of that year he did not mention his synchronization procedure. This derivation was completely based on light speed invariance and the relativity principle, so Einstein noted that for the electrodynamics of moving bodies the aether is superfluous. Thus, the separation into "true" and "local" times of Lorentz and Poincaré vanishes all times are equally valid and therefore the relativity of length and time is a natural consequence.

In 1908, Hermann Minkowski introduced the concept of a world line