Observers in different inertial frames can disagree about the time separating two events.

Question

Quizplus · Accepted Answer

This is one of the postulates of special relativity, known as the relativity of simultaneity. Since the speed of light is constant in all inertial frames, events that are simultaneous in one frame will not be simultaneous in another frame that is moving relative to the first frame. This can lead to disagreements about the time separating two events.

Observers in Different Inertial Frames Can Disagree About the Time