The Two Postulates

A postulate is a proposition that is assumed to be self evident. Its truth is taken for granted and so can be used as a starting point for deducing other truths.

Einstein’s special theory of relativity is based on the two postulates below.

1. First postulate

The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.

2. Second postulate (invariance of c)

As measured in an inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.

The first postulate says that the laws of physics apply for any non accelerating reference frame. If you drop a ball in a moving train it falls to the floor at 9.8 m/s².

The second postulate says that the velocity of light, c, is constant regardless of what reference frame you happen to be in.

Once we accept these two postulates the incredible deductions from them are quite simple to derive.