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Author Topic: Does gravity get out of a black hole?  (Read 6743 times)

Offline Bill S

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Does gravity get out of a black hole?
« on: 23/07/2015 22:28:54 »
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.

1.) How does gravity get out of a black hole?

It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.

2.) Why is the speed of gravity restricted to the speed of light?

Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.

3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?

If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.

I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.


 

Offline PmbPhy

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Re: Does gravity get out of a black hole?
« Reply #1 on: 24/07/2015 03:40:42 »
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.

1.) How does gravity get out of a black hole?

It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.

2.) Why is the speed of gravity restricted to the speed of light?

Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.

3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?

If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.

I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
Have you read the FAQ on this: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

It will address most of your questions.
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #2 on: 24/07/2015 08:18:22 »
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.

1.) How does gravity get out of a black hole?

It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.

2.) Why is the speed of gravity restricted to the speed of light?

Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.

3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?

If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.

I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.

I think Pete's link summed it up. It is a very complex situation to try to understand. We have no experimental evidence for any of this. However, GR predicts various things that have been confirmed by evidence. So I would say that there should be a high level of confidence in current views on the matter.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #3 on: 24/07/2015 12:31:49 »
Thanks for the link.  The link from there to virtual particles looks as though it will be worth reading when I have time.
How much is the thinking about these things likely to have changed over the past 20 years?
 

Offline David Cooper

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Re: Does gravity get out of a black hole?
« Reply #4 on: 24/07/2015 18:35:05 »
What about a case where a black hole moves? How does the space ahead of it know to curve as the black hole approaches? You can look at it from the point of view of a frame of reference moving with the black hole and the problem disappears, but if there's another black hole moving in the opposite direction, that breaks the solution. Some kind of signal will have to exceed the speed of light in order to procuce the required curving. To make the problem easier to spot, picture two black holes moving towards each other at 99.999% the speed of light (which should be allowed in a thought experiment). Length contraction will lead to the event horizons being flattened into planes like two plates coming together face to face. How is the space between them going to curve fast enough not to distort in a manner that produces a different distortion if you calculate it again from a different frame of reference? Any change in the shape will reveal an absolute frame. The only way to avoid showing up an absolute frame is to have something that causes the curvature change be transmitted faster than the speed of light.

(With LET this is not such a problem as there is no curvature creating extra space for light to travel through, so light is moving slower than normal in a gravity well instead and gravity is allowed to propogate at the unslowed speed of light through the fabric of space.)
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #5 on: 24/07/2015 22:56:06 »
After reading Pete’s link, I question I ask myself is: In the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?  If it is not, how does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?   
 

Offline David Cooper

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Re: Does gravity get out of a black hole?
« Reply #6 on: 24/07/2015 23:43:37 »
The same problem occurs with two heavy stars moving towards each other, but the distortions are less obvious as the communications aren't so dramatically slowed. It's better to use black holes when thinking about it as it makes it easier to run the simulations in your head when you know that no communications from inside the event horizon can advance ahead of the event horizon at all.
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #7 on: 25/07/2015 17:08:20 »
Black hole do move. The super massive black holes at the centres of galaxies move with those galaxies. What if two galaxies change trajectories during a merger. If the black holes at the centres follow the change in trajectory what does this tell us? If they are supposed to react more slowly because of time dilation then this breaks that hypothesis. I don't know the answer as I have never seen a galactic merger.
 

Offline David Cooper

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Re: Does gravity get out of a black hole?
« Reply #8 on: 26/07/2015 01:19:16 »
Actually, there is no problem so long as a signal is somehow allowed to get out of a black hole, and while that does mean going faster than light, it is the same for GR as it is for LET - the path out of the black hole is just physically longer, so the problem is the same for both theories.

If gravity is a curvature of space caused by a mass and all the mass is at the centre of a black hole, when you move that black hole along you have to have a mechanism to adjust the curvature of the space ahead of the black hole. If you don't want any signals to go faster than light, you'd have to have the leading part of the "dent" somehow take up the shape of the part of the dent ahead of it and pass its own shape to the part just behind it without any guidance from the centre, but if you tried to implement a mechanism of that kind, it would go wrong when two black holes collide because the dents would just keep going (looking like normal black holes but with no content) while the singularies merge and stop.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #9 on: 29/07/2015 17:53:01 »
Rather than start a new thread with this question, I thought I could tack it on here.

https://en.wikipedia.org/wiki/Vacuum_solution_%28general_relativity%29


“The fact that the gravitational field itself possesses energy yields a way to understand the nonlinearity of the Einstein field equation: this gravitational field energy itself produces more gravity.”

Why would this not result in “run-away” increase in gravity?
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #10 on: 30/07/2015 00:15:32 »
If you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect. This could be thought of in a similar way to gluon interactions. Where a gluon can absorb or emit another gluon.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #11 on: 31/07/2015 21:10:28 »
Quote from: Jeffrey
If you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect.

That makes sense to me, but are we sure that energy would decrease in successive generations, and if it did, would that necessarily bring the process to a halt? 
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #12 on: 01/08/2015 18:38:49 »
Quote from: Jeffrey
If you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect.

That makes sense to me, but are we sure that energy would decrease in successive generations, and if it did, would that necessarily bring the process to a halt?

I can't answer that question at the moment. What I can say is that we do not see an infinite amount of gravitational energy from sources in the universe around us. I am also sure that conservation laws must prevent this.
 

Offline evan_au

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Re: Does gravity get out of a black hole?
« Reply #13 on: 03/08/2015 10:32:13 »
Quote from: BillS
In the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?
The gravitational acceleration at the event horizon will be far greater than the gravitational acceleration at the surface of the original star.
My reasoning:
  • Let's take a star with radius R m
  • If we call the gravitational acceleration at the surface of this star A m/s2.
  • If the entire mass of that star collapses into a black hole, the gravitational acceleration at distance R from the center of the black hole will still be A m/s2
  • Let's call the Schwarzchild radius of the black hole S m.
  • The gravitational acceleration at the surface of the black hole will be A(R/S)2.
  • ...And the tidal forces grow even faster than an inverse square law... 

Quote
how does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?
Stars in close binary orbits often suck loosely-held gas from the less-dense star, and dump it onto the denser star. This atmospheric stripping starts well before their distance is equal to the sum of their radii. This frequently causes a Type 1a supernova. But the stars are still at a fairly large distance when their stellar atmospheres merge.

Black holes have a much smaller radius than their parent stars, so they can approach to much closer distances, with much faster orbital velocities, but with no transfer of gas between them. They merge when their event horizons touch, which is at a relatively small distance.
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #14 on: 03/08/2015 18:50:50 »
Quote from: BillS
In the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?
The gravitational acceleration at the event horizon will be far greater than the gravitational acceleration at the surface of the original star.
My reasoning:
  • Let's take a star with radius R m
  • If we call the gravitational acceleration at the surface of this star A m/s2.
  • If the entire mass of that star collapses into a black hole, the gravitational acceleration at distance R from the center of the black hole will still be A m/s2
  • Let's call the Schwarzchild radius of the black hole S m.
  • The gravitational acceleration at the surface of the black hole will be A(R/S)2.
  • ...And the tidal forces grow even faster than an inverse square law... 

Quote
how does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?
Stars in close binary orbits often suck loosely-held gas from the less-dense star, and dump it onto the denser star. This atmospheric stripping starts well before their distance is equal to the sum of their radii. This frequently causes a Type 1a supernova. But the stars are still at a fairly large distance when their stellar atmospheres merge.

Black holes have a much smaller radius than their parent stars, so they can approach to much closer distances, with much faster orbital velocities, but with no transfer of gas between them. They merge when their event horizons touch, which is at a relatively small distance.

If we take e18d015eb7528189bcb30399f0440938.gif then as r approached rs the result will tends towards 1. If we take the maximum value of A to be equal to an instantaneous velocity of c at r=rs then that states in a simple way the relationship between escape velocity and and g. However this cannot be true since g depends upon distance from source and the density of that source. Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes. To balance the books there is something missing in your line of reasoning.

NOTE: Also as initially r > rs then would't that give superluminal speed outside the horizon?
« Last Edit: 03/08/2015 18:52:54 by jeffreyH »
 

Offline evan_au

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Re: Does gravity get out of a black hole?
« Reply #15 on: 04/08/2015 11:50:45 »
Quote from: jeffreyH
Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.
Spaghettification (ie acceleration due to gravity differing significantly between your head and your feet) is inevitable when approaching a stellar-mass black hole.

However, the rate of change of gravitational acceleration is much gentler near a supermassive black hole, and I understand that reaching the event horizon in one piece is theoretically possible (provided that you don't get minced and/or fried by an accretion disk)....

One of the largest known black hole candidates is in the center of NGC4889, with an estimated mass of  21 billion solar masses, and a Shwarzchild radius rs≈6x1013m. If my quick calculation is correct, the acceleration due to gravity at the Schwarzchild radius is about 10 times Earth-normal (as seen by a distant observer).

So I suggest that it is unlikely that you can find a black hole in our part of the universe that has a gravitational attraction as low as Earth's surface 10m/s at rs.
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #16 on: 04/08/2015 23:04:52 »
Gravity is an interaction between waves of light in the infrred spectrum, its all around us everywhere in space, There is a big mystery around what light is and how it travels, I explain that under TOE fun. Also this also clears up how light waves interact, The slit experiment caused a lot of confusion my theory explains it clearly and simply ;)

Prove it.
 

Offline jeffreyH

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Re: Does gravity get out of a black hole?
« Reply #17 on: 04/08/2015 23:10:59 »
Quote from: jeffreyH
Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.
Spaghettification (ie acceleration due to gravity differing significantly between your head and your feet) is inevitable when approaching a stellar-mass black hole.

However, the rate of change of gravitational acceleration is much gentler near a supermassive black hole, and I understand that reaching the event horizon in one piece is theoretically possible (provided that you don't get minced and/or fried by an accretion disk)....

One of the largest known black hole candidates is in the center of NGC4889, with an estimated mass of  21 billion solar masses, and a Shwarzchild radius rs≈6x1013m. If my quick calculation is correct, the acceleration due to gravity at the Schwarzchild radius is about 10 times Earth-normal (as seen by a distant observer).

So I suggest that it is unlikely that you can find a black hole in our part of the universe that has a gravitational attraction as low as Earth's surface 10m/s at rs.

The interesting region appears to be very near the horizon of a black hole using the Schwarzschild metric. Although the Kerr metric is more realistic it is also a lot harder to work with. I am talking in Planck lengths here and applicable to any size of black hole.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #18 on: 05/08/2015 23:14:12 »
Comparisons between Schwarzschild and Kerr black holes is interesting, but is there any evidence for non-rotating BHs anywhere in the Universe?  Is it even possible, under the constraints of gravity, for a body to form by accretion without rotating?   
 

Offline Mordeth

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Re: Does gravity get out of a black hole?
« Reply #19 on: 09/08/2015 14:39:22 »
Comparisons between Schwarzschild and Kerr black holes is interesting, but is there any evidence for non-rotating BHs anywhere in the Universe?  Is it even possible, under the constraints of gravity, for a body to form by accretion without rotating?

Black holes do not form by accretion.  They form due to the collapse of a massive star.  Accretion is how the black hole grows.  There is no evidence of a non-rotating black hole, although it is not forbidden.  Even one photon hitting a non-rotating black hole would cause it to spin due to conservation of angular momentum.  It is also assumed that a black hole retains the angular momentum of the star that formed it. Therefore, one would expect most, if not all actual black holes are Kerr black holes ie., rotating. The math used to derive a Schwarzschild black hole is simply far easier to deal with and the Schwarzschild solution is a useful teaching aid.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #20 on: 09/08/2015 23:49:23 »
Hi Mordeth, good to meet a fellow nit-picker. :)

I should have said:

Is it even possible, under the constraints of gravity, for a body that may later collapse to a BH, to form by accretion without rotating?
 

Offline Mordeth

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Re: Does gravity get out of a black hole?
« Reply #21 on: 10/08/2015 02:06:58 »
Hi Mordeth, good to meet a fellow nit-picker. :)

I should have said:

Is it even possible, under the constraints of gravity, for a body that may later collapse to a BH, to form by accretion without rotating?
Hi Bill,

Highly unlikely but theoretically possible.  The issue is not with gravity but with the conservation of angular momentum and the distribution and spin of particles. Consider a cloud of dust. If all particles of the cloud were evenly distributed and the cloud was a perfect sphere with no angular momentum, then it could collapse via accretion into something that is non-rotating.  This object could form a non-rotating black hole if it's mass collapses to within its Schwarzchild radius.
 

Offline Bill S

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Re: Does gravity get out of a black hole?
« Reply #22 on: 10/08/2015 14:56:48 »
Quote from: Mordeth
Consider a cloud of dust. If all particles of the cloud were evenly distributed and the cloud was a perfect sphere with no angular momentum, then it could collapse via accretion into something that is non-rotating.

As you say; very unlikely, I don't know enough about the physics of this sort of situation to know if it is even possible, but it raises some interesting thoughts.

Could there be a perfectly spherical cloud of dust if accretion was not already acting on it?

Would there have to be a particle that was absolutely central to avoid rotational movement?

If gravity = spacetime curvature, would that curvature initiate rotation?

That's just a start, there would then be questions about the initiation/conservation of angular momentum, but let's not go there (yet). :)
 

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Re: Does gravity get out of a black hole?
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