Maybe this could help...
The Principle of Relativity: All steady motion is relative and cannot be detected without reference to an outside frame. Or in other words, It is not possible to tell whether you are moving at a constant velocity, or stationary without reference to an outside frame.
Consider the question, "If i was in a train moving at the speed of light, would i be able to see my own reflection."
If the aether model was correct, the answer would be No. This is due to the fact that the light would never be able to catch up to the mirror to return as a reflection (the speed of light being constant meaning it could not travel any faster than the train). But this places a contradiction on the law outlined above. If the person inside the train was not able to see their reflection, they would then know that they are moving.
Without violating the principle of relativity, it could be said that the person inside the train would see their reflection, and that a person from the outside would see the light ray travelling at 2c. Einstein suggested that instead, the light ray would travel at a constant c, and that speed and distance become relative to which frame of reference they are viewed from (demonstrated in Einstein's other thought experiments).
