the postulates of special relativity
In 1905 Albert Einstein rebuilt the whole of physics starting from just two simple-sounding assumptions, called the postulates of special relativity. A postulate is a starting statement you assume to be true and then follow to see where it leads. The everyday image behind the first one is a smooth train ride: with the blinds down and the ride perfectly smooth, no experiment you do inside the carriage — bouncing a ball, pouring water — can tell you whether the train is parked or gliding along at a steady 300 km/h.
The two postulates are these. First, the principle of relativity: the laws of physics are exactly the same in every inertial reference frame, that is, in every lab moving at constant velocity without spinning or speeding up. So there is no such thing as being 'truly at rest' — only moving relative to something else. Second, the invariance of the speed of light: light travels through empty space at the same speed, c, for every inertial observer, roughly c = 3.00 x 10^8 m/s, no matter how fast the light source moves and no matter how fast you move toward or away from it. This second postulate is the shocking one, because it clashes head-on with everyday common sense about how speeds add up.
Almost everything strange in special relativity — the relativity of simultaneity, time dilation, length contraction, E = m c^2 — follows logically from these two modest-looking statements. Einstein did not tack on extra assumptions; he insisted on taking these two seriously at the same time, and the rest of the theory is the price of doing so.
A spaceship moving at half the speed of light switches on a headlight. Both the crew on board and an observer watching from a planet measure the beam racing away at the same speed c — not 1.5c. That single stubborn fact forces every other relativistic effect.
Postulate two in action: light's speed is the same for everyone, whatever they measure it against.
The postulates apply to inertial frames only — labs moving at constant velocity with no acceleration and no gravity. Extending relativity to accelerating frames and to gravity is the job of general relativity, a separate and later theory.