I realize this must a very fundamental issue and perhaps already answered but I couldn’t find a double.

Let A be an object near the event horizon - as seen by us - of a black hole, and B an object near the black hole (BH) but somewhat further out. I speculate now that B sees the BH with its own event horizon, closer to the BH than the one we see.

I assume therefore that B could see A after its passage through our event horizon. Would not B be able to gain information about A while in this zone (between our event horizon and that of B), and subsequently relay that information to us?


1 Answer 1


I find this kind of thing basically impossible to reason about without looking at a Penrose diagram. Here's the Penrose diagram of a black hole.

Penrose diagram of black hole

I have a nonmathematical explanation of Penrose diagrams in my book Relativity for Poets, which is free online.

The event horizon is by definition the boundary of the region from which light rays can't escape to infinity. To see the interior of a black hole, you would have to have some events in the hole's interior in your past light cone. This can only happen if you're inside the horizon.

Relaying doesn't help, because cause and effect are confined to the light cone.

Your observers A and B are able to see different amounts of the exterior universe, but neither can see the interior or even the event horizon itself.

  • $\begingroup$ Thanks I take it we all have the same event horizon, determined only by the properties of the black hole. I also take it from your comment that relaying information perhaps might be possible near the edge of what is called the observable universe. $\endgroup$ Mar 3, 2019 at 20:21
  • $\begingroup$ @MikaelJensen: The edges of the diagram are not the edges of the observable universe, they're idealized points at infinity, similar to vanishing points in artistic depictions of perspective. $\endgroup$
    – user4552
    Mar 3, 2019 at 22:27

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