You're in a sealed train. You can't tell if you're moving or not. At the other end of the train, a light goes on and reaches you at a velocity c = length of train/time it takes to cross the train. Since light's propagation velocity is independent of the train's velocity (light's propagation velocity is relative to its medium), the light will always have that train's length to cross in the same time. But let's say you have two light detectors at your end, one outside the train and the other inside. You have two sources of light, outside and inside, that go on together when they're side by side. If the train is moving toward the light you'd expect the outside light to hit the outside detector first because the train's motion has shortened the distance the light has to travel compared to the inside light which always has to traverse the full length of the train. Remember, the train's motion does not affect the speed of light inside the train or the distance that light has to traverse. The problem is the MMX proved that the light outside the moving train hits the end simultaneously on the outside as the inside light does. This makes no sense from a standard definition of relative velocity. The train's velocity has no effect on the light speed and the light is traversing two different distances in the same time. So Einstein copied Lorentz and said the train must contractinside to match its length outside. Of course this is nonsense. What is happening is the time inside the train is slowed relative to the time outside the train allowing the light more time to cover the greater distance so it can arrive simultaneously with the light covering the shorter distance outside the train. I hope you can see how wrong Enistein was in his crazy assumption about how the length of the train inside contracted while the outside remained the same.
Now relativists will lie to you and say light travels as a photon and not a wave. The photon as bullet would be affected by the movement of the train. It's not because a wave propagates at a velocity relative to its medium. But since light's medium is electromagnetic and not material (it's a vacuum) The vacuum inside the sealed train can't be pushed relatively to the vacuum outside the train. If the medium can't be moved, and the relative velocity of the light to the medium can't be changed, then the relative velocity of the light to the train can't be changed by the train's movement. The MMX verified the earth registers no velocity relative to light's medium whatever its nature.
Yes ralfcis, thanks as always for the great discussion on this leads to more and more questions without perfect answers. At the time of the MMX there was more adherence to the idea of aether - I read that the Lorentz invariances were better proof of isotropics. Victoremedvil was talking earlier about dark energy (I haven't heard much about anti matter recently) Others can talk about all that but I'm going to have to simply accept the isotropic nature of light and read your relativity algebra to see if that helps.
I have also been wondering if redshift only applies to universal expansion measurements and not to a local experiment- but that is another topic.
Talking of vacuums and ether, I have been wondering about transparency and how light travels through glass - but not less dense materials such as cardboard. It is thought to slow down as it passes through!! - and of course speeds up as it passes back outside - so much for isotropics! Maybe space is an ether and glass too!
Question: (I may post this one too to see what comes back)
If two flash light beams are shone towards each other from opposite walls of a room, the photons will pass each other going in opposite directions. An observer standing next to the beams at the centre of the room would measure photon speed in both directions at light speed at the same time. What would be the relative speed of said photons?