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Physics

Simultaneity

Quick fact

Einstein showed that there is no universal 'now' — two events that are simultaneous for one observer will not be simultaneous for another moving relative to them, provided the events are separated in space.

Why this is interesting

Imagine two fireworks exploding at the same moment — but depending on where you stand, one might appear to flash before the other. Now imagine Earth and a distant star: could two events be simultaneous for you but not for someone moving at high speed?