Einstein began not by asking how fast time flows, but how distant clocks can be said to agree.
Year
1905
People
Albert Einstein, Mileva Maric, Hendrik Lorentz, Henri Poincare
Ideas at stake
special relativity, simultaneity, speed of light, reference frame
Synchronizing clocks was an engineering problem
In 1905 Einstein examined electrical patents in Bern while railways, telegraphs, and city clock networks made distant simultaneity practical. Classical physics assumed one universal time. Electromagnetism insisted that light in vacuum has the same speed for moving observers.
Einstein questioned the word everyone else had treated as obvious: simultaneous.
Imagine flashes sent from the middle of a fast train toward both ends. A passenger at the center says they arrive together. An observer on the platform sees the front moving away and the rear moving toward the light. Since both beams still move at the same speed, the arrival events are not simultaneous in the platform frame.
This is not merely delayed information. Space and time are divided differently by different inertial frames.
Relativity does not say everything is relative. It builds stricter invariants, including the speed of light. What disappeared was the hidden universal clock that everyone had assumed without measuring.
