We will derive this later. Another consequence is a relativistic Doppler effect. The classical formula cannot be valid for light, since it can only depend on the relative speed between source and observer. If the source is moving toward us, change u to -u. Hnec if an object is moving away from us, we see a lower frequence.
Blue light becomes green or red, depending on u.
Hence we need the assumption that the laws of physics are the same everywhere in the universe. Fun to think about. We have seen that there are some interesting things happening when an observer predicts what is happening to an observer who is moving at a constant velocity with respect to . If the velocity is large, there are measurable effects on time and length. If we can find the mathematical mapping we have a model for the physics related to shifts from one inertial frame to another.