While reading about Galilean transformation and electromagnetism (as a prelude to the implications of the special theory of relativity on quantum mechanics, which I’m reviewing since I never did that well in my quantum mechanics class at Princeton), I learned something about ether that I didn’t know before. To me, ether was always either a classification for organic compounds, an item that restored manna to RPG characters, or an archaic word for “space.” Now, I just learned that ether was actually the name given by early physicists to the medium through which electromagnetic radiation was thought to propagate (just as sound, for example, propagates through air).

As the Michelson-Morley experiment would later show, electromagnetic waves are indeed capable of propagating without a propagation medium (see explanation below). This in turn allowed Einstein to develop his special theory of relativity as we know it today. So, the “ether” coined by those early physicists was nothing more than an imaginary construct created to help people of that time understand electromagnetic phenomena. Fascinating, isn’t it?

The Michelson-Morley experiment didn’t explicitly disprove the existence of ether. It did, however, show that light travels at the same speed in perpendicular directions, which, assuming ether did exist, wouldn’t be possible unless the ether frame moved in sync with the Earth’s rotation, which would be a preposterous claim. (They believed the ether frame was rooted either in the solar system’s center of mass or in the center of the universe.) It was Einstein who later used these experimental results to assert that there is no ether frame, which means the velocity of light is only relative to the observer’s own frame, which would then result in the famous concept of a “constant speed of light, c.” Einstein then used this idea to arrive at his famous postulate:

The laws of electromagnetic phenomena, as well as the laws of mechanics, are the same in all inertial frames of reference, despite the fact that these frames move with respect to each other. Consequently, all inertial frames are completely equivalent for all phenomena.

This was an incredibly bold statement at the time, because it meant that Maxwell’s equations and Galilean transformations could not both be correct (one of them had to be wrong). Even bolder, Einstein chose to modify the Galilean transformation, which meant he was challenging the fundamental equations of Newtonian physics!