Relativity of simultaneity

Intermediate

Events simultaneous in one frame are not in another.

Two events simultaneous in one frame generally happen at different times in a frame moving relative to it. There's no absolute 'now' across space.

Worked example

Lightning strikes at both ends of a moving train are simultaneous for the ground observer, but not for passengers.

Common mistake

Events at the same place and time are simultaneous for all.

In more detail

Events that are simultaneous in one frame may not be simultaneous in another moving frame. Simultaneity depends on the observer's motion.

Synchronising clocks across moving frames requires relativistic care.
It is essential for designing satellite time systems.
Physicists explain that two lightning strikes can be simultaneous for one observer but not for another moving past.

Can two events be simultaneous for everyone?

Only if they occur at the same place and time.

Does it matter for everyday life?

Not directly, but it becomes important at high speeds and large distances.

Can simultaneity really depend on the observer?

Yes. For events separated in space, the time order can differ between frames, though cause and effect are never reversed.