>>10888626Okay, basically before SR people didn't know about lorentz transformations. IN SR, the speed of anything travelling at the speed of light is Lorentz invariant, meaning that light is observed at the same speed in all inertial frames. You can see this by examining the velocity addition formula
where u' is the speed of an object with speed u in the unboosted frame, where v is the boost. Now let u'=c, and we see that
Compare that to a Galilean transformation, which holds true in classical physics
clearly if something is at speed c, under a boost its speed is c+v.
Now Maxwell's equations were known about before SR, but have the structure of SR already built into them. After a series of manipulations on Maxwell's equations, you can determine the speed of electromagnetic waves in a vacuum, and the speed just comes out as c without any reference to a medium. This is fine if you know about SR, where anything travelling with speed c is the same in all reference frames, but prior to SR nothing we knew of was like this, for instance water waves clearly appear to travel faster towards you if you're moving in the opposite direction to them in a boat. The only sensible way to measure the speed of water waves (or sound waves, or any wave in a fluid) is to measure it in the comoving frame of the fluid, (the one in which the fluid is stationary). So the speed of light that appeared in Maxwell's equations was presumably the speed in the comoving frame, but we didn't know about a comoving frame for light. A fluid-like medium was therefore posited in the frame of which the speed of light was c, the luminiferous ether, but attempts to measure the planet's velocity relative to the ether all found the same speed of light, because relativity is true, and the ether doesn't exist.
