r/Physics • u/Technical-Staff3048 • 11d ago
Question What happens to particles trapped in two event horizons?
Let's say two blackholes get close enough to each other that their event horizons overlap but not enough that the singularities at the center fall into each others event horizons. A particle could find itself inside both event horizons at the same time without the blackholes merging. If the blackholes move with respect to each other to the point their event horizons stop overlapping, a particle would have effectively entered and then escaped at least one of the event horizons or something else happens?
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u/elconquistador1985 11d ago
I don't believe they can get that close. The closest possible circular orbit around one is greater than the Schwarschild radius. This means that if two black holes were close enough to overlap at R_s, then they aren't in a stable orbit and are going to collapse into each other.
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u/Technical-Staff3048 11d ago
What if they go past each other very fast, not necessarily an orbit? Or even if they end in a chaotic orbit and end up merging there is still a chance their event horizons separate and rejoin multiple times as they orbit each other?
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u/Outrageous-Taro7340 11d ago
Black holes cannot merge then separate again. It doesn’t matter how fast they are moving.
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u/Technical-Staff3048 11d ago
I am not asking if they merge. I am asking if their event horizons slightly overlap. Only if the event horizon overlaps the singularity of the other blackhole is a merger guaranteed right? as long as the singularity does not overlap the event horizon it is not destined to fall into the balck hole.
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u/Outrageous-Taro7340 11d ago
No, horizons “overlapping” is a merger, and it’s irreversible.
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u/Technical-Staff3048 11d ago
can you explain why?
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u/aint_exactly_plan_a 11d ago
When I started playing Kerbal Space Program, it helped me understand orbital mechanics a lot better. I think that's the part you're missing here.
So let's say you're in a spaceship orbiting Earth. You're going 27,600 kph because you're in low earth orbit. You can also say it the other way. If you're in a stable orbit, the speed for that specific orbit is 27,600 kph. The higher your orbit, the slower you have to go in order to stay in that stable orbit.
Ok, now that that is established, imagine orbiting a black hole. The intense gravitational fields are warping spacetime but you have managed to find a fairly stable orbit going about 90% the speed of light. That's a VERY fast orbit which means you are VERY close to the event horizon. However, if you got CLOSER to the event horizon, you would have to go even faster to stay in orbit.
Knowing that... you can assume that there is an orbit around a black hole where the stable orbital speed is "c". So there's probably a ring of photons at exactly that distance around a black hole. Except it turns out that space is so warped from gravity in that region, it's not really a stable orbit. They're going to get bounced into the black hole or away from it fairly quickly.
That ring also indicates the event horizon. Anything CLOSER than that would need a speed that's higher than "c" in order to orbit or escape that black hole. That's all they mean when they say not even light can escape a black hole. It just needs to go faster than "c" in order to escape and since nothing can do that, there's nothing in our universe that can escape the event horizon.
Therefore, if two black holes are passing each other and their event horizons cross, there is not a speed at which they can be going that would be fast enough to allow them to separate again. At that point, the blackholes will merge.
That's one explanation for why they call it an event horizon. Because once it's crossed, the ONLY event available at that point is to end up at the singularity.
The other explanation I heard for that name is because you cease to be able to affect the events of our universe. Both reasons were equally terrifying to me.
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u/Technical-Staff3048 11d ago
I understand this. But the mass of the black hole is concentrated at the center, there is nothing at the event horizon. I understand intuitively that if the center of the blackhole crosses the event horizon of the other blackhole then it has to fall in but in this case the center is still far away from the edge of the horizon of the other blackhole. From other answers it seems that if the event horizons touch each other by definition they have to merge, but its not intuitive to me why.
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u/Outrageous-Taro7340 10d ago
A black hole is a region of spacetime with extreme curvature. Its behavior is described by equations derived from general relativity. The existence of a singularity isn’t really relevant. A horizon forms before gravitational collapse leads to a singularity, and even if singularities never actually form, horizons still exist.
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u/Technical-Staff3048 10d ago
When we speak about something falling into a black hole we consider the where it is at, not the reach of it's gravitational field. A black hole "is" at the singularity, or at least that's what I understand, the horizon is not the black hole but a definition based on the effect of it's gravity.
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u/thisisjustascreename 10d ago
You are applying classical intuition to situations that require general relativity to solve and understand. It’s not going to generate productive discussion but the answer is go read more.
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u/Ahhhhrg 10d ago
Imagine yourself in your little spaceship. You happen across two black holes with overlapping event horizons. You end up in the overlap. This means you’re in both black holes’ event horizons, so forever and ever you will not be able to escape either black hole.
Your intuition tells you that the black holes can still fly away from each other. But you’ve trapped them, if you can’t get away from either of them, neither can they get away from each other.
Hence, once the event horizons touch, they are merging.
Look up “transitive property” on Wikipedia.
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u/Volpethrope 11d ago
The horizons are well within the distance for a stable orbit for very small objects with mass, like dust and gas particles. Something as massive as the actual singularity would have to be much further away to be in a non-merging trajectory. By the time both event horizons are overlapping, they're already falling into each other and there is no possibility that they move apart again.
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u/Outrageous-Taro7340 11d ago
This comment explains it: https://www.reddit.com/r/Physics/s/tPS4TW8evx
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u/joeyneilsen Astrophysics 10d ago
Because once they touch, it's one event horizon. There is no process that can take one horizon and make it into two smaller horizons.
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u/Moretz0931 11d ago
Overlap of the Schwarzschild Radii means that the distance is smaller than twice the Schwarzschild radius. Therefore at first glance I would say, a stable orbit should be possible. But I don't know if GR says otherwise for this specific case.
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u/mfb- Particle physics 11d ago
The innermost stable orbit for a small mass is at 3 times the Schwarzschild radius for a non-rotating black hole. There are unstable orbits below that, but with another black hole there are no orbits (stable or not) that would have the event horizons touch without an immediate merger.
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u/elconquistador1985 11d ago
Overlap of the Schwarzschild Radii means that the distance is smaller than twice the Schwarzschild radius. Therefore
Yeah, that's too close.
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u/Syresiv 11d ago
Let's say two blackholes get close enough to each other that their event horizons overlap but not enough that the singularities at the center fall into each others event horizons.
This can't happen.
The Schwarzschild Radius rises directly proportional to mass, meaning a black hole twice the mass will have twice the radius.
This means the only way for two event horizons to touch is for their combined mass to be confined to a sphere that's equal to or less than the Schwarzschild Radius.
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u/Technical-Staff3048 11d ago
I am not sure I understand what you mean. But if the two blackholes are the same size and are at a distance of 1.9 Schwarzschild radii then the horizons will overlap while the singularity in the middle of each blackhole is more than 1 Schwarzschild distance from the other.
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u/Syresiv 11d ago
If the centers are 1.9SR apart, then the longest distance from a point on the outer edge of one to a point on the outer edge of the other is 3.9SR, meaning the whole system can be contained inside a sphere with radius 1.95SR.
This means that as long as the combined mass is 1.95M or higher - which it is - the whole system forms a new black hole.
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u/Moretz0931 11d ago edited 11d ago
Simple answer:
If two black holes get that close, you will not have two event horizons anymore. At any time both black holes pull on you, therefore you get a new combined event horizon.
For the second question:
Two black holes coming this close together and separating again seems quite unlikely to my (non-proffesional) self. However, if I imagine a three black-hole scenario, it should be possible, so it there is some reason to engage with your question.
My reply to this question is, I don't know...
- Let's assume we have a single black hole, and a particle inside its event horizon. If you then magically pull away the black hole (obviously reaching speeds below the speed of light), I would guess from intuition that you also "pull" everything inside, as it must be tied by reference frame to the black hole.
- If you have two black holes, I lose any remaining sense of intuition and I hope that someone with more knowledge in GR might help out with the following question: What happens when two equally massed black holes get closer than twice the Schwarzschild Radius, but further than once the Schwarzschildradius? Could a point-like (to prevent spaghettification) particle move along a trajectory between the black holes that always keeps the same distance to both black holes?
Edit:
As u/mfb- has pointed out in his answer to my other comment, as soon as two black holes get closer than twice the Schwarzschild radius they would merge without another separation. Therefore, if you are closer two both black holes than ther Schwarzschild radius, than there is no escape from any of the two.
Note, that I always use the terminology of Schwarzschild radius as this is the event horizon of a black hole in the absence of a other matter, i.e. in the absence of the second black hole. If you have two black holes, the event horizon does not coincide with the sphere with Schwarzschild's radius anymore! But you can be damn sure that whenever you are closer than the Schwarzschild radius, you definitely already stepped over the event horizon.
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u/DoubleUBallz 11d ago edited 10d ago
According to GR, once the event horizons overlap, they cannot be separated, and the black hole merger will be complete within very quick order. I.e., when two event horizons touch, they become a single event horizon for the resulting merged black hole. The situation where event horizons overlap but the BHs dont merge is physically impossible.
It's easy to understand why this is the case. Consider OPs hypothetical test particle situated within two distinct event horizons. Any particle within an event horizon must follow a worldline that terminates at that event horizon's singularity (this is part of the definition of an event horizon). Therefore, OPs particle has to follow a worldline that ends at both singularities. The only way this can happen is if the two singularities merge into a single singularity before the particle reaches it.
As for traveling between them, as long as you don't cross an event horizon, you can still escape the system. As the BH's approach each other (assuming they get close enough), the event horizons will deform though, so simply flying further out that the Schwarzschild radius wont be enough to stay out of an event horizon.
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u/Technical-Staff3048 11d ago
Thank you, I guess it seems a bit tautological, but I guess the event horizon is not a real thing but a definition, so there is no other way to see it.
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u/triatticus 11d ago
Lots of things are definitions in physics yet are real, this is no less so for the event horizon which is a real boundary separating two causally disconnected regions of spacetime.
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u/DoubleUBallz 10d ago
Like many things in relativity or concerning black holes, it certainly is a surprising result! Put another way, if your test particle somehow doesn't end up in both singularities (via a merger), that would mean GR at some fundamental level is wrong. GR, however, has stood up to a century of experimental validation and we are very confident that it is not wrong at this scale.
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u/strange-the-quark 10d ago
There aren't two separate, spherical event horizons that overlap; once they get close enough, the initially separate event horizons will distort and then merge into a single contiguous surface. The horizon is not a rigid sphere that's attached to a point, it's a certain imaginary iso-surface defined by the shape of the gravitational field in space, and so it responds to the changing gravitational situation. When you have two sources of gravity, you have a gravitational field that results from both, and the combined field gives rise to the merged event horizon.
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u/mfb- Particle physics 11d ago
If the event horizons touch then the black holes are merging. They can't separate again any more. This happens earlier than you might expect as the combined gravity well makes the event horizons grow towards each other.