Timeline for If two black hole event horizons overlap (touch) can they ever separate again?
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| Jun 27, 2019 at 17:02 | comment | added | Florin Andrei | @TheLuckless A college-level class in relativity would provide the definitive answers to all those questions and more. And those would be true answers, not pop-sci metaphors. | |
| Jun 27, 2019 at 16:55 | comment | added | TheLuckless | Cleaner details on why touching EHs merge both blackholes into a single blackhole may help this answer be easier to understand. - IE, why is it considered a single EH as soon as they deform enough to 'touch', rather than one EH where anything goes to Blackhole A, EH where anything goes to BH-B, and an 'overlapping zone' of both EHs where 'extremely weird stuff probably happens...' | Confusion may arise from thinking like "EH is 'an area of space', and black holes can shrink, which suggests space can 'escape' a blackhole, so why can't one EH 'escape' another EH?" | |
| Jun 27, 2019 at 6:42 | comment | added | Robert | Can you explain how evaporation figures into this? If the black holes are shrinking - and small ones shrink fast - couldn't they shrink apart? Particles caught in both would be destroyed or go wherever it is everything else in a black hole goes when it evaporates. | |
| Jun 27, 2019 at 1:23 | comment | added | Florin Andrei | @Michael No particle is needed for the merger to complete once it has started. All of the above are just intuitive models. | |
| Jun 26, 2019 at 23:22 | comment | added | Michael | @LorenPechtel That is a truly fascinating explanation - basically even a single virtual particle getting trapped in the overlap of two event horizons would be enough of a fixed point of sorts to "lasso" both black holes and irrevocably bind them together. It's not the interaction of anything we would normally think of as "real" (matter, energy, etc.) but more like both holes are getting snagged on the overlapping spacetime itself. | |
| Jun 26, 2019 at 16:34 | comment | added | Loren Pechtel | @Rob Another way of looking at the merger problem: In a simplistic world the space grabbed is entirely empty, but in the real world it's certain to contain particles (if nothing was falling in you still have virtual particles.) Imagine one particle in the overlap zone. It's part of A and can never be pulled away. It's also part of B and can never be pulled away. You have a real world example of an infinitely strong object--even a yank that makes the Death Star look like a candle can't break it. | |
| S Jun 26, 2019 at 13:24 | history | mod moved comments to chat | |||
| S Jun 26, 2019 at 13:24 | comment | added | called2voyage♦ | Comments are not for extended discussion; this conversation has been moved to chat. | |
| Jun 26, 2019 at 9:33 | comment | added | PM 2Ring | @Aron According to the proper time of the infalling matter. That is, if an indestructible clock is inside the EH, it won't tick for long before it reaches the centre of the BH. | |
| Jun 26, 2019 at 8:54 | comment | added | Aron | @PM2Ring Rapidly? According to, who, specifically where. :P | |
| Jun 26, 2019 at 1:48 | comment | added | Rob | @Draco18s and SteveLinton - thank you both very much. The explanation about the lagrange point between the holes and the image finally made this 'click' for me! | |
| Jun 25, 2019 at 23:48 | comment | added | Draco18s no longer trusts SE | @Paulpro No, that's the point where the "bridge" between the two egg-shaped-protrusions from the original two black holes touch and combine their event horizons and become "one" blackhole. | |
| Jun 25, 2019 at 20:28 | comment | added | Paul | @Draco18s Does that mean that in theory something moving in that exact direction (would that be practically impossible, because the center of mass of the two black holes is not stationary) would become a third black hole at the center of mass of the two black holes, eventually causing three black holes to merge at the same time? | |
| Jun 25, 2019 at 19:05 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Jun 25, 2019 at 18:23 | comment | added | Draco18s no longer trusts SE | @Fax The egg-shaped regions prevent things from leaving too. Anything "moving perpendicular" to the "boundary" between the two black holes still needs enough momentum to overcome the combined gravity pulling it back towards the Lagrangian point between the two black holes (remember, you need to fight against both, even if you could escape if there was only one of them). If the escape velocity at the Lagrange point exceeds the speed of light, then the Lagrange point is, itself, inside the combined event horizon and the black holes merge. | |
| Jun 25, 2019 at 15:39 | comment | added | PM 2Ring | @Guimoute Black holes aren't full of matter like your diagram depicts. GR says that any matter inside the EH (event horizon) must rapidly fall to the centre of the BH, and once it crosses the EH it's causally disconnected from the outside universe. (We don't know exactly what happens at the very core of a BH, we need a proper theory of Quantum Gravity for that). So we only need to consider the gravitational field outside the EH. Please see How does the gravity get out of a black hole? | |
| Jun 25, 2019 at 15:22 | comment | added | PM 2Ring | @Rob Increasing the relative speed of the BHs doesn't help them escape each other. In fact, it has the opposite effect.The source of the gravitational field in the Einstein field equations of GR is the stress-energy-momentum tensor. So the additional KE & momentum increases the gravity in the region where the BHs meet. | |
| Jun 25, 2019 at 14:52 | comment | added | Steve Linton | @Fax images.vice.com/motherboard/content-images/article/no-id/… gives a bit of an idea. Each black hole is a region of very low gravitational potential. Their combined effect makes the potential low in the region between them. | |
| Jun 25, 2019 at 14:31 | comment | added | Guimoute | I do not understand that answer. If two event horizons touch, there is nothing to trap there. It's too virtual frontiers intersecting. I thought the only thing that counts is if one of the black hole matter is in the other black hole's EH. Like so: i.imgur.com/Zsj3two.png | |
| Jun 25, 2019 at 12:21 | comment | added | Fax | Interesting. The egg-shaped event horizons are quite unintuitive. I would have thought that the space in the middle between two equally powerful attractors would allow any matter travelling perpendicularly to escape with ease, i.e. the event horizons would repel one another and be squashed. Why is the opposite the case? | |
| Jun 25, 2019 at 8:53 | comment | added | Steve Linton | @Rob It seems like your mental model basically has the mass and the event horizon having locations in space just like normal objects. That's not how it is. Both the singularity and the event horizon are aspects of extremely curved space-time and their behaviour can only be understood, even approximately, in that curved space time. Look at youtube.com/user/SXSCollaboration for some simulations that take this into account | |
| Jun 25, 2019 at 7:56 | comment | added | Rob | I guess what's difficult for me to reconcile in my head is that if the singularity (assuming all the matter is located at a single point) doesn't cross another event horizon - why should it be unable to escape? I understand the event horizons merge, however, if we think of the two singularities (surely they don't instantaneously merge?) having their own schwarzschild radius, why should their intersections spell doom for their respective singularities? If a sun partially crossed an event horizon, I would imagine only the part of the sun that crossed the event horizon would be trapped forever | |
| Jun 25, 2019 at 7:42 | comment | added | Florin Andrei | @Rob The center is not privileged. Forget the center. Once the bridge has been created, for all intents and purposes it's one black hole. There is no "overlap", your image is wrong - the two entities have merged already, there's a single event horizon, not two (see the answer I've linked at the end). And you cannot split chunks off an event horizon, no matter what you do. Forget the 0.999c, that's nothing. Most people don't realize how truly scrambled is the spacetime within the event horizon. There really is no way out, it's not a figure of speech. | |
| Jun 25, 2019 at 6:14 | comment | added | Rob | For example, in this picture: i.sstatic.net/Kgkhy.png - If each blackhole was travelling at 0.999c in opposite directions, surely the centers here would be able to escape the other event horizon? And as an additional question: in the overlapping region, would there not be an area which was not part of either event horizon, due to the gravitational fields of both blackholes 'cancelling' out? | |
| Jun 25, 2019 at 6:10 | comment | added | Rob | If the two EHs touch, the centers of the blackholes are still not inside each other's event horizon. Depending on their sizes, the centers may be quite a long way away from the other's event horizon... so surely if they're going fast enough, they can escape even after the event horizons merge? | |
| Jun 25, 2019 at 5:58 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Jun 25, 2019 at 5:52 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Jun 25, 2019 at 5:45 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Jun 25, 2019 at 5:15 | history | edited | Florin Andrei | CC BY-SA 4.0 |
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| Jun 25, 2019 at 0:03 | history | answered | Florin Andrei | CC BY-SA 4.0 |
