One of the great controversies in history, back when we were still grappling with relativity, was the question “if light is a wave, what is the medium?” Whatever this medium was, it was given the name the “ether”. People tried to figure out what the ether was, and thought it would be the canvas on which the stuff of the universe was painted. Some reasoned that if a background ether was there, we were unlikely to be stationary with respect to this higher-level fixed coordinate system, and so we ought to be able to detect our movement relative to the ether as the planet rotated, orbited around the sun, which is in turn dashing through the galaxy. This movement which they determined must be there if there was an ether, was dubbed the “ether wind”.
The epic fail, celebrated as a victory, was the experiment Michelson & Morley did to allegedly prove that there was no ether wind. They reasoned that if they measured the speed of light in two directions at right angles, they should be different because one would be across the ether wind, and the other would be along it. They took an interferometer, which is a pair of mirrors mounted on a rigid steel frame so that they would face each other. They used lasers to measure the spacing, counting the number of wavelengths of light, then rotated the apparatus and counted again. No difference in light speed, they said, ergo there is no ether wind, ergo no such thing as ether.
The epic fail comes in when they failed to realize that the atoms in the steel beam were held at a certain spacing by atomic-scale electromagnetic forces, which acted at the speed of light. In other words, their ruler would change size exactly as much as the thing they were trying to measure.
So what is the medium that light travels through?
Okay, light is a self-propagating electromagnetic wave. Forget the magnetic part, it’s a weak side effect. Something is being polarized, the positive and negative charges pulled further apart, which creates a field that polarizes the next bit of space and so on. The energy travels along through this polarizable medium. So what is the medium? What is there in space that can be polarized? Well, all those protons and electrons out there filling space, of course. Even deep space, we realize now, is filled with ionized electron wave forms, and protons. Light passing through a molecule does not have to be absorbed or emitted. The passing light wave simply distorts the electron waves in their orbitals, which then spring back into position as the wave passes.
If there really was a true vacuum, there would be nothing there to polarize. The light would come to the end of the road and conservation of energy would force it to bounce and go back the way it came. You know that toy with the row of suspended marbles? You drop the first one and the shock runs down the row until the last one goes flying up, then it comes back down and sends the pulse of energy back down the row. Light hitting a vacuum would be like that.
When Einstein was looking at absorption and emission of light in an electric field (like a laser beam), he could mathematically derive the probability per unit time that the electron would hop from one orbital to another, absorbing or emitting a photon of light. But when he tried to calculate the probability of an isolated atom spontaneously changing state and emitting a photon in a vacuum, the answer was zero. This is a consequence of the Spherical Harmonic coordinate system, where each orbital is orthogonal and independent of each other by design. He found that to simulate reality, he had to fudge and put in background thermal radiation to stimulate the event. This gave equations that worked, but what did that math manipulation mean? Simply that an atom isolated alone in the universe never would emit a photon, but that an atom will only exchange energy if it has something else to interact with.
The take-home here? Light energy exists as distortions of the electronic structure of the atoms and molecules it is passing through. The speed of light is a measure of how quickly that energy propagates along. As it gets thicker (more polarizable) the light slows down, a phenomenon that is easily measured as index of refraction. This is why the pencil seems to bend when you put it in a glass of water. As matter gets more tenuous and approaches zero density, light speed approaches that theoretical value of speed of light in a vacuum. But as we said above, light would never actually enter a total vacuum, there being nothing there to polarize. So light energy transmission through the local material in space never quite gets up to the theoretical speed of light in a perfect vacuum.
Light does not skip from one place to another, passing through atoms on the way. Light energy is by its nature the passing of field distortions from atom to atom, in a continuous process that happens at the speed of light. If you expect to find photons of light, traveling in isolation through empty space, such a thing does not and can not exist. I don’t care if the theoreticians can write an equation for that perfect wave traveling in an empty universe, it does not exist.
So another take-home: Einstein was wrong and there is a preferred point of reference. That would be the inertial frame of reference of the medium through which the light is passing. A subtle, but important point for later.
[© Copyright 2016 by Gerald Keep. All Rights Reserved.]