I'm trying to understand SR better however I think I've got myself confused in some ways. Firstly, I can't think of exactly why a mechanical wave like sound should follow gallilean relativity (besides that F = ma is used on the "mechanical pieces" of the wave and we know F = ma obeys gallilean relativity), yet maxwell's equations don't. What's different regarding these two waves? Secondly is a bit about the twin paradox (Sorry ;]): We assume the standard "paradox" where one twin goes out (straight line, short/quick accelerations) and comes back. One twin will feel accelerations but as I take it, it's the velocity that will "age" the twin, but what is difference between the velocity of the true departing twin perceived by the "stationary twin" (say, on earth) and the velocity of the "stationary twin" as perceived by the twin who feels accelerations?