The photon: issues of integrity

Abstract

A richly diverse range of phenomena and applications, frequently in the context of laser applications, owe their means of operation to the properties of the photon. Yet, since the arrival of the laser, the distinctive and often paradoxical nature of the photon has become more than ever evident, and what the optics community now understands by a ‘photon’ has become richer – certainly less simple, than Einstein’s original conception. There has been a marked expansion in the pace of development since the now familiar derivative term ‘photonics’ first emerged, and in much current theory any dividing line between ‘real’ and ‘virtual’ photons proves to be illusory. So if, in this technical sense, no photon can ever be regarded as entirely real, one is drawn to deeper questions of whether the photon is ‘real’ in the broader sense of reality. Some would argue that electromagnetic fields are closer to irreducible reality. Yet whether we elect to describe optical phenomena in terms of fields or photons, neither represents what is actually measured. The surest ground has to be found where theory is cast in terms that explain or predict actual observations, under given conditions. It is consistent with the path integral formulation of quantum mechanics that derivations should not prescribe what intervenes between setup and measurement, but instead allow for all possibilities. Indeed, one of the beauties of the associated mathematics is its capacity to home in on possibilities that most closely conform to post-event physical interpretation. Still, we can ask: how much do or can we know about the photon itself? How much information could this entity contain or convey? And, how essential is a photonic formulation of theory? This study focuses on some of the key issues.