The Postulates of Special Relativity

On June 30, 1905 Einstein formulated the two postulates of special relativity:

1. The Principle of Relativity
The laws of physics are the same in all inertial frames of reference.

2. The Constancy of Speed of Light in Vacuum
The speed of light in vacuum has the same value c in all inertial frames of reference.

The speed of light in vacuum c (299792458 m/s) is so enormous that we do not notice a delay between the transmission and reception of electromagnetic waves under normal circumstances.

The speed of light in vacuum is actually the only speed that is absolute and the same for all observers as was stated in the second postulate.