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### Is it possible to escape from a black hole using another black hole?

Posted: Sat Apr 27, 2019 4:57 pm UTC
If a probe entered a blank hole event horizon, then immediately after, the black hole passed by another black hole in such a way that a pocket of reduced gravity is created in the section of the event horizon where the probe is located, would be possible for the probe to surf that gravity pocket and escape from the black holes?

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sat Apr 27, 2019 9:21 pm UTC
you left relativity when you wrote "immediately after". time has a completely different meaning once past the event horizon.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sat Apr 27, 2019 9:57 pm UTC
Fine, not immediate, the velocity of the other object toward you is much faster than your infalling v.

GR is nonlinear so this question is hard. I'm not sure how the horizon gets distorted. Hmmm.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sat Apr 27, 2019 11:40 pm UTC
D-503 wrote:
a pocket of reduced gravity is created

The force on the probe might be reduced, but that doesn't mean the energy (velocity) it would need to escape is reduced.

Something at the center of Earth would feel weightless, but escape velocity there is even higher than it is on the surface.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 4:21 pm UTC
What the OP's question suggested to me is two co-orbiting black holes with equal mass, some time before they coalesce, but after the event horizons overlap. If you could manage to aim you ship in along such a path that you remained equidistant from the black holes, could you get through the overlapped event horizons?

I have no frikkin' idea what would happen, but that's what I visualized when I read the question. My gut feeling is "maybe".

Then again ... perhaps the time between when the event horizons touch, and the singularities merge, is just too small to allow passage along the hypothesized trajectory. The typical "chirps" that LIGO picks up suggest things happen pretty fast near the end of the merger.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 5:00 pm UTC
Heimhenge wrote:What the OP's question suggested to me is two co-orbiting black holes with equal mass, some time before they coalesce, but after the event horizons overlap. If you could manage to aim you ship in along such a path that you remained equidistant from the black holes, could you get through the overlapped event horizons?

I have no frikkin' idea what would happen, but that's what I visualized when I read the question. My gut feeling is "maybe".

Then again ... perhaps the time between when the event horizons touch, and the singularities merge, is just too small to allow passage along the hypothesized trajectory. The typical "chirps" that LIGO picks up suggest things happen pretty fast near the end of the merger.

I'm thinking of a scenario where the black holes are moving fast enough that they do not combine in the end and the probe escapes by staying between them as they move apart from eachother. If they did combine I'm pretty sure areas where their event horizons overlap would be part of a shared event horizon.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 5:24 pm UTC
If they're not capturing each other, one's not picking things out of the event horizon of the other.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 6:28 pm UTC
The stability of two black holes in orbit around each other is short lived ... even if both have the requisite speed for a circular orbit. Emission of gravitational waves causes the system to lose energy, so the death spiral is inevitable. And as I said, the closer they get the faster this happens.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 8:18 pm UTC
I'm imagining a one time fly by where the black holes end up traveling away from each other rather than in orbit. Is there a theorem that if their event horizons overlap they must combine with rather regardless of velocity? It seems like an escape should be theoretically possible as long as the central singularities are not within the event horizons of each other.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 10:24 pm UTC
You carelessly dropped your probe past an event horizon. No amount of acceleration can allow it to escape. Your first idea is thus to try accelerating it, via a nearby gravitational source. Why exactly is that supposed to help?

Looking at simulations of black hole mergers, it appears as if the area between two black holes is even less escape-friendly than usual; the event horizons aren't shrinking, they're growing. You aren't doing your probe any favors by bringing another one of those damn black holes around.

The properties of event horizons also tell us what happens when two event horizons overlap: any particles within the overlap must get closer to the centers of both black holes, and for that to happen those centers need to get closer to each other. The black holes will merge. Heimhenge's ship will be lost, and the insurance will refuse to pay as the pilot is deemed grossly negligent for flying into not just one, but two event horizons.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Sun Apr 28, 2019 11:55 pm UTC
Is it possible to have a stellar mass black hole inside a supermassive black hole, such that some objects are outside both, some are inside both, and some are inside the supermassive black hole but outside the stellar mass black hole? Obviously they will merge eventually, but before they do . . .

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 1:20 am UTC
Tub wrote:You carelessly dropped your probe past an event horizon. No amount of acceleration can allow it to escape. Your first idea is thus to try accelerating it, via a nearby gravitational source. Why exactly is that supposed to help?

Looking at simulations of black hole mergers, it appears as if the area between two black holes is even less escape-friendly than usual; the event horizons aren't shrinking, they're growing. You aren't doing your probe any favors by bringing another one of those damn black holes around.

The properties of event horizons also tell us what happens when two event horizons overlap: any particles within the overlap must get closer to the centers of both black holes, and for that to happen those centers need to get closer to each other. The black holes will merge. Heimhenge's ship will be lost, and the insurance will refuse to pay as the pilot is deemed grossly negligent for flying into not just one, but two event horizons.

I would describe my idea more as using a gravitational source to warp the event horizon so the probe is no longer inside it rather than to accelerate it out.

I phrased my question about overlapping event horizons in a flawed way. What I meant is to refer to is the situation where the event horizon spheres that would occur in the absence of other gravitational sources overlap rather than the actual distorted event horizons overlapping.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 1:51 am UTC
You seem to be imagining the event horizon shrinking away from the probe as some of the black hole's gravity is canceled out by the gravity of the other more distant black hole. But I don't think that's actually happens in close approaches of black holes. On the contrary, I think the event horizon tends to expand. This effect is most obvious when you look at simulations of black hole mergers, but a similar, subtler effect should happen even if one black hole is just passing by the other on a hyperbolic trajectory. The reason was given above by gmalivuk: bringing another black hole in only makes the gravitational well deeper, requiring even more energy to escape. There may be less gravitational force at that point, but that doesn't mean you can escape the black hole from there. I think the example of the center of the Earth was illustrative.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 3:05 am UTC
Good old fashioned Newtonian potential "reaches out" to nearby masses, which by analogy makes me think event horizons would do the same thing, even without seeing GR simulations of what happens. It's like two drops of water merging, not a drop of water and a drop of oil.

If something is on the other side of an event horizon, then by definition it is not in the causal past of any future event in the outside universe. If it can escape, then it was never behind an event horizon in the first place.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 10:27 am UTC
Eebster the Great wrote:Is it possible to have a stellar mass black hole inside a supermassive black hole, such that some objects are outside both, some are inside both, and some are inside the supermassive black hole but outside the stellar mass black hole? Obviously they will merge eventually, but before they do . . .

A merger is a gradual process, with both holes approaching each other at a finite speed, so it seems sensible to assume that one such intermediate step is as you described. But the terminology doesn't allow it.

If you take "inside a black hole" to mean "inside the outermost event horizon", then you'll discover that both of your objects have created a combined horizon that's larger than the sum of the previous horizons (the mass is proportional to the radius, not the volume). These horizons cannot be separated. There's no place where you can permanently escape one of the black holes but not the other - everything inside that event horizon is by definition inside both black holes. From the outside perspective, they're now a single black hole, even before their centers meet.

You can try arguing about some of the inner event horizons (remember, every spherical surface below the outer event horizon is also an event horizon), or you can measure the distance to each center, or you can determine for each particle which center it'll reach first. That's kinda complicated, but it might allow you to make a statement similar to the one you were trying to make.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 10:34 am UTC
If you pass through a surface and then escape to an arbitrary distance. The surface you passed was not an event horizon. So no.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 2:55 pm UTC
Given that the black holes would be passing each other at extreme speed (i.e. high Lorentz factor), there is likely to be some severe frame dragging involved as well.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 3:56 pm UTC
Tub wrote:
Eebster the Great wrote:Is it possible to have a stellar mass black hole inside a supermassive black hole, such that some objects are outside both, some are inside both, and some are inside the supermassive black hole but outside the stellar mass black hole? Obviously they will merge eventually, but before they do . . .

A merger is a gradual process, with both holes approaching each other at a finite speed, so it seems sensible to assume that one such intermediate step is as you described. But the terminology doesn't allow it.

If you take "inside a black hole" to mean "inside the outermost event horizon", then you'll discover that both of your objects have created a combined horizon that's larger than the sum of the previous horizons (the mass is proportional to the radius, not the volume). These horizons cannot be separated. There's no place where you can permanently escape one of the black holes but not the other - everything inside that event horizon is by definition inside both black holes. From the outside perspective, they're now a single black hole, even before their centers meet.

You can try arguing about some of the inner event horizons (remember, every spherical surface below the outer event horizon is also an event horizon), or you can measure the distance to each center, or you can determine for each particle which center it'll reach first. That's kinda complicated, but it might allow you to make a statement similar to the one you were trying to make.

But suppose I am in a spaceship in orbit about a stellar mass black hole a few Schwarzschild radii from its center. Now suppose the black hole is falling into a supermassive black hole. Surely I should not see anything strange happen to the black hole as we cross the event horizon of the supermassive black hole, right? Just like nothing strange should happen to any other orbit.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Mon Apr 29, 2019 4:48 pm UTC
Eebster the Great wrote:But suppose I am in a spaceship in orbit about a stellar mass black hole a few Schwarzschild radii from its center. Now suppose the black hole is falling into a supermassive black hole. Surely I should not see anything strange happen to the black hole as we cross the event horizon of the supermassive black hole, right? Just like nothing strange should happen to any other orbit.

Correct, you're experiencing nothing special when passing the event horizon. That's true for single black holes as well, or cosmological event horizons. Did you know there's an event horizon in your room right now? Did you notice passing through it?

Still, the volume of a black hole being defined the way it is, your ship is now in both black holes. That has no special physical meaning for the crew right this very moment, it just restricts their possible future timelines. Even though your distance to the center of the smaller black hole hasn't decreased, you can no longer escape it; hence you're inside.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue Apr 30, 2019 8:38 pm UTC
D-503 wrote:If a probe entered a blank hole event horizon, then immediately after, the black hole passed by another black hole in such a way that a pocket of reduced gravity is created in the section of the event horizon where the probe is located, would be possible for the probe to surf that gravity pocket and escape from the black holes?

My understanding of black holes admittedly comes mostly from movies and video games, but wouldn't the resulting tidal forces of two black holes in such close proximity simply rip the probe to shreds? Barring that - say you've got an indestructible probe - wouldn't it just be pulled into the other one as soon as it exited the first one? Seems like the window'd be pretty slim to actually escape both.

All in all, this sounds like a terrible plan.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue Apr 30, 2019 8:44 pm UTC
The tidal force directly between two black holes is just the sum of the tidal forces from each one of them. If the probe can avoid spaghettification from one, it can avoid it from both by being "only" twice as strong.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Wed May 01, 2019 2:51 am UTC
Or the probe could just be microscopic. Not much tidal force across a one micron robot.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Wed May 01, 2019 7:56 pm UTC
There is one sense in which you could (at least temporarily) 'escape' the original black hole. When the second black hole gets close enough, it will be possible to escape the Schwarzchild radius of the original singularity, and even to reach a point that is outside the Schwarzchild radii of both individual singularities. The problem is that by the time the two singularities are that close to each other, the event horizon is no longer a sphere defined by the Schwarzchild radii, and is in fact larger than the combination of the event horizons that would exist for each singularity in isolation.

So despite falling into the event horizon of an individual singularity, you could conceivably die by spaghettification as you fall into a different singularity, and thus claim to have "escaped" the original singularity. Of course, eventually the two singularities will merge, but you don't really exist as a distinct thing anymore at that point, so in some sense you've still escaped.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Wed May 01, 2019 9:11 pm UTC
I'm not sure any of that is true.

Past the event horizon, it's not simply that it's impossible to leave the event horizon, but rather that "away from the singularity" actually means "backward in time".

I know a second black hole means Schwarzschild no longer quite applies, but since the second singularity necessarily approaches the first one extremely quickly, I wouldn't be surprised if it remained impossible to move away from either singularity once inside the or joint event horizon.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Wed May 01, 2019 11:16 pm UTC
Ok so what would happen if a black hole made of ice collided with a black hole made of lava (regular black hole)? Both black holes would be the same size and ice-made hole's temperature would be -1000 degrees of celsius and normal hole's temperature +1000 degrees of celsius.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Wed May 01, 2019 11:31 pm UTC
easy they cancel out, you get big black hole made of solid rock

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 8:00 am UTC
Eebster the Great wrote:Or the probe could just be microscopic. Not much tidal force across a one micron robot.

In these thought experiments, the best kind of probe is always a photon.

Eebster the Great wrote:Ok so what would happen if a black hole made of ice collided with a black hole made of lava (regular black hole)?

The temperature of a black hole as seen by the outside observer is determined by hawking radiation, which depends on mass and nothing else.

The temperature on the inside of a black hole is simply unknown. If you simultaneously dump ice and lava into a black hole it's conceivable that they'll be able to interact and equalize while they're still far away from the center. But as the masses approach the center, something is going to happen to them, and they'll no longer be matter as we know it. We don't know if they continue to have a property that resembles temperature. We don't know if the weird spacetime near the center allows different masses to interact with each other in a way that allows for equalization.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 8:10 am UTC
Tub wrote:
Eebster the Great wrote:Ok so what would happen if a black hole made of ice collided with a black hole made of lava (regular black hole)?
The temperature of a black hole as seen by the outside observer is determined by hawking radiation, which depends on mass and nothing else.
OK I get that, but what if instead of putting in mass you put in ice and lava? Who would win?

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 9:17 am UTC
You have massless ice? I'm sure that's going to have excellent culinary applications. Can you make it tasty while keeping it at 0 calories?

I'm not sure if I'm misunderstanding your question, or if you stopped reading halfway through my last post. Gravity wins. Your feeble contenders will be destroyed.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 11:01 am UTC
Here are some relevant Physics Stack Exchange questions.

Black hole collision and the event horizon. The top answer there has links to several diagrams.

Why stop at lava vs ice when you can do matter vs antimatter?
What happens if you dump antimatter into a black hole?

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 11:12 am UTC
Those are very interesting, but it seems my question is still a subject of serious debate among researchers.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Thu May 02, 2019 3:51 pm UTC
If you put in lava rather than cold matter, the extra heat energy would register as mass-energy right?

You can increase the mass of a BH without adding any matter at all, you can just add energy (photons) and the mass increases IIRC.

And I was discussing the antimatter question recently too - but isnt it identical to adding matter? Any reaction that happens behind an EH is irrelevant, no?

Although I also recall something about the exact relationship between AM and gravity is unknown, as we have not been able to create enough antimatter to observe its behaviour in gravity (too small amounts are subject to diffusion, thermal effects etc which massively overpower the effect of gravity on tiny amounts. Or something.)

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Fri May 03, 2019 8:49 pm UTC
Heimhenge wrote:What the OP's question suggested to me is two co-orbiting black holes with equal mass, some time before they coalesce, but after the event horizons overlap. If you could manage to aim you ship in along such a path that you remained equidistant from the black holes, could you get through the overlapped event horizons?
If we have two non-rotating black holes A and B, both with Schwarzschild radii of r, once we bring them within 2r of each other, thy will be unable to escape their combined pulls and fall to the common center to form a new black hole C.

In the situation you describe, the probe in near the common center already and will be unable to escape the Schwarzschild radii of A, B or C.

In a sense, the probe will fall into A; in a sense, the probe will fall into B; in a sense, both A and B will fall onto the probe. Relativity shows us the we can have these multiple descriptions of motion all be correct.
D-503 wrote:I'm thinking of a scenario where the black holes are moving fast enough that they do not combine in the end and the probe escapes by staying between them as they move apart from eachother.
It's not possible for the black holes to be moving that fast.

Increasing the velocity of the black holes makes them heavier, and the attractive force scales faster than the centrifugal force.
Eebster the Great wrote:Is it possible to have a stellar mass black hole inside a supermassive black hole, such that some objects are outside both, some are inside both, and some are inside the supermassive black hole but outside the stellar mass black hole? Obviously they will merge eventually, but before they do . . .
Kind of. Let's call the original supermassive black hole A, the stellar one B, and the combined one C.

To an observer at a distance, they would see B and a probe around B, both fall into A. After that, the observer would just see C.

The probe would see B red-shift, time-dilate and move away. This can be determined by determining how the past light cone of the probe intersects with B: As the probe is closer to B, it's pas light cone is more heavily bent "outwards"; as it's more bent, the total proper distance to B is increased. Asa larger section of the probes time corresponds to a smaller section of B's time, B appears time dilated.
Tub wrote:You have massless ice?
Yeah, it floats and everything

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 1:41 am UTC
Heimhenge wrote:What the OP's question suggested to me is two co-orbiting black holes with equal mass, some time before they coalesce, but after the event horizons overlap. If you could manage to aim you ship in along such a path that you remained equidistant from the black holes, could you get through the overlapped event horizons?
If we have two non-rotating black holes A and B, both with Schwarzschild radii of r, once we bring them within 2r of each other, thy will be unable to escape their combined pulls and fall to the common center to form a new black hole C.

In the situation you describe, the probe in near the common center already and will be unable to escape the Schwarzschild radii of A, B or C.

In a sense, the probe will fall into A; in a sense, the probe will fall into B; in a sense, both A and B will fall onto the probe. Relativity shows us the we can have these multiple descriptions of motion all be correct.

If black holes come within their combined Schwarzschild radii of each other are they always destined to merge regardless of their spin and sizes?

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 2:23 am UTC
I think the event horizons of Kerr black holes are oblate spheroids, or something similar, and the minor radius is smaller than the Schwarzschild radius. This should in principle allow two Kerr black holes that are rotating extremely quickly to pass by each other without their event horizons overlapping but such that the distance between the centers is less than the sum of their Schwarzschild radii.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 3:39 am UTC
Is it not more important whether they pass within where the event horizon of their combined mass and rotation (and charge, but that's basically always negligible) would be?

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 11:39 am UTC
Eebster the Great wrote:I think the event horizons of Kerr black holes are oblate spheroids, or something similar, and the minor radius is smaller than the Schwarzschild radius. This should in principle allow two Kerr black holes that are rotating extremely quickly to pass by each other without their event horizons overlapping but such that the distance between the centers is less than the sum of their Schwarzschild radii.

Both the Kerr metric and the Schwarzschild metric are valid only in empty, non-expanding, globally flat universes with a single black hole. If you do something drastic like adding another nearby black hole, neither the Schwarzschild metric nor the Kerr metric will make accurate predictions about the size and shape of the event horizon. The actual event horizons may very well exceed the Schwarzschild radii as the black holes get closer.

And there's reason to believe that they do. Assume two black holes with masses M1 and M2, and Schwarzschild radii of Rs(M1) and Rs(M2). If the distance of their centers is d <= Rs(M1) + Rs(M2), then their combined mass of (M1+M2) is within a sphere of r = Rs(M1)+Rs(M2) = Rs(M1 + M2). In other words, the combined mass is located within its own Schwarzschild radius.

I'm not sure if that calculation is accurate in non-euclidean situations like those, but I'd still bet that your black holes combine, no matter how fast they rotate.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 11:56 am UTC
D-503 wrote:
Heimhenge wrote:What the OP's question suggested to me is two co-orbiting black holes with equal mass, some time before they coalesce, but after the event horizons overlap. If you could manage to aim you ship in along such a path that you remained equidistant from the black holes, could you get through the overlapped event horizons?
If we have two non-rotating black holes A and B, both with Schwarzschild radii of r, once we bring them within 2r of each other, thy will be unable to escape their combined pulls and fall to the common center to form a new black hole C.

In the situation you describe, the probe in near the common center already and will be unable to escape the Schwarzschild radii of A, B or C.

In a sense, the probe will fall into A; in a sense, the probe will fall into B; in a sense, both A and B will fall onto the probe. Relativity shows us the we can have these multiple descriptions of motion all be correct.

If black holes come within their combined Schwarzschild radii of each other are they always destined to merge regardless of their spin and sizes?

Given the "reaching out" that the even horizons do toward each other, and the fact that gravitational waves shed a lot of energy, I suspect it's inevitable even beyond that. However, be careful talking about "Schwarzschild radius" and "regardless of their spin" in the same thought. The Schwarzschild metric doesn't apply to spinning black holes, and none of the "basic" black hole solutions apply to a pair of them.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 12:30 pm UTC
The black holes themselves are in motion and have angular momentum about their barycenter. I don't see why it would be impossible to warp the horizons enough so that they don't meet. The Schwarzschild radius is just a number here.

### Re: Is it possible to escape from a black hole using another black hole?

Posted: Tue May 07, 2019 7:54 pm UTC
So basically, even if you had two black holes magically pinned in place, and tried to zing a photon exactly down the line directly between them, it might go past the center of mass, but it couldn't make it out the other side to infinity.

IOW: There may be an open question as to which singularity you'll hit first, but they both agree that you're not going home.