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Why can two events be simultaneous for one observer but not for another observer?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In the theory of special relativity, the fact that two events can be simultaneous for one observer but not another arises because simultaneity is an observer-dependent notion rather than an absolute one.1

This phenomenon occurs because Albert Einstein postulated that the speed of light is constant for all inertial observers. To maintain this constant velocity, space and time cannot be absolute; instead, they must change based on the observer's state of motion.23

When two observers are in relative motion, their respective frames of reference define time and distance differently. Because the timing of an event depends on the observer's perspective, events that appear to happen at the same instant to a stationary observer may occur at different times for an observer moving relative to those events. Consequently, there is no perspective-independent "present," and judgments about whether events are simultaneous are relative to one's physical point of view.231