ABSTRACT
CONTENTS 18.1 Introduction ................................................................................................ 272 18.2 QED Formulation ....................................................................................... 272 18.3 Retarded Electric Fields and Photon Transversality ............................ 275 18.4 Quantum Pathways ...................................................................................277 18.5 Spin and Photon Angular Momentum ...................................................277 Conclusion ............................................................................................................ 278 Acknowledgment ................................................................................................ 279 References ............................................................................................................ 279
It has been observed that every photon is, in a sense, virtual-being emitted and then sooner or later absorbed. As the motif of a quantum radiation state, the photon shares these characteristics of any virtual state: that it is not directly observable; and that it can signify only one of a number of indeterminable intermediates, between matter states that are directly measurable. Nonetheless, other traits of real and virtual behavior are usually quite clearly differentiable. How “real”, then, is the photon? To address this and related questions it is helpful to look in detail at the quantum description of light emission and absorption. A straightforward analysis of the dynamic electric field, based on quantum electrodynamics, reveals not only the entanglement of energy transfer mechanisms usually regarded as “radiative” and “radiationless”; it also gives significant physical insights into several other electromagnetic topics. These include: the propagating and non-propagating character in electromagnetic fields; near-zone and wave-zone effects; transverse and longitudinal character; the effects of retardation, manifestations of quantum uncertainty and issues of photon spin. As a result it is possible
to gain a clearer perspective on when, or whether, the terms “real” and “virtual” are helpful descriptors of the photon.
