Einstein's Clocks and Poincare's Maps

As Einstein put it: “We have to take into account that all our judgments in which time plays a role are always judgments of simultaneous events.

Instead, kinematics had to come first, that is, how clocks and rulers behaved in constant, force-free motion.

Building on this traditional use of the relativity principle in mechanics, Einstein in his 1905 paper raised relativity to a principle

Almost three hundred years earlier, Galileo had similarly questioned frames of reference. Picturing an observer in a closed ship’s cabin, borne smoothly across the seas, Galileo reasoned that no mechanical experiment conducted in a below-deck laboratory would reveal the motion of the ship

Einstein’s goal was to produce that single account, one that did not refer to the ether at all, but instead depicted the two frames of reference

As Einstein reframed the problem there was one single phenomenon: coil and magnet approached each other, lighting the lamp.

As far as he was concerned, one observable phenomenon demanded one explanation.

So the standard account gave two explanations depending on whether one viewed the scene from the point of view of the magnet or the point of view of the coil.

But in its then-current interpretation, electrodynamics (the theory that included Maxwell’s equations—describing the behavior of electric and magnetic fields—and a force law that predicted how a charged particle would move in these fields) gave two different explanations of what was happening.

Einstein began his relativity paper with the claim that there was an asymmetry in the then-current interpretation of electrodynamics, an asymmetry not present in the phenomena of nature.