Einstein had to "guess" several things in order to derive relativity, but they were very educated guesses. There was no reason to assume a priori that the laws of physics would be the same in all inertial frames at speeds near that of light. It was clear from the measurements that classical theory was wrong, but the question was which part? He had to guess which aspects of classical theory to keep and which to trash.Integral said:Just for the record, Einstein did not just "guess" this relationship. It is a derived result of aplying the Lorentz transforms to the kinematics equations of physics.