Greg Egan on Nostr: What if we ask, instead, what redshift the astronaut who fell in first would measure ...

What if we ask, instead, what redshift the astronaut who fell in first would measure for the light coming down to them, either from the spacecraft, or a beacon you're carrying when you jump down after them? The second diagram plots those redshifts. Here, the black dots mark the points where the first astronaut crosses the event horizon, and where they hit the singularity. In this case, the light shining down on them from your beacon when they cross the horizon will have been emitted well before you crossed the horizon yourself, and in fact (for the particular scenario plotted here) they will hit the singularity before they see any light from you from after you fell through the horizon. Maybe you won't actually cross it, and another friend will use the spacecraft to swoop down and rescue you.

For much more about this kind of thought experiment:

https://www.gregegan.net/SCIENCE/FiniteFall/FiniteFall.html