Simultaneity describes the occurrence of two events at the same time. In accord with the postulates of special relativity, simultaneity is a perception governed by the speed of light that depends upon the reference frame and the relationship of observers to phenomena. Accordingly, simultaneity is a relative phenomenon rather than an absolute and is affected by time dilation.

In 1916, German-American physicist Albert Einstein wrote, "There is hardly a simpler law in physics than that according to which light is propagated in empty space. Every child at school knows that this propagation takes place in straight lines with a velocity c = 300,000 km/sec... Who would imagine that this simple law has plunged the conscientiously thoughtful physicist into the greatest intellectual difficulties?" Those intellectual difficulties, as described by Einstein, concern the relativity of simultaneity, and observer-dependent time.

In order to understand the relativity of...