Einstein has two theories of relativity- The Special Theory of Relativity (SRT) and The General Theory of Relativity (GRT). Each theory is based on an assumption about light which follows from the mutual employment of two postulates. This assumption states that light, when it travels through empty space, travels at the constant velocity of c with respect to the reference systems (or observers) identified in the first postulate. Einstein?s second postulate remains the same in each theory but the first postulate is generalized from SRT to GRT via his "Principle of Equivalence". Einstein?s second postulate is called The Principle of the Constancy of the Velocity of Light. In SRT, the first postulate is called The Special Principle of Relativity (SPR). In GRT, the first postulate is called The General Principle of Relativity (GPR). Einstein introduces his second postulate as a new law of optics. The second postulate says that light travels at the constant velocity of c, when it travels through empty space. The first postulate identifies the reference systems within which the laws of optics (and therefore Einstein?s second postulate) can claim validity.
This article aka "Einstein?s First Postulate"
