Special relativity theory says simultaneity is relative, meaning that different observers will not agree on what happened first and what second. Does it then make sense to say that looking at distant stars, we see them how they looked "billions of years ago" and not how they look now? Does it make sense to talk about what these stars look like now? How do we define this "now" if simultaneity is relative?

Arectemieryf0 2022-07-14 Answered
Special relativity theory says simultaneity is relative, meaning that different observers will not agree on what happened first and what second. Does it then make sense to say that looking at distant stars, we see them how they looked "billions of years ago" and not how they look now? Does it make sense to talk about what these stars look like now? How do we define this "now" if simultaneity is relative?
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Answers (1)

cindysnifflesuz
Answered 2022-07-15 Author has 19 answers
Consider two points A and B in spacetime. If they are separated by a spacelike interval, then indeed different observers can disagree on which happened earlier. However, there are two other possibilities. A could be inside the future light cone of B, in which case all observers agree that A is in B's future. Similarly, B can unambiguously be in A's future.
Things may get more complicated in general relativity, but this idea still holds. The distant galaxies we see are in our past, and no shift of reference frame can alter this. Causality is preserved.

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