There are two basic postulates of special theory of relativity:
I. The principle of relativity The natural physcial laws preserbe or retain their form for all inertial frames, ie, they retain the same form relative to all observers in a state of relative uniform motion.
This postulate is an extension of the conclusion drawn from Newtonian Mechanics, ie, the velocity is not absolute but it is relative as is evident from the failure of Michelson-Morley experiment performed to determine the velocity of earth from ether.
II. The principle of constancy of velocity of light The velocity of light in vacuum is independent of the velocity of observer or the velocity of the source, ie, it has same value in all inertial frames.
This postulate is not true according to Galilean transformations, but it is verified experimentally that the velocity of light calculated by any mass remains constant. It is this postulate, which draws a demarcation line between classical theory and the theory of relativity given by Einstein.
The constancy of velocity of light requires the following axioms as to introduce the transformation laws:
- The velocity of light c must have the same value in all inertial frames.
- The transformations should be linear and approaching to Galilean transformations for low velocities, ie, v << c.
- The transformation laws should be independent of the 'absolute time' and 'absolute space' notions.