Measure a light beam and you get 299,792,458 m/s — standing still, running towards it, or chasing it at nine tenths of that yourself. Every other strange thing in relativity is forced by keeping that one number fixed.
If light’s speed cannot differ between observers, then the things used to measure speed — distance and time — have to differ instead. Bounce a photon between two mirrors and set the clock moving: the photon must travel a diagonal, which is longer, and it cannot speed up to compensate. So the tick takes longer. Pythagoras on that triangle gives the whole of special relativity.
It never fails — but at everyday speeds it is unmeasurably small, which is why nobody noticed for two centuries. A passenger jet gains around a nanosecond an hour. Newton is not wrong at 70 mph, he is indistinguishable from right, and the corrections only become load-bearing for satellites, particle accelerators and anything reaching a serious fraction of light speed.
Estimate with the rule, then check it against the calculator that models it properly.
Open Relativity →Measure a light beam and you get 299,792,458 m/s — standing still, running towards it, or chasing it at nine tenths of that yourself. Every other strange thing in relativity is forced by keeping that one number fixed. If light’s speed cannot differ between observers, then the things used to measure speed — distance and time — have to differ instead.