Einstein's special theory of relativity is founded on just two simple postulates: 1. The principle of relativity: The laws of physics are the same in every inertial frame of reference.
2. The principle of constancy of the speed of light: The speed of light is the same throughout all inertial frames of reference, and it is independent from the motion of the source.
One of the most important consequences of the special theory of relativity is the postulate regarding the relativity of length: length is not absolute, but depends on the body's velocity relative to a given frame of reference.
The concept of relativity of length is based on the assumption that the speed of light is the same in all inertial frames of reference, and on the principle of relativity of time intervals.