Galilean relativity

Galilean relativity is the common-sense way of relating two inertial frames that physicists used before Einstein. It rests on two assumptions that feel obviously true: time flows at the same rate for everyone, and velocities simply add. Walk forward at five kilometres per hour on a train doing a hundred, and the ground says you move at one hundred and five.

In this picture a single universal clock ticks for the whole cosmos, so two events that are simultaneous for one observer are simultaneous for all. Lengths, time intervals, and the order of events are absolute, shared by every frame. To convert positions from one frame to another you just subtract the steady drift between them — the so-called Galilean transformation.

For everyday speeds this scheme is superbly accurate, which is why it ruled physics for centuries and still guides engineers today. It only breaks down when speeds climb toward that of light: then velocities stop adding straightforwardly, clocks fall out of step, and simultaneity becomes frame-dependent. Galilean relativity is the gentle, low-speed limit of Einstein's deeper theory.