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So far as I know, if we have 2 blackholes A and B, both having their own event horizons, then what's inside the event horizon of A remains completely unknown to us, and same for what's inside the event horizon of B.

Now, suppose A and B coalesce (merge). So we now have a single blackhole C, with its own new event horizon.

This would mean that what was inside the event horizon of A now "knows" what is inside the event horizon of B. Would this mean that all event horizons of all blackholes are actually "linkable" together since they might all merge at some point?


Edit: After watching the suggested video, I can try rewording the question:

Suppose Alice and Bob are outside blackholes A and B.

Alice jumps into blackhole A, and Bob jumps into blackhole B.

Bob never sees Alice crossing the event horizon (according to the video below; it's the same Bob than in the video: he cannot see Alice crossing the event horizon until "t=infinity"), but Bob crosses the event horizon of blackhole B (not part of the video).

Let's say that the two blackholes are also falling to one another so let's say they are "now" (that concept is probably misplaced) merging. So the two event horizons now become one.

Are Alice and Bob reunited despite Bob never seeing Alice cross the event horizon of A? How would such situation (2 blackholes) be drawn using the diagram in the video in answer below?


This might be related to Do black hole singularities actually merge? and What happens to the information on the event horizons of two merging black holes? but I'm unable to get my answer from there... but I'm only a computer engineer, I'm not a physicist, so the answer might be lying there and I don't see it.

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    $\begingroup$ related: astronomy.stackexchange.com/questions/32410/… $\endgroup$ – Florin Andrei Sep 5 '19 at 7:46
  • $\begingroup$ Bear in mind that Alice and Bob rapidly fall towards the centres of their black holes, as do any photons they happen to emit. So you need SMBHs if you want them to last for more than a few milliseconds. But I'm not totally clear on what happens to the photon paths once the merging starts. $\endgroup$ – PM 2Ring Sep 5 '19 at 16:56
  • $\begingroup$ FWIW, you can get the proper time of a freefalling body to reach the centre of a black hole, plus other interesting info, from the Hawking radiation calculator. $\endgroup$ – PM 2Ring Sep 5 '19 at 16:59
  • $\begingroup$ Afaik there is some theorem saying that the total surface of the event horizons never decrease in any black hole configurations. $\endgroup$ – peterh - Reinstate Monica Sep 5 '19 at 18:00
  • $\begingroup$ @PM2Ring Right, but if I get the video below right, then once Alice crosses the EH of a blackhole, she can go at the speed of light (or so) right after having corssed it, and so, she will have "plenty" of time before reaching the singularity (she will reach it, but later [from her point of view] than if free falling)? Suppose Bob does the same after crossing his BH EH, and "then" the BH EH touches each other and start merging. Are now both Alice and Bob (or their "photons" since they are at almost the speed of light) "causaly" relatable? $\endgroup$ – Xenos Sep 6 '19 at 7:31
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We're also not physicians, not even Astrophysicians, but we're physicists.

One of the more famous physicists, Leonard Susskind, discusses in his excellent youtube lecture series on general relativity (I believe it was episode 7, which I've linked) that during a black hole merger the Schwarzschild surface (not a sphere anymore) deforms and 'comes out to get you'.

Otherwise one would run into the problem that early research into black holes had, when they were still called Collapsars. This problem was that a Collapsar would eternally collapse due to the gravitational time dilation and never actually form a Schwarzschild radius that the static solutions indicate should exist. Then also, black holes could never merge.

But this problem has been solved with the realization that the Horizon deforms during a merger. The Horizons themselves then merge, and A and B come into causal contact.

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  • $\begingroup$ I'll probably need to rewatch that video, and reread the comment of @Florin Andrei but so far, I'm not sure I get the link between EH shape and my question: say Alice and Bob are outside blackholes A and B. Alice jumps into A, Bob jumps into B and they cross each EH. So Alice can't see Bob, and Bob can't see Alice. Now blackholes merge. So do Alice now see Bob (inside the new EH of A+B)? It's like having 3 isolated spacetimes (EH-A, EH-B and the rest of the universe) becoming two (EH-(A+B) and the universe) so we "broke" the EH-A/EH-B isolation? $\endgroup$ – Xenos Sep 5 '19 at 15:38
  • $\begingroup$ A and B come into casual contact? That's an odd way to state that. Any reason you used the qualifier casual? $\endgroup$ – Magic Octopus Urn Sep 5 '19 at 16:30
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    $\begingroup$ @MagicOctopusUrn Causal != casual $\endgroup$ – PM 2Ring Sep 5 '19 at 16:52
  • $\begingroup$ @PM2Ring I mean I figured it had a connotation here, and was asking what that connotation was. That did not help very much. $\endgroup$ – Magic Octopus Urn Sep 5 '19 at 17:50
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    $\begingroup$ @MagicOctopusUrn If A and B are in causal contact, that means that A can cause an effect on B, and vice versa. The word "casual", which you used, has a quite different meaning to "causal". $\endgroup$ – PM 2Ring Sep 5 '19 at 17:57
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At the moment that two black hole interiors merge into one, there's a bridge between the event horizons, as you can see in simulations like this one (20 second silent YouTube video).

While it may look like the protrusions on the event horizons form shortly before the merge, they're actually present (though very small) all the way back to the formation of the horizons. Event horizons "know" which mergers will happen. This is possible because they're defined by reference to a future singularity, not by anything that could be measured locally.

The spacetime diagram of the merging event horizons is shaped like a pair of pants. The points at the end of the protrusions form the inseam of the pants (that's standard terminology, not my invention). The inseam is spacelike. It's possible to pick a foliation of spacetime in which the whole inseam appears before any other part of the horizon/interior, or in which the crotch appears first followed by the legs. In these foliations, there is always only one connected interior region. So it's not objectively (general-covariantly) true that there were ever two separate black holes.

As far as I know, the singularities of the black holes can't be clearly distinguished either. The singularity of realistic black holes is spacelike, so it can't have different states before and after a merger.

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  • $\begingroup$ This is fascinating. Can you supply any more references to this analysis of merging black holes that isn't completely buried in differential geometry? $\endgroup$ – Steve Linton Aug 10 at 10:53
  • $\begingroup$ @SteveLinton Dieter Brill, Black Hole Horizons and How They Begin looks like a good introduction. He uses the term "crease set" for the generalization of what I called the "inseam". I have seen "inseam" in papers, though... $\endgroup$ – benrg Aug 10 at 16:40

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