Relativity in Physics

Relativity begins with a failed experiment. Physicists expected the Earth's motion through space to change the measured speed of light, built an interferometer precise enough to see the difference, and found nothing at all. Taking that null result seriously forces a single change to the rules, that the speed of light is the same for everyone, and almost everything else follows. If light will not oblige by going faster when you chase it, then time and length have to give way instead, so moving clocks run slow and moving objects shorten. The blocks give the older Galilean rules first so the contrast is visible, then the Lorentz factor and the two effects it controls, the transformations, and the energy and momentum relations that replace their Newtonian versions once speeds approach that limit.