For over a hundred years now we have accepted that the speed of light is the same in all frames of reference. What I'm wondering is - how was this determined?
I'm aware of the Michelson and Morley experiment, but that only showed that the speed of light doesn't depend on the movement of the light source. As in - it's not like a cannonball being shot out of a moving cannon.
But here's another thought - what if light is like sound, a wave travelling inside a medium? And that medium itself also can have a velocity? For example, take the classical example of two people - one inside a train, and the other standing on the platform. When each one of them measures the speed of sound, they'll get the same value. When one makes the sound and the other tries to measure the speed this particular sound has in their vicinity (like the Michelson and Morley experiment), they will also get the same value.
In this setup both people will also conclude that the speed of sound is the same no matter how fast the source of the sound is moving. There can be a Doppler effect (also observed for light), but the speed of sound itself will be constant.
That's because the sound waves travel through air (or, briefly, the material of the train carriage), and the air inside the carriage moves relative to the air outside. In essence, sound speeds up when it enters the carriage, and slows down when it exits it. But since you cannot measure sound from afar, you also cannot see this effect.
Now, obviously this is not how the world works and it has been thoroughly tested by now, but I'm wondering - how was this possibility eliminated? Which experiments contradicted with it?