In all of the articles on the newly discovered gravity waves, none I have read mentions how the interferometer principle that detected them started Einstein thinking about relativity back in the early 1900s.
Back then, they knew light was a wave, but just what was waving? They didn’t know, so they called it the “ether,” or “aether.” It had to be something, so they would just give it a name and eventually they would discover more about it. Whatever it was, it was the universal, static atmosphere of space that carried the light and through which the planets rotated.
The interferometer had already been invented. It could compare two beams of light and detect the difference in the position of their waves, which in terms of distance was incredibly small. If their position was exactly the same (in phase), their intensity would add, if a wavelength off, they would subtract, or “interfere.”
The interferometer had been tried to determine the movement of the earth through space in 1887 by Albert A. Michelson. The earth obviously rotates around the sun, and the sun rotates around the center of the milky way galaxy, so what is the total movement? Maybe they would find the galaxy is also rotating around something else.
Their idea was cleaver. If you swim toward an ocean wave, it will approach you more quickly, swim away from it and it approaches more slowly. Take a beam of light and split it in two. Send one beam down a tube and bounce it back from a mirror. Do the same with the second beam, but set its tube at right angles to the first. Both beams start out in sync, but would not be when they arrived back because one would be more-or-less in line with the earth’s movement and the other would not. The one in line would hit the mirror as it was moving away and have to travel farther. The interferometer could detect the difference. It might take some twisting of the apparatus to get it aligned just right, but the apparatus then was small and could be turned easily. (The illustration here is simplified. Interferometry can get very complicated, which explains why the paper describing the discovery of the gravitational waves had over 1,000 authors. Michelson was awarded the Nobel prize in 1907.)
When they actually tried it, they were stunned to find there was no difference, no matter how they twisted it. The beams of light returned in sync in any direction. This defied logic and remained a puzzle of physics. Einstein made the jump by saying the speed of light is always the same, independent of the relative movement of the source and the observer. It seems illogical, but that is the way it is. The result was Einstein’s theory of relativity, and trashing the concept of a static ether permeating all of space. There is nothing fixed in space where you can say something is moving or not. “Ether” is only used now in a poetic sense.
So, when a light wave waves, what, then, is waving? The answer is: Nothing.
(The importance of gravitational waves is that they are not electromagnetic waves. Everything else we can detect passing through space, such as light, radio waves, microwaves, and X-rays, are all electromagnetic waves in one form or another. Electromagnetic waves are trapped within their own dimensional home and can never cross over to other dimensions. We will never receive a radio signal from another dimension, and neither can we send one. Gravitational waves are not so constrained. Studies are already underway to detect other dimensions by anomalies of gravity. So far, they have found none, another puzzle waiting for another Einstein.)