Relativity of Simultaneity
Two spatially separated events can share the same time coordinate in one inertial frame and receive different time coordinates in another. Simultaneity is frame-dependent, not universal.
Can two events be simultaneous for one observer but not another?
Yes. The disagreement is not caused by signal delay or faulty clocks. It comes from how different inertial frames assign coordinates to the same pair of events.
Below we use units with . The platform assigns both events .
Keep the events fixed. Change only the observer.
The two flashes are the same physical events in every frame. Only their coordinate labels change.
In the platform frame both events have t = 0. In the moving frame their t′ values separate as soon as v is nonzero.
Reverse the frame velocity and the event that was earlier becomes later. For spacelike-separated events, no invariant time order exists.
A moving frame slices spacetime into “now” differently.
For the platform, the horizontal line through both events is one instant of time. A moving inertial frame uses a tilted simultaneity line. The light cone stays fixed while the frame's space and time axes tilt together.
This statement concerns assigned event coordinates, not visual appearance.