Photon Angular Momentum
In several paragraphs that precede this chapter, we have studied aspects related to the angular momentum of the electromagnetic field. Thus, in the framework of the microscopic theory of classical electrodynamics the global (Sect. 2.6.3) and local (Sect. 2.7.4) angular momenta of the field were discussed, and the angular momentum flow associated with particle interactions analyzed. In our treatment of transverse electromagnetic multipole waves (Sect. 5.4), further insight was obtained from a study of the differential equation which spherical harmonics satisfy. In an Interlude (Sect. 5.4.2), the classical angular field momentum operator was introduced, and related to the angular momentum operator appearing in the quantum theory of massive particles. An important step forward in our understanding of the structure of the field angular momentum was taken in Sect. 9.3.4. There it was shown that the part of the classical field momentum which contains the longitudinal electric field can be considered as a contribution to the particle orbital angular momentum.