing term ensures that, in the massless limit, the longitudinal mode is removed from the spectrum and only the two transverse modes survive. The system, coupled to a classical conserved current, is quantized in the canonical formalism. The classical field configurations due to time-independent electric charges and currents are represented by coherent states of longitudinal and transverse photons, respectively. The occupation number in these states is finite. In particular, the number of longitudinal photons bound by an electric charge q is given by N=q ²/(16πħ).