WHAT IS LIGHT?
One of the great underlying questions that has remained throughout the evolution of physics, the true nature of light is still not completely known to us. I view light as energy in its truest form, manifesting itself when a physical system or interaction, operating under the conservation of energy, requires it to.
We’ve all heard that light behaves as both a particle and a wave. Here is a brief and qualitative discussion of both formulations.
The wave theory of light is derived from classic electrodynamics, and hinges on the work done by physicists such as Michael Faraday and James Clerk Maxwell. The basic theory states that light results from a coupled disturbance of the electromagnetic field. What this means is that, since a changing electric field induces a magnetic field and a changing magnetic field in turn induces an electric field, any single disturbance can result in a chain reaction: an energy-carrying wave that can travel crazy distances and carry energy into or out of systems. This wave, through strong theoretical and experimental proof, was found to be what we perceive as light.
The experimental observation that called for a reworking of the theory of light in the early twentieth century was performed by Albert Michelson and Edward Morley in 1887. Just as sound waves must have a substance to propagate through (the atmosphere on Earth, for example) it was predicted that, in order for light to propagate through the Universe, the Universe must be filled with some mysterious medium, or ether. Using refined interferometry and the movement of the Earth around the sun, Michelson and Morley were able to prove that such an ether does not exist.
How can light exist as a wave if there is nothing to wave through? This, along with observations that showed energy being absorbed and emitted by systems in discrete units, led physicists such as Max Planck and Albert Einstein to suggest that light exists not as a wave, but rather as a particle, with each particle carrying a discrete amount of energy through the Universe.
Remarkably, both of these theories are correct at the same time. The wave theory of light is solidly derived from a sound physical basis and explains a number of phenomena observed throughout the Universe and in the lab. Likewise, the particle theory of light explains phenomena such as the photoelectric effect and fits in nicely with modern physics.
For me, the easiest way to reconcile this contradiction is to look toward the future. We know that something can’t be both a particle and a wave, and yet we’ve found something that is. Both particle and wave formulations are our attempts to understand the dynamics of light, and they are obviously lacking. The true nature of light must transcend both of these formulations. It will be elegant and beautiful, it will be complex and powerful. Physics still has unanswered questions.