The Postulates of Special Relativity

Simultaneity

One of the most important concepts in special relativity is the one of simultaneity. Two physical events that occur simultaneously in one inertial frame are only simultaneous in any other inertial frame if they occur at the same time and at the same place. Time is relative!

The two figures to the left, seen from two different inertial frames, help clarify the concept of simultaneity:

Top figure:
In the inertial frame of the wagon, the lamps are switched on simultaneously and the two light impulses reach the girl at the same time.

Bottom figure:
In the inertial frame of the observer outside the wagon, it seems that the left lamp is switched on first, although for the girl in the wagon the lamps are switched on simultaneously.