Dear PF Forum, Before I go further, I'm stuck and need clarification. Two postulates of SR 2. the velocity of light is the same for all inertial observers. Okay..., this one I can understand, at least can grasp. 1. the laws of physics are the same in all inertial frames of reference; What laws of physics that it means? Time and distance, I already know that it differs. And they say mass, too What about temperature? When things move, will we see that waters in that space ships boils when it's cooled? So, what kind of laws that are involved? Thanks.