<p>We're also not physicians, not even Astrophysicians, but we're physicists. </p>

<p>One of the more famous physicists, Leonard Susskind, discusses in his excellent <a href="https://www.youtube.com/watch?v=BdYtfYkdGDk&amp;t=3531s" rel="noreferrer">youtube lecture series on general relativity</a> (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'.</p>

<p>Otherwise one would run into the problem that early research into black holes had, when they were still called <a href="https://en.wikipedia.org/wiki/Collapsar" rel="noreferrer">Collapsars</a>. 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.</p>

<p>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.</p>

CC BY-SA 4.0
2019-09-05T15:38:58.423Z
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?
Xenos
CC BY-SA 4.0
2019-09-05T16:30:47.690Z
A and B come into casual contact? That's an odd way to state that. Any reason you used the qualifier casual?
Magic Octopus Urn
CC BY-SA 4.0
2019-09-05T16:52:53.453Z
@MagicOctopusUrn Causal != casual
PM 2Ring
CC BY-SA 4.0
2019-09-05T17:50:31.877Z
@PM2Ring I mean I figured it had a connotation here, and was asking what that connotation was. That did not help very much.
Magic Octopus Urn
CC BY-SA 4.0
2019-09-05T17:57:46.990Z
@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".
PM 2Ring
CC BY-SA 4.0
2019-09-05T18:27:02.690Z
@PM2Ring thank you, that was the clarification I wanted, still pretty generic but gives me more to go on.
Magic Octopus Urn
CC BY-SA 4.0
2019-09-04T11:59:26.990Z
2019-09-04T14:23:45.497Z
2019-09-04T14:23:45.497Z
AtmosphericPrisonEscape
<p>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 <a href="https://www.youtube.com/watch?v=4nM6kf2OAFw" rel="nofollow noreferrer">this one</a> (20 second silent YouTube video).</p>
<p>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 &quot;know&quot; 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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2020-08-10T10:53:07.853Z
This is fascinating. Can you supply any more references to this analysis of merging black holes that isn't completely buried in differential geometry?
Steve Linton
CC BY-SA 4.0
2020-08-10T16:40:01.413Z
@SteveLinton [Dieter Brill, *Black Hole Horizons and How They Begin*](https://doi.org/10.1080/21672857.2012.11519694) 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...
benrg
CC BY-SA 4.0
2020-08-09T18:50:23.607Z
2020-08-09T18:50:23.607Z
benrg
<p>The big unjustified assumption is that, once Bob and Alice are both in the &quot;merged hole&quot; they can &quot;see&quot; eachother.</p>
<p>As between the outside and inside of a BH, there is a reversal of roles of the time and radial dimensions. What was time outside becomes space inside and the radial dimension inside is the new time dimension</p>
<p>So you have to ask many more questions like
&quot; Are there electrons, neutrons and protons inside a BH?&quot;</p>
<p>&quot; Are Alice and Bob therefore still &quot;matter&quot; inside a BH, and from whose perspective?&quot;</p>
<p>&quot;Is there light and other electromagnetic propagation inside?&quot;</p>
<p>I can't answer what becomes of an object falling into a BH although I have a suspicion that the whole idea of &quot;falling in&quot; is fallatious , but I can answer this: If the BH  is big enough so that the fall time to the center is longer than the age of our universe, then there can not only exist laws of physics like our own inside ( based on swapping r snd t with an appropriate tensor transformation) but also the internal universe would apparently be an expanding one starting with a Big Bang at the event horizon and ending with The Big Rip at the center.</p>

CC BY-SA 4.0
2022-09-22T12:49:20.530Z
2022-09-22T12:49:20.530Z
Paul
<p>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.</p>

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

<p>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?</p>

<hr>

<p>Edit: After watching the suggested video, I can try rewording the question:</p>

<p>Suppose Alice and Bob are outside blackholes A and B.</p>

<p>Alice jumps into blackhole A, and Bob jumps into blackhole B.</p>

<p>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).</p>

<p>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.</p>

<p>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?</p>

<hr>

<p>This might be related to <a href="https://astronomy.stackexchange.com/questions/13809/do-black-hole-singularities-actually-merge">Do black hole singularities actually merge?</a> and <a href="https://astronomy.stackexchange.com/questions/13895/what-happens-to-the-information-on-the-event-horizons-of-two-merging-black-holes">What happens to the information on the event horizons of two merging black holes?</a> 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.</p>

CC BY-SA 4.0
2019-09-05T07:46:37.643Z
related: https://astronomy.stackexchange.com/questions/32410/if-two-black-hole-event-horizons-overlap-touch-can-they-ever-separate-again
Florin Andrei
CC BY-SA 4.0
2019-09-05T16:56:54.683Z
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.
PM 2Ring
CC BY-SA 4.0
2019-09-05T16:59:37.840Z
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](https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator).
PM 2Ring
CC BY-SA 4.0
2019-09-05T18:00:56.333Z
Afaik there is some theorem saying that the total surface of the event horizons never decrease in any black hole configurations.
peterh
CC BY-SA 4.0
2019-09-06T07:31:10.593Z
@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?
Xenos
CC BY-SA 4.0
2019-09-06T07:47:17.870Z
They won't have plenty of time, though. Once you cross the EH, all motion *must* take you towards the core, although it's possible to increase the time a little, as discussed [here](https://physics.stackexchange.com/q/426143/123208). OTOH, that question is only about Schwarzschild BHs. Real BHs have a lot of angular momentum: most SMBHs are spinning at a significant fraction of lightspeed, which makes things more complicated, and may permit longer trajectories.
PM 2Ring
CC BY-SA 4.0
2019-09-06T07:51:05.780Z
I *think* that if Alice & Bob cross the EHs just before the EHs touch then Alice & Bob *might* be causally connected, but I don't know enough GR to do the necessary maths (or to write a numerical simulation to test it), which is why I didn't post an answer when you first posted this question.
PM 2Ring
CC BY-SA 4.0
2019-09-07T17:35:47.977Z
How would Alice and Bob reach a  black hole? All those we know of are far out of range and always will be.Assuming they got round that little difficulty,why would they be mad enough to dive into it? Spacecraft crews are highly selected,and mentally deranged crew members would be weeded out at an early stage.Assuming Alice and Bob somehow manage to fool the selection board (highly unlikely) why would other crew members not restrain them? By putting an ordinary working jacket on back to front,the empty arms could be tied behind their  backs,thus converting an ordinary jacket into a strait jacket
Michael Walsby
CC BY-SA 4.0
2019-09-09T08:39:37.543Z
@MichaelWalsby To tell the Truth, Alice and Bob are not actually people. It's a Beauty-full nickname I gave to two Charm-ing particles, to cheer them Up when they're feeling Down. You never name your favorite particles?! That's Strange...
Xenos
<black-hole>
Does blackhole merging break their event horizon segregation?
Xenos
# Does blackhole merging break their event horizon segregation?

(Asked by: username_0 on 2019-09-04)

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?](https://astronomy.stackexchange.com/questions/13809/do-black-hole-singularities-actually-merge) and [What happens to the information on the event horizons of two merging black holes?](https://astronomy.stackexchange.com/questions/13895/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.

**Tags:** black-hole
> **Comments:**
> **username_1**: related: https://astronomy.stackexchange.com/questions/32410/if-two-black-hole-event-horizons-overlap-touch-can-they-ever-separate-again  
> **username_2**: 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.  
> **username_2**: 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](https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator).  
> **username_3**: Afaik there is some theorem saying that the total surface of the event horizons never decrease in any black hole configurations.  
> **username_0**: @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?  
> **username_2**: They won't have plenty of time, though. Once you cross the EH, all motion *must* take you towards the core, although it's possible to increase the time a little, as discussed [here](https://physics.stackexchange.com/q/426143/123208). OTOH, that question is only about Schwarzschild BHs. Real BHs have a lot of angular momentum: most SMBHs are spinning at a significant fraction of lightspeed, which makes things more complicated, and may permit longer trajectories.  
> **username_2**: I *think* that if Alice & Bob cross the EHs just before the EHs touch then Alice & Bob *might* be causally connected, but I don't know enough GR to do the necessary maths (or to write a numerical simulation to test it), which is why I didn't post an answer when you first posted this question.  
> **username_4**: How would Alice and Bob reach a  black hole? All those we know of are far out of range and always will be.Assuming they got round that little difficulty,why would they be mad enough to dive into it? Spacecraft crews are highly selected,and mentally deranged crew members would be weeded out at an early stage.Assuming Alice and Bob somehow manage to fool the selection board (highly unlikely) why would other crew members not restrain them? By putting an ordinary working jacket on back to front,the empty arms could be tied behind their  backs,thus converting an ordinary jacket into a strait jacket  
> **username_0**: @MichaelWalsby To tell the Truth, Alice and Bob are not actually people. It's a Beauty-full nickname I gave to two Charm-ing particles, to cheer them Up when they're feeling Down. You never name your favorite particles?! That's Strange...  

---

## 3 answers

### Answer by username_5 on 2019-09-04. Score: 14

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](https://www.youtube.com/watch?v=BdYtfYkdGDk&t=3531s) (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](https://en.wikipedia.org/wiki/Collapsar). 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.

> **Comments:**
> **username_0**: 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?  
> **username_6**: A and B come into casual contact? That's an odd way to state that. Any reason you used the qualifier casual?  
> **username_2**: @MagicOctopusUrn Causal != casual  
> **username_6**: @PM2Ring I mean I figured it had a connotation here, and was asking what that connotation was. That did not help very much.  
> **username_2**: @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".  
> **username_6**: @PM2Ring thank you, that was the clarification I wanted, still pretty generic but gives me more to go on.  

---

### Answer by username_7 on 2020-08-09. Score: 1

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](https://www.youtube.com/watch?v=4nM6kf2OAFw) (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.

> **Comments:**
> **username_8**: This is fascinating. Can you supply any more references to this analysis of merging black holes that isn't completely buried in differential geometry?  
> **username_7**: @SteveLinton [Dieter Brill, *Black Hole Horizons and How They Begin*](https://doi.org/10.1080/21672857.2012.11519694) 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...  

---

### Answer by username_9 on 2022-09-22. Score: 0

The big unjustified assumption is that, once Bob and Alice are both in the "merged hole" they can "see" eachother.

As between the outside and inside of a BH, there is a reversal of roles of the time and radial dimensions. What was time outside becomes space inside and the radial dimension inside is the new time dimension

So you have to ask many more questions like
" Are there electrons, neutrons and protons inside a BH?"

" Are Alice and Bob therefore still "matter" inside a BH, and from whose perspective?"

"Is there light and other electromagnetic propagation inside?"

I can't answer what becomes of an object falling into a BH although I have a suspicion that the whole idea of "falling in" is fallatious , but I can answer this: If the BH is big enough so that the fall time to the center is longer than the age of our universe, then there can not only exist laws of physics like our own inside ( based on swapping r snd t with an appropriate tensor transformation) but also the internal universe would apparently be an expanding one starting with a Big Bang at the event horizon and ending with The Big Rip at the center.


---

