Einstein's two postulates of special relativity

Inertial frames of reference

• An inertial frame of referenceA reference frame that is either at rest or moving at a constant velocity. Newton's laws hold without modification in any inertial frame. An accelerating frame is non-inertial. is one that is either at rest or moving with a constant velocity (not accelerating).
• In any inertial frame, Newton's first law holds: an object with no resultant force acting on it remains at rest or continues in a straight line at constant speed.
• A car travelling at constant velocity on a straight road is an inertial frame. A car going round a bend or braking is not.

The two postulates (1905)

Why these postulates matter

• The first postulate is essentially an extension of Galilean relativity (which said the same for mechanics) to all of physics, including electromagnetism and optics.
• The second postulate is more radical. In everyday experience, velocities add: if you walk at 5 km/h on a train moving at 100 km/h, your speed relative to the ground is 105 km/h. But for light, a torch on the train still emits light at $c$ relative to both the train and the ground.
• Crucially, these two postulates together lead to time dilation, length contraction, and mass-energy equivalence, all of which have been confirmed experimentally.
• The Michelson-Morley null result is fully consistent with the second postulate: there is no preferred frame, and $c$ does not depend on direction or the observer's motion.