ABSTRACT
The concept of simultaneity is of central importance in the theories of relativity, being an essential prerequisite for defining the synchronization of clocks and the measurement of time. As emphasized by Einstein, the time of an event can be reliably attributed only by consulting a ‘local’ clock, adjacent to the event. Then, one has to specify a method to synchronize such clocks at different locations, to build a consistent physical theory. That was how the constancy of the velocity of light in all inertial frames became the pivotal point of relativity. After postulating the invariance of the velocity of light, Einstein asserted that if all clocks are synchronized using light, in the rest frame of the clocks, they are all properly synchronized, independent of any common uniform velocity they may have. In other words, they are synchronized as if they are all at rest.
