IS LIGHT A WAVE OR A PARTICLE? Contributed by Marlon and ZapperZ. In our ordinary world, “wave” and “particle” behavior are two different and opposite characteristics. It is difficult for us to think that they can be one of the same. Is light a particle or a wave? The simple, naïve answer to that is “both” or “neither”. Light, or photon, was never defined as a “particle” the way we normally define a particle. Light is not defined to have a definite boundary in space like a ping-pong ball, or a grain of sand. Instead, light is defined as having quanta of energy. So the discreteness is not defined as discrete object in space, but rather in the energy it can carry. Already, this is not your regular “particle”, and should not be confused as such. Secondly, in quantum mechanics, the description and properties of light has only ONE, single, consistent formulation, not two. This formulation (be it via the ordinary Schrodinger equation, or the more complex Quantum Electrodynamics or QED), describes ALL characteristics of light – both the wave-like behavior and the particle-like behavior. Unlike classical physics, quantum mechanics does not need to switch gears to describe the wave-like and particle-like observations. This is all accomplished by one consistent theory. So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.