3 ms·

Incorrect. Relativistic differences in simultaneity between observers have nothing to do with light travel time between them and everything to do with different

Incorrect. Relativistic differences in simultaneity between observers have nothing to do with light travel time between them and everything to do with different velocities. The other side your living room, and even the other side of the galaxy, is moving close enough to the same speed that simultaneity is perfectly well defined between them. So we can confidently assign times in the past to events based on what we see in the present, in the knowledge that they will be consistent with times assigned by other observers in different locations at the same speed. The only wrinkle in the scenario of a distant galaxy is cosmic expansion inducing a different reference frame by causing motion relative to Earth. However, if the redshift isn't more than a few percent then the time dilation won't be either, so we can be pretty sure it will be in the past by any definition that isn't moving a fair fraction of c relative to us. Don't confuse light travel time with relative simultaneity. Relativity does weird things to time but that's not one of them.

narag 5y ago
Don't forget they're talking about two black holes merging, so there are actually extreme relativistic effects to consider.
palmtree3000 5y ago
"Incorrect" is way too strong here. I agree that a reference frame that has about the same velocity as the stuff you care about is in some sense privileged. But the fact that we haven't seen the merger happen yet means that it's outside our light cone, which means there exists a reference frame where the merger hasn't happened yet. that is to say, t_{this_coversation} < t_{merger}, in that frame.
- andrewflnr 5y ago
  It is not too strong. GP specifically claimed that simultaneity depends on "where you're standing". This is wrong in all reference frames. As for timing of distant events, GP didn't make any reference to other reference frames at arbitrary velocity, only different positions. In our reference frame, which they were implicitly talking about, the merger (assuming the prediction is correct) was in the past, unambiguously.

narag 5y ago

Don't forget they're talking about two black holes merging, so there are actually extreme relativistic effects to consider.

palmtree3000 5y ago

"Incorrect" is way too strong here. I agree that a reference frame that has about the same velocity as the stuff you care about is in some sense privileged. But the fact that we haven't seen the merger happen yet means that it's outside our light cone, which means there exists a reference frame where the merger hasn't happened yet. that is to say, t_{this_coversation} < t_{merger}, in that frame.

andrewflnr 5y ago
It is not too strong. GP specifically claimed that simultaneity depends on "where you're standing". This is wrong in all reference frames. As for timing of distant events, GP didn't make any reference to other reference frames at arbitrary velocity, only different positions. In our reference frame, which they were implicitly talking about, the merger (assuming the prediction is correct) was in the past, unambiguously.