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# Getting out of a black hole...

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### #1 deanbrown3d

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Posted 14 February 2019 - 01:10 AM

I’ve been doing a thought experiment for a while now, about escaping from a black hole: Suppose you’re in black hole A, which has an event horizon of diameter 2 light-hours. Suppose also that you are 1 mm inside this event horizon. There’s no chance of getting out at all; all is lost and hopeless. Your relatives are never going to see you again unless they follow you in. But you will at least have a chance to see what cannot be seen by any outside observer.

Well now, suppose a second black hole B comes along. It’s not going to hit A directly – but it will glance by, at a closest distance of some number of light days, and then continue on in an unbound trajectory never to return.

Now, remember you’re in A. While B is close, the attraction of you to the center of gravity of A is partially counterbalanced by the attraction of you towards B. The event horizon of A has been pushed back slightly (where it’s closest to B), and you are now able to use all your energy to propel yourself towards B. If you get it just right, you’ll end up in a position where you are being pulled fairly equally from both A and B. Remaining in this equilibrium position, making adjustments with your last bit of rocket fuel, you wait it out until A and B are far enough apart that you are no longer within either’s event horizon. You have escaped and give your spouse a hug.

Graphics simulation:

http://deanbrown.org...lack_holes.html

My math is not up to this quantitatively, but the simulation here shows random trajectories for glancing black holes (sometimes for fun they might be repulsive white holes. Ignore those cases for now). Suppose you’re one of the text dots that makes it into the event horizon of one of the two black holes and suppose its event horizon is 50 pixels wide on your screen. Some of the dots get within 50 pixels, and then get out again completely free by the gravitational pull from the other black hole. If I were one of those dots, I’d thank my lucky stars!

### #2 Rock22

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Posted 14 February 2019 - 01:55 AM

Oh, you meant alive.  I figured anyone's matter getting sucked into a black hole might be jettisoned out in a different energy state in the relativistic jets.  I've also heard that the spaghettified individual might still "exist" as information.  But that's still dead, IMHO.

### #3 bobzeq25

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Posted 14 February 2019 - 02:32 AM

Consider the black hole is a depression of space time.  That depression goes down infinitely far.  After you get 1mm inside the event horizon, you can't hang there, you'll slide down to infinity (and beyond? <grin>).

B can indeed shrink the event horizon of A.  But it won't stop A from being a black hole.  And you've disappeared into it.

### #4 deanbrown3d

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Posted 14 February 2019 - 08:03 AM

@bobzeq25 true if you were just being sucked in directly. But what if you're a particle in orbit around the black hole, at near-relativistic speed, getting pulled closer and closer over some long period of time - say 1000 years. At some point you will cross over the EH and still have plenty of time to remain in your spiraling orbit before you meet your singularity. That is the period of the thought experiment, and where an external influence is assumed to occur.

### #5 Mark326

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Posted 14 February 2019 - 09:07 AM

A layman physics dummy opinion.

General Relativity tells us no particle with mass can travel at/beyond the speed of light in a vacum.

Being 1mm inside the EH in effect would be the same as being at the singularity in terms of escaping the blackhole since the mass can no longer reach a escape velocity, i.e. faster than C.   No outside force will be able to propel or slingshot you beyond the speed of light.

All of this not withstanding the fact you (the particles that in aggregate whos reference frame is known as Dean) would have already been pulled apart as they accelerated at near realativistic speed.

### #6 bobzeq25

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Posted 14 February 2019 - 09:49 AM

A layman physics dummy opinion.

General Relativity tells us no particle with mass can travel at/beyond the speed of light in a vacum.

Being 1mm inside the EH in effect would be the same as being at the singularity in terms of escaping the blackhole since the mass can no longer reach a escape velocity, i.e. faster than C.   No outside force will be able to propel or slingshot you beyond the speed of light.

All of this not withstanding the fact you (the particles that in aggregate whos reference frame is known as Dean) would have already been pulled apart as they accelerated at near realativistic speed.

That's actually somewhat equivalent to my idea.

deanbrown3d's point is, I believe, that the presence of a nearby singularity will lower the wall (event horizon) between the singularity and the universe to provide a refuge where one could go.  Then, as the singularities separate, one could escape the refuge.

I don't think such a refuge would exist, mathematically, nor do I think it would be possible to occupy it for any significant time, in the unlikely event it did exist.

### #7 llanitedave

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Posted 14 February 2019 - 10:49 AM

@bobzeq25 true if you were just being sucked in directly. But what if you're a particle in orbit around the black hole, at near-relativistic speed, getting pulled closer and closer over some long period of time - say 1000 years. At some point you will cross over the EH and still have plenty of time to remain in your spiraling orbit before you meet your singularity. That is the period of the thought experiment, and where an external influence is assumed to occur.

A particle cannot remain in orbit inside the event horizon.  All the space-time curves lead towards the singularity.  I think in your scenario, the boundaries of the event horizon can indeed be temporarily  pushed back, but that's only relevant to particles that have not yet fallen in.  There would be no occupied zone of orbiting particles just inside the event horizon.

### #8 deanbrown3d

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Posted 14 February 2019 - 11:20 AM

@dave - if I shine my laser pointer near a black hole, it's path can be bent a little, but that's all. Point it a right at the center of the black hole, and the photons enter the EH and singularity. Between these two extremes, photons can take any trajectory. A photon that enters the EH does not suddenly do a 90-degree about turn and head directly to the singularity. It will continue for some time within the EH on its trajectory. This is the time period in discussion. At the right angle, some photons could exist there for long periods of time in their orbit. External dynamic gravitational changes can influence the shape of the EH away from a perfect sphere, and a photon or particle in that orbit could be released.

### #9 Mark326

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Posted 14 February 2019 - 01:15 PM

Found this paper...

https://arxiv.org/pd...338v1.pdf#page2

once again, a layman reading of it indicates to me,

1. For passing Black Holes to have any effect on the other, they would have to be close enough to be part of a bound system.    I.e. they will eventually merge.  Section on Two Body problem GR

2. I think your question falls somewhere in “The Blackhole Scattering Problem” section. “ There is no-known natural mechanism in the universe that can accelerate black holes to ultra-relativistic velocities, and hence the black hole scattering problem is largely a thought experiment that can probe a very interesting regime of Einsteins’ theory.”

Dean, not trying to be argumentative but you are changing the “goal post” parameters of your original topic.

1MM inside the event horizon to now just outside event horizon. Also have changed from an object with Mass (you) to a Massless Particle (Photon).

For original post, no particle with Mass can travel faster than light in a vacum if we go with GR assertions. 1MM inside the EH is 1MM too far to escape. In the event of second BH merger, your now inside a larger steller mass BH... whatever that means at the singularity inside.  If second Black Hole passes by at a distance far enough to escape its own eventual merger, it would have no effect.  In the case of a Massless photon, same applies..Think Hawking radiation offers an escape route when your wave length stretches far enough. Quantum Tunnelling maybe your escape hatch.

Going with your modified ever changing thought experiment parameters, Outside the event horizon traveling in some orbital velocity where speed of light C has not been reached, you could still escape.  Requires additional potential energy to achieve escape velocity, 1. Turn on your thrusters, 2. Get a gravitational energy boost from some passing object.  The energy needed would be enormous as GR tells us the closer to C you approach M increases.

Interesting thought experiment... guess without being able to do the theoretical calculations, GR gets the last word.

### #10 EJN

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Posted 14 February 2019 - 03:20 PM

I figured anyone's matter getting sucked into a black hole might be jettisoned out in a different energy state in the relativistic jets.

This seems to be a common misconception. Relativistic jets do *not* originate from inside a black hole,

they are formed from material in the accretion disk which *never* has fallen beyond the event horizon.

B can indeed shrink the event horizon of A.

Actually, no. In the mathematics of General Relativity, the area of the event horizon *never*

decreases, it can only remain the same or increase. Once you are inside the event horizon there is no way

out. Period.

The event horizon is not a material object, but a geometric boundary between the region where light

can escape, and where light cannot escape. Once you have crossed this boundary, there is no possible

world-line which can get you back out without traveling faster than light.

The only way out is Hawking radiation, but that is a quantum mechanical process of virtual particles

formed right at the event horizon, but since it is too weak to be directly observed from earth, it

remains a working hypothesis, not an observationally verified theory.

A particle cannot remain in orbit inside the event horizon.  All the space-time curves lead towards the singularity.  I think in your scenario, the boundaries of the event horizon can indeed be temporarily  pushed back, but that's only relevant to particles that have not yet fallen in.  There would be no occupied zone of orbiting particles just inside the event horizon.

This is correct. Additionally, for a rotating black hole, there is a region outside the event horizon

called the static limit. Once you pass that, you are forced into orbiting in the same direction as the

black hole rotation. This is due to frame-dragging by the rotating black hole. Frame dragging has been

measured even in Earth orbit, it happen with all rotating bodies.

A static, non-rotating (Schwarzschild metric) black hole is an idealized model and probably does not

exist in the real world. Rotating black holes are described by the Kerr metric.

Edited by EJN, 14 February 2019 - 04:25 PM.

### #11 gavinm

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Posted 14 February 2019 - 03:27 PM

I think the key words in this situation (as mentioned above by EJN) is that the event horizon is not a physical barrier. I have read many times that if you crossed the event horizon, you wouldn't even know it, until you got so close to the singularity that tidal forces stretch you. There isn't instant 'destruction' when you pass it, so who's to say that if you are just inside the Schwarzschild Radius and that radius changes.......??

### #12 DaveC2042

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Posted 14 February 2019 - 05:32 PM

A)  To expand on llanitedave a little, inside a black hole, space takes on the 'one-way' characteristic of time.  So you are unavoidably dragged 'down' to the singularity in the same way that you are dragged forward in time normally.  So waiting for the surface to be deformed past you doesn't work.

B)  Without getting into the maths, I'm confident that if you did it, you'd find that the surface does not really move in the sense you seem to be thinking of, and what would happen is that the bits of spacetime where the surface and you are, are both deforming together, so you always wind up staying inside the deformed surface.

C)  I completely understand the desire to get to the 'physics' without getting bogged down in the 'maths'.  Unfortunately, GR is one of those things that doesn't really permit that except at the most simplistic level.  The maths is a fundamental aspect of the physics, and produces non-intuitive physical results that can't really be understood without a proper understanding of the maths.

Not trying to be rude or elitist here, by the way.  I find the maths very hard myself, and it's a long time since I studied GR - so happy to be told I'm wrong by actual experts.

Edited by DaveC2042, 14 February 2019 - 05:34 PM.

### #13 deanbrown3d

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Posted 14 February 2019 - 06:02 PM

the surface and you are, are both deforming together

Isn't the event horizon being pushed towards A while you are being attracted gravitationally (i.e. accelerated) towards B?

### #14 EJN

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Posted 14 February 2019 - 06:27 PM

Isn't the event horizon being pushed towards A while you are being attracted gravitationally (i.e. accelerated) towards B?

No it is just the opposite, the event horizon is pulled towards B and so are you, but less so, because you

are further away from B and closer to the center of mass (dreaded singularity) of A.

And since for every action there is an equal and opposite reaction, the event horizon of B is pulled towards

A also.

Think of it like a very close binary star system, where both stars are elongated and the major axis

of the elongation points to the other star.

But thinking in terms of "pushing" and "pulling" is not really correct, it is really just a manifestation

of spacetime curvature. Also since gravity is *always* an attractive force, it cannot "push" anything.

This is a spacetime diagram of a black hole merger. Note as they approach the event horizons elongate

towards each other. Even if they did not merge, but only pass very close, they would elongate

in the same way.

Edited by EJN, 14 February 2019 - 07:00 PM.

### #15 gavinm

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Posted 14 February 2019 - 06:55 PM

That's a nice diagram - very simple to understand

### #16 deanbrown3d

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Posted 14 February 2019 - 07:21 PM

Yes but there is more to it than that. Mark's paper above goes into the different collision scenarios. Some of them are not direct mergers, they go passed each other and their gravitational fields interact. It's like your picture above but in the reverse time direction, going down. You're in one net gravitational field (or event horizon), which divides into two. If you are in the right point, you have two gravitational fields pulling you, but the net effect is zero and you can remain there as they separate.

### #17 EJN

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Posted 14 February 2019 - 08:07 PM

Yes but there is more to it than that. Mark's paper above goes into the different collision scenarios. Some of them are not direct mergers, they go passed each other and their gravitational fields interact. It's like your picture above but in the reverse time direction, going down. You're in one net gravitational field (or event horizon), which divides into two. If you are in the right point, you have two gravitational fields pulling you, but the net effect is zero and you can remain there as they separate.

If you are referring to FIG. 6 in the paper, the green line is where they approach close and separate again,

but if you read closely a common AH (apparent horizon) never forms in that scenario; what you say above is

partially incorrect (bolded), the 2 are not equivalent, the net gravitational field at any point can be

expressed as a vector sum of the 2 fields, but the event horizons remain separate.

This can't be overemphasized.

So there can be a point where the gravitational fields are equal but opposite, but that point is not

within either horizon.

The dashed blue line indicates where the inspiral results in a common AH, and once they merge they

cannot, ever, un-merge (unless they were traveling backwards in time, which would be quite a trick).

The dashed blue line ends where the horizons merge (this is stated in the caption). The green line

trajectory never reaches the radius where the horizons merge, this is apparent from even a casual

look at the plot.

At one extreme, k = 0, there will be a
head on collision; at the other, k ≈ 1, the black hole trajectories will be deflected by some amount, though
ultimately they will fly apart and separate. At intermediate values of k there should be a significant amount of
close-interaction of the black holes, and then they will either merge or separate (to possibly merge at some time
in the future). What appears to happen near the threshold value of k between these two distinct end-states
is the black holes evolve toward an unstable near-circular orbit, remain in that configuration for an amount of
time sensitively related to the initial conditions, then either plunge toward coalescence or separate.

Edited by EJN, 14 February 2019 - 08:43 PM.

### #18 deanbrown3d

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Posted 14 February 2019 - 11:29 PM

It's really weird what is being reported for this question. I've found the same exact question asked by a few other people, and the answers follow the same pattern over and over again:

90% of the answers blatantly state that when the black holes merge, so-and-so happens, so there's no escape. Well that's not the scenario. The question is when there is no merger, and the black holes just glance passed each other at a distance and velocity which does not lead to a merge.

Of the remaining 10%, 9% of them answer that "Once inside an event horizon - by definition - you cannot get out". This again is answering the wrong scenario, and it's so frustrating that it's done so many times. These answers examine the points within the resultant event horizon(s). They do not discuss the crux of the matter which is that the event horizon can change shape dynamically. They say that the event horizon does change shape, but if you are still in it then there is no escape.

The final 1% of the answers are the interesting ones that contemplate what happens when the EH is distorted dynamically, and potentially while you are at its edge. From what I've read so far, it is by no means certain what the answer is. I'm still looking for a paper that has done the specific calculation.

Anyway, thanks all for the amazing discussion here!

### #19 llanitedave

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Posted 15 February 2019 - 01:17 PM

It's really weird what is being reported for this question. I've found the same exact question asked by a few other people, and the answers follow the same pattern over and over again:

90% of the answers blatantly state that when the black holes merge, so-and-so happens, so there's no escape. Well that's not the scenario. The question is when there is no merger, and the black holes just glance passed each other at a distance and velocity which does not lead to a merge.

Of the remaining 10%, 9% of them answer that "Once inside an event horizon - by definition - you cannot get out". This again is answering the wrong scenario, and it's so frustrating that it's done so many times. These answers examine the points within the resultant event horizon(s). They do not discuss the crux of the matter which is that the event horizon can change shape dynamically. They say that the event horizon does change shape, but if you are still in it then there is no escape.

The final 1% of the answers are the interesting ones that contemplate what happens when the EH is distorted dynamically, and potentially while you are at its edge. From what I've read so far, it is by no means certain what the answer is. I'm still looking for a paper that has done the specific calculation.

Anyway, thanks all for the amazing discussion here!

It's not the wrong scenario, according to your original post, because you were hypothesizing a particle inside an event horizon that becomes "freed" when the event horizon contracts past its position.  What we've been trying to explain is that simply can't happen, if the event horizon is gravitationally distorted, so are any particles inside it, and thus they will always remain inside it, and always take a pathway inward.

### #20 gavinm

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Posted 15 February 2019 - 02:48 PM

The event horizon of A has been pushed back slightly (where it’s closest to B)

is wrong. The event horizon will be pulled out towards B, so it will get bigger and you still have no escape (demonstrated in the graphic from EJN)

### #21 Astroman007

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Posted 15 February 2019 - 03:12 PM

While an interesting thought experiment (and accompanied by good graphics), I really don't think that escaping the immediate vicinity of a black hole alive and in one piece is possible in the real universe, unfortunately.

### #22 Astroman007

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Posted 15 February 2019 - 03:14 PM

It's really weird what is being reported for this question. I've found the same exact question asked by a few other people, and the answers follow the same pattern over and over again:

90% of the answers blatantly state that when the black holes merge, so-and-so happens, so there's no escape. Well that's not the scenario. The question is when there is no merger, and the black holes just glance passed each other at a distance and velocity which does not lead to a merge.

Of the remaining 10%, 9% of them answer that "Once inside an event horizon - by definition - you cannot get out". This again is answering the wrong scenario, and it's so frustrating that it's done so many times. These answers examine the points within the resultant event horizon(s). They do not discuss the crux of the matter which is that the event horizon can change shape dynamically. They say that the event horizon does change shape, but if you are still in it then there is no escape.

The final 1% of the answers are the interesting ones that contemplate what happens when the EH is distorted dynamically, and potentially while you are at its edge. From what I've read so far, it is by no means certain what the answer is. I'm still looking for a paper that has done the specific calculation.

Anyway, thanks all for the amazing discussion here!

Well, in that case, survival and / or escape seems even less likely.

Edited by Astroman007, 15 February 2019 - 03:14 PM.

### #23 Pess

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Posted 16 February 2019 - 08:37 AM

I’ve been doing a thought experiment for a while now, about escaping from a black hole: Suppose you’re in black hole A, which has an event horizon of diameter 2 light-hours. Suppose also that you are 1 mm inside this event horizon. There’s no chance of getting out at all; all is lost and hopeless. Your relatives are never going to see you again unless they follow you in. But you will at least have a chance to see what cannot be seen by any outside observer.

Well now, suppose a second black hole B comes along. It’s not going to hit A directly – but it will glance by, at a closest distance of some number of light days, and then continue on in an unbound trajectory never to return.

Now, remember you’re in A. While B is close, the attraction of you to the center of gravity of A is partially counterbalanced by the attraction of you towards B. The event horizon of A has been pushed back slightly (where it’s closest to B), and you are now able to use all your energy to propel yourself towards B. If you get it just right, you’ll end up in a position where you are being pulled fairly equally from both A and B. Remaining in this equilibrium position, making adjustments with your last bit of rocket fuel, you wait it out until A and B are far enough apart that you are no longer within either’s event horizon. You have escaped and give your spouse a hug.

Graphics simulation:

http://deanbrown.org...lack_holes.html

My math is not up to this quantitatively, but the simulation here shows random trajectories for glancing black holes (sometimes for fun they might be repulsive white holes. Ignore those cases for now). Suppose you’re one of the text dots that makes it into the event horizon of one of the two black holes and suppose its event horizon is 50 pixels wide on your screen. Some of the dots get within 50 pixels, and then get out again completely free by the gravitational pull from the other black hole. If I were one of those dots, I’d thank my lucky stars!

Once you pass the Event Horizon, you can never cross back.

Let's consider the fate of a photon traveling at 'c'.  It blunders across the EH. It still has velocity of 'c' but that ain't gonna cut it but if the photon crossed at an angle it can have its trajectory bent into an orbit around the singularity deep down inside.

The photon still exists (in one form or another) but is trapped in a circular path inside the EH around the singularity which is a point at the center.

In fact I imagine there is a lot of energy and some matter in an unimaginable maelstrom beyond any EH. It is just that none of it has the velocity exceeding 'c' that would permit it to be thrown back out.

Now add to your thought experiment. Black Holes evaporate via radiating Hawking radiation.  This radiation carries away energy (mass) from a Black Hole over a period of time. Thus, every single BH is shrinking and contracting its EH.  However, NOTHING will ever pass back out of the BH that was originally sucked across the border of death (EH).

Pesse (It's like a Big Wall that says, 'Thou shall not pass!) Mist

### #24 deanbrown3d

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Posted 16 February 2019 - 10:11 AM

Once you pass the Event Horizon, you can never cross back.

...

Now add to your thought experiment. Black Holes evaporate via radiating Hawking radiation.  This radiation carries away energy (mass) from a Black Hole over a period of time. Thus, every single BH is shrinking and contracting its EH.  However, NOTHING will ever pass back out of the BH that was originally sucked across the border of death (EH).

Thanks for your answer, but you're in the 9% group as discussed above - you only consider a single static black hole. Please think about the crux of the matter which is the effect of the dynamic and external gravitational field. I agree with your response as it applies to a static black hole, but that's not the scenario.

### #25 Mark326

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Posted 16 February 2019 - 10:45 AM

The only way I see this comming to an agreed upon conclusion is experimental testing, to that end I propose a GoFundMe account to launch Dean into a Blackhole to test his hypothesis.

JK

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