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$\begingroup$ The question "when does event B happen" is frame dependent, so you cannot say absolutely that event B didn't happen when you were at A, different observers will disagree about what you said. Event B didn't happen while you were at A, but it did happen for another observer, so the existence of the event in space-time is fixed isn't it? $\endgroup$Davyz2– Davyz22025-12-31 11:29:49 +00:00Commented 21 hours ago
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8$\begingroup$ No! The order of events on a world line is not frame dependent. When I am celebrating my 67th birthday I am not celebrating my 68th. That is true in every frame. $\endgroup$Professor Sushing– Professor Sushing2025-12-31 11:33:52 +00:00Commented 21 hours ago
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$\begingroup$ the order is invariant but the particular time at which the two events happen is dependent, right? isn't your 68th birthday a single event in spacetime on which different observers will disagree on when it happens? when you celebrate the 67th birthday, that event didn't yet happen but this is only true in your frame, right? $\endgroup$Davyz2– Davyz22025-12-31 11:45:50 +00:00Commented 21 hours ago
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2$\begingroup$ No, you are not thinking it through. My 68th birthday happens on my 68th birthday, and always after my 67th, regardless of what frame you adopt. Different observers will say my 68th birthday is simultaneous with other events happening elsewhere, and will allocate different time coordinates to it, but they will all agree it is not taking place on my 67th birthday. $\endgroup$Professor Sushing– Professor Sushing2025-12-31 12:28:54 +00:00Commented 20 hours ago
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$\begingroup$ @Davyz2 Don't forget that if events A & B are timelike separated in one frame then they are timelike separated in every frame, and have the same order in every frame. Frame change can only affect the ordering of events with spacelike separation. See physics.stackexchange.com/a/844818/123208 $\endgroup$PM 2Ring– PM 2Ring2025-12-31 21:20:21 +00:00Commented 11 hours ago
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