eli5 Is it correct to think of events happening on the far side of the universe or in a different galaxy as occurring “at the same time”?
In: Physics
For any two events where there is enough time for light from event A to reach event B (whether it actually does or not), then event A precedes event B for all observers.
For events where this is not the case, events A and B will be simultaneous for some observers, event A will occur before event B for some observers and event B will occur before event A for some observers.
The closer together in time and further apart in space two events are, the wider this window is, but any two events that can be described as truly simultaneous will not be from some perspective, even if very close together.
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