1. The problem statement, all variables and given/known data 2.Explain in your own words what Einstein means by “the principle of relativity”, and why it seems to be incompatible with the theory of electromagnetism (or “the law of propagation of light”). 2. Relevant equations Not an equation but Einsteins answer can be seen here in section 7 http://en.wikisource.org/wiki/Relat...ion_of_Light_with_the_Principle_of_Relativity 3. The attempt at a solution Alright, here's my attempt. I'm posting it here because I get the feeling I may have entirely missed the point Einstein was trying to put across, and would like correcting if that is the case... Einsteins “Principle of relativity”, is that for a Galileian co-ordinate system moving relative to another Galileian co-ordinate system (used as the reference body) in a straight line, the general laws of physics apply relative to the moving co-ordinate system in the exact same way as they do with respect to the reference body co-ordinate system. However the Dutch astronomer De Sitter, by observing double stars, was able to show that the velocity of propagation of light cannot depend on the velocity of motion of the body emitting the light. Therefore, applied to the principle of relativity, if light is traveling relative to the reference body co-ordinate system in the same direction as the moving co-ordinate system, then as viewed from the moving co-ordinate system the light would be traveling at the accepted velocity of light minus the velocity of the moving co-ordinate system (ie; slower than the speed of light). So the law of propagation of light could not apply to both the stationary co-ordinate system and the moving co-ordinate system, so either the principle of relativity or the law of propagation of light must be wrong. It's pretty badly worded at the moment, but I'd just like to know if my explanation is along the right lines. The deadline for this question is tomorrow afternoon so help before then would be much appreciated.