Postulates of special relativity

Special relativity rests on just two statements, and almost everything strange about it follows from taking them literally. The first is that the laws of physics are the same in every inertial frame — any laboratory drifting at constant velocity is as good as any other, and no experiment performed inside one can tell you that you are the one moving. The second is that light in vacuum travels at the same speed c for every observer, no matter how the source or the observer is moving.

The second postulate is the one that breaks everyday intuition. If you run after a departing bus, the bus's speed relative to you drops. If you chase a light pulse at 0.8c, its speed relative to you is still exactly c — not 0.2c. This is not a measurement error and not an approximation that better instruments would expose; it is what experiments actually find, and it is the assumption the rest of the theory is built on.

Something has to give, and what gives is the assumption that all observers share one universal clock and one universal ruler. Holding c fixed for everyone forces time intervals and lengths to depend on who is measuring them — which is exactly where time dilation and length contraction come from. Note what does not give: the two observers still agree on which events happened and on the laws governing them. "Everything is relative" is a slogan, not a consequence of the postulates.