- #1

Stephanus

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What does "at the same time" mean for two observers?

I've read https://www.physicsforums.com/threads/criteria-of-simultaneity.699560/. But I want to ask a simpler question.

1. Does "At the same time" mean different for two COMOVING observers than two REST observers?

And about this picture:

2. Can we say in picture 1 that E1 and E7 are at the same time?

3. Or do we have to convert it to picture 2 then we'll know that it's E1 and E2 are at the same time?

4. Regarding Pic 2. If E1 and E2 are at the same time, Blue (B) will never now about E2 until E2A will it?

5. Is at the same time for B is E2A and E2?

6. Is at the same time for B is E1 and E2?

Thanks for any help