The nature of light: what are photons?

Introduction What do we mean by interference—or the superposition effect— of light? For centuries we have used the word ‘interference’ to describe the dark-bright bands recorded when we superpose two coherent light beams at a small angle on a detector. Indeed, holograms are such fringes caused by superposition of an ‘object beam’ with a coherent and uniform reference beam recorded on a very-high-resolution photographic plate. Physicist Sir Roger Penrose recently underscored the lack of reality in our current theories about how the world works.1 We would respond that reality is whatever we can actually sense and measure, even if that amounts to only a tiny fraction of the enormously complex jigsaw puzzle we call the cosmos. In our everyday experience, light beams pass through each other unperturbed without interacting (interfering) with each other unless we try to detect them. Taking this mundane observation into account increases the potential for innovative applications in technology, while resolving the problem of wave-particle duality for photons.