These are the starting points for all of special relativity and you should make sure you understand and remember them:
● The Principle of Relativity: the laws of physics are the same in all inertial (non-accelerating) frames. There are no privileged observers.
● The Principle of Invariant Light Speed: observers in all inertial frames agree upon the speed of light. In other words, no matter how fast you’re going, you always see light to travel at the same speed, c. The speed of light is ALWAYS the same.
The second postulate may appear to be obtainable from the first, since Maxwell showed that the speed of light is determined by fundamental physical constants, which by the principle of relativity are the same in all frames.
However, the important point is that the second postulate means that no matter how quickly you are travelling, you always see light travelling at the same speed, whether it is moving in the same direction as you or the opposite direction.
There are many consequences of adopting these postulates, but the two most important ones are that time is relative and nothing can go faster than the speed of light. Newtonian physics assumes that all observers agree on the times at which events occur. Einstein blew this out of the water, as we shall see.