Do black holes have a centre?

[geordief]

Have been watching the BBC'S Universe(    )

One of the 5 parts is about BHs.

 

Apparently they may radiate away to nothing(?) and what "next" is unknown.

 

But I wonder about what might go on at the centre. Might "centre"  not be a meaningful concept?

 

Anyway, the force of gravity at the centre of the Earth (if it was a complete homogeneous sphere would be zero ,wouldn't it? Why does that not apply to a BH?

Edited December 2, 2021 by geordief

[swansont]

We don't know what the inside of a BH actually looks like. The simplest solution predicts a singularity, and that's unphysical.

Geometrically speaking, yes, it has a center. The simplest case should also have spherical symmetry.

[geordief]

1 hour ago, swansont said:

We don't know what the inside of a BH actually looks like. The simplest solution predicts a singularity, and that's unphysical.

Geometrically speaking, yes, it has a center. The simplest case should also have spherical symmetry.

Is it more accurate to talk of a central region rather than any central point?

And are all systems asymmetric to a degree in the finest detail? (so no physically realistic  completely simple case)

Or can the asymmetry be squeezed out in some limit?

[swansont]

It's the physics ideal case. Completely static, no changes to anything. I would expect some sort of asymmetry if you got to a small enough resolution, for any real system. 

[beecee]

6 hours ago, swansont said:

We don't know what the inside of a BH actually looks like. The simplest solution predicts a singularity, and that's unphysical.

Hasn't this "unphysical" prediction been rejected by scientists? If by unphysical, you mean a singularity of infinite density and curvature. And of course ( my reasoning now) GR tells us nothing about the quantum/Planck region, where this prediction takes place...so in effect it can be reasonably discarded. Which then leaves a physical singularity with a surface of sorts, (in an unknown state of existence)  at or below the quantum/Planck level.

Of course all predictions inside the EH are essentially unknown.

7 hours ago, geordief said:

Anyway, the force of gravity at the centre of the Earth (if it was a complete homogeneous sphere would be zero ,wouldn't it? Why does that not apply to a BH?

With any Kerr BH, (the most likely real life example) we would have a "ring singularity" If one were to enter the BH at the polar regions and survive spaghettification, one could pass directly central through this ring singularity without any catastrophic effects. The gravitation would be equally acting on all sides.

[skydelph]

GR is a fundamental theory, which predicts black holes together with many observed physical phenomena in the universe.

As long as we are not able to observe anything beyond the event horizon, the theoretical model accepts singularity: a single point for a non-rotating BH and a ring singularity for a rotating BH.

Though the singularity is only a mathematical abstraction.

Physically a black hole has only three independent physical properties: mass, electric charge, and angular momentum.

‘...for real black holes under the laws of modern physics is currently an unsolved problem.’ - Wikipedia.

[beecee]

4 hours ago, skydelph said:

Though the singularity is only a mathematical abstraction.

As far as I am aware, a singularity can be defined as where infinite spacetime curvature and densities take place (this version is generally rejected by most scientists) and a singularity defined by where our laws of physics and GR are non applicable. 

https://www.universetoday.com/84147/singularity/

The Hawking Singularity Theorem added to this by stating that a space-like singularity can occur when matter is forcibly compressed to a point, causing the rules that govern matter to break down. Hawking traced this back in time to the Big Bang, which he claimed was a point of infinite density. However, Hawking later revised this to claim that general relativity breaks down at times prior to the Big Bang, and hence no singularity could be predicted by it.

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The highlighted part by me, can I believe also apply to the core of a BH. The predicted singularity obviously applies to singularities as defined by infinite spacetime curvatures and densities.

[Kevin_Hall]

As I know singulatiry is considered to be the centre of a black hole. Though it's a little bit naive to present BH as a ball. It's actually a creature of a spacetime, but not space. Near the event horizon, time dilation becomes divergent. Again, to the outside observer, any process near the event horizon appears to slow to a halt. In fact, this is why the horizon remains forever in the future: if you could track an infalling particle, it would take forever, by your reckoning, for it to reach the event horizon.
And if we suppose, that we are brave enough to enter a BH, the moment we cross the horizon it becomes a moment in the past. Not a apherical or surface, just a moment in time.
And your future now means the singularity. But like the horizon, the singularity is also not a location in space. It is a future moment in time and actually, once you are past the horizon, you find yourself in a collapsing pocket universe.

[skydelph]

On 12/3/2021 at 11:54 PM, beecee said:

Hawking traced this back in time to the Big Bang, which he claimed was a point of infinite density. However, Hawking later revised this to claim that general relativity breaks down at times prior to the Big Bang, and hence no singularity could be predicted by it.

What is applied to the big bang, may be applied to a black hole n a speculative way.
'the main difference is that a black hole singularity is the end of space time (and pulls matter in) and the big bang singularity is the beginning of space time (where matter and space were made 'real')'

The big bang - is an accepted starting point of the universe, an it is not the same of what black holes are. Prior to the big bang hypothesis are around in major, which hare not really the subject of GR.
Thank you.

[beecee]

10 hours ago, skydelph said:

What is applied to the big bang, may be applied to a black hole n a speculative way.

I see it as more then just speculative. We essentially  know nothing about space and time at the instant of the BB, where our laws break down and where infinite densities are approached and we also know nothing about the mathematical singularity at the core of a BH for the same reasons. Both occurred or occur at the quantum/Planck level. Any matter/energy entering a BH, is essentially broken down into its most basic fundamentals, the same basic fundamentals that evolved through phase transitions and false vaccums at t+10-45 seconds. https://www.uu.edu/dept/physics/scienceguys/2001Aug.cfm#:~:text=When matter falls into or,the rest of space-time.&text=Once inside the black hole's,be squeezed into the singularity.

extract:

"When matter falls into or comes closer than the event horizon of a black hole, it becomes isolated from the rest of space-time. It can never leave that region. For all practical purposes the matter has disappeared from the universe. Once inside the black hole’s event horizon, matter will be torn apart into its smallest subatomic components and eventually be squeezed into the singularity. As the singularity accumulates more and more matter, the size of the black hole’s event horizon increases proportionally".

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Again, while we certainly cannot be sure 100% of what happens inside any BH EH,  we can I believe trust GR to a large extent, and at the same time understand that while it predicts both the BB and BH singularities as regions of infinite densities and spacetime curvatures, it also fails us at these regions. That tells me that we can reasoanbly trust its predictions once inside the EH, at least up to the quantum/Planck level where it fails us.

https://www.space.com/what-happens-black-hole-center

 

"It could be that deep inside a black hole, matter doesn't get squished down to an infinitely tiny point. Instead, there could be a smallest possible configuration of matter, the tiniest possible pocket of volume.

This is called a Planck star, and it's a theoretical possibility envisioned by loop quantum gravity, which is itself a highly hypothetical proposal for creating a quantum version of gravity. In the world of loop quantum gravity, space and time are quantized — the universe around us is composed of tiny discrete chunks, but at such an incredibly tiny scale that our movements appear smooth and continuous.

This theoretical chunkiness of space-time provides two benefits. One, it takes the dream of quantum mechanics to its ultimate conclusion, explaining gravity in a natural way. And two, it makes it impossible for singularities to form inside black holes.

As matter squishes down under the immense gravitational weight of a collapsing star, it meets resistance. The discreteness of space-time prevents matter from reaching anything smaller than the Planck length (around 1.68 times 10^-35 meters, so…small). All the material that has ever fallen into the black hole gets compressed into a ball not much bigger than this. Perfectly microscopic, but definitely not infinitely tiny.

This resistance to continued compression eventually forces the material to un-collapse (i.e., explode), making black holes only temporary objects. But because of the extreme time dilation effects around black holes, from our perspective in the outside universe it takes billions, even trillions, of years before they go boom. So we're all set for now"

10 hours ago, skydelph said:

'the main difference is that a black hole singularity is the end of space time (and pulls matter in) and the big bang singularity is the beginning of space time (where matter and space were made 'real')'

And both exist where GR fails us and logically where matter/energy should exist at its most fundamental levels.eg: quarks/electrons. 

10 hours ago, skydelph said:

The big bang - is an accepted starting point of the universe, an it is not the same of what black holes are. Prior to the big bang hypothesis are around in major, which hare not really the subject of GR.
Thank you.

Yes, of course, but as per a BH singularity, we know nothing about those regions that exist at the quantum/Planck level and the instant of the BB. The BB for example, only starts aligning with current physics and GR at around 10-45th seconds. No one yet has mentioned anything about pre BB times, if that is at all a reasoanble concept, since space and time, (as we know them) evolved at that point. Whatever state it existed before the instant of the BB, (or the post quantum/Planck era) is unknown, as is the state of space and time at a BH singularity and the same quantum/Planck era that follows that.

But I'm only a poor old bastard that has now retired, so while  what I say are my own thoughts (reasonable I hope) it is of course based on much of what I have read and researched with regards to reputable data and references on the subject.

The point to remember is that most cosmologists and scientists today, reject the singularity as defined by infinite spacetime curvatures and densities as unrealistic. I'm inclined to agree with that for whatever it is worth.

And all open to modifications and/or error corrections from our own online knowledgable people.

Edited December 6, 2021 by beecee

[joigus]

The singularity of a BH lies in the future, rather than at the centre. So it's a time, not a position, from what I know. Now, you can call that the centre, for convenience, but it's a time, not a radius. Time and radius change roles when you cross the event horizon. That's what the maths says. What does that mean? I don't know.

There are many things about black holes that I would like to understand better. Is the Schwarzschild black hole anything to go by, or is it just a freak of the equations of relativity for being so unrealistically simple?

The only thing I can say is that theorists keep discussing them and the role they play in physics, including giant ones, microscopic ones that may exist, etc. There is no unanimous agreement about them. That's all I can say.

The best thing about black holes is probably that they create conflict in our theories. I hope that means that research in black holes will usher in the next revolution in physics, but not much is certain about them except one thing: astrophysical black holes do exist.

[J.C.MacSwell]

On 12/2/2021 at 8:36 AM, geordief said:

(if it was a complete homogeneous sphere would be zero ,wouldn't it? Why does that not apply to a BH?

Under what conditions, or history, would the BH's mass distribution be that of a homogenous sphere?

[beecee]

21 minutes ago, joigus said:

The singularity of a BH lies in the future, rather than at the centre. So it's a time, not a position, from what I know. Now, you can call that the centre, for convenience, but it's a time, not a radius. Time and radius change roles when you cross the event horizon. That's what the maths says. What does that mean? I don't know.

OK, I can understand that...but I still prefer my more layman's description.😉 What does the changing of time and space mean? I'll leave that to swansont or Marcus to give a better description. 

27 minutes ago, joigus said:

There are many things about black holes that I would like to understand better. Is the Schwarzschild black hole anything to go by, or is it just a freak of the equations of relativity for being so unrealistically simple?

And of course the simplest solution, albeit totally unlikely solution, making it easy for math ignorant people like myself to understand somewhat.

30 minutes ago, joigus said:

The only thing I can say is that theorists keep discussing them and the role they play in physics, including giant ones, microscopic ones that may exist, etc. There is no unanimous agreement about them. That's all I can say.

In some respects OK, but most agree re the SMBH Sag1 at the center of the MW, and aLIGO has validated mid size ones. Quantum BH's seem to be the biggest querie or uncertainty.

35 minutes ago, joigus said:

The best thing about black holes is probably that they create conflict in our theories. I hope that means that research in black holes will usher in the next revolution in physics, but not much is certain about them except one thing: astrophysical black holes do exist.

In time, the continued discoveries of gravitational waves might just be the revolution you talk about. While obviously a weird result caused by the extremities of gravity, they were actually first realized with Newtonian mechanics in the late 1700's, as you probably are aware, and I'm just teaching granny how to suck eggs.😊

[geordief]

1 hour ago, J.C.MacSwell said:

Under what conditions, or history, would the BH's mass distribution be that of a homogenous sphere?

I don't know.It was just my naive assumption that they would tend towards that state in the long run and that the more massive they were the more homogenous they would become

 

 

I thought all the matter inside a black hole was the same and even that matter no longer existed but that the black hole was considered as an extreme curvature in spacetime (without understanding what that or much else besides  actually  meant)

[joigus]

13 minutes ago, beecee said:

OK, I can understand that...but I still prefer my more layman's description.😉 What does the changing of time and space mean? I'll leave that to swansont or Marcus to give a better description. 

I'm looking forward to that description. This swapping of the radial coordinate with the time coordinate is a general feature of horizons, though, AFAIK. Back in the sixties, Penrose and Hawking proved singularities to be an inevitable consequence of GR spacetimes, and formulated a conjecture of 'cosmic censorship', that there are no naked singularities ever; they're always hidden behind a horizon, with this funny swapping going on.

But horizons appear from the POV of a far-away observer, from the POV of the free-falling observer nothing funny is going on. Looking at the maths, though, the falling observer is doomed. There is a point on their future worldline that means 'the end of space-time' for them. And it's in their future, because the coordinate that monitors time, from the perspective of the far-away observer (the one that carries the negative sign in the metric), is in their radial approach coordinate, rather than the 't' one that describes time outside. I know it's strange, but that's what the maths say.

 

13 minutes ago, beecee said:

And of course the simplest solution, albeit totally unlikely solution, making it easy for math ignorant people like myself to understand somewhat.

Even leaving aside the problem of evaporation, there are many problems with the Schwarzschild solution. An important one being that it is past-eternal. And astrophysical black holes presumably are not. Don't forget the Schwarzschild solution is static. The way you deal with that in GR is by directly deleting this freak eternal past, modelling a spherical shell of collapsing matter, and patching up the solutions à la Penrose.

My approach to any physical problem is very mathematically-biased, but that's the only thing I can do with my toolkit.

7 hours ago, beecee said:

In time, the continued discoveries of gravitational waves might just be the revolution you talk about. While obviously a weird result caused by the extremities of gravity, they were actually first realized with Newtonian mechanics in the late 1700's, as you probably are aware, and I'm just teaching granny how to suck eggs.😊

Yes, I am aware. But Laplace's 'black holes' were very different. Laplace realised that there can be objects that are capable of swallowing any light trying to escape from them, as a result of an escape-velocity calculation, but back then the speed of light was not considered to be the universal limit that today we know to be. From the Laplacian perspective, there's no problem in those photons being detected outside the 'horizon' if given proper initial data. They can't reach spatial infinity, that's all.

Gravitational waves open up an invaluable tool to understand black-hole collisions and the like, but the physics community seems to agree that further theoretical clarification of what goes on in them is necessary.

Are the words 'in them' even meaningful? In a way, all about a BH is coded in its surface. Some people like to think they are bridges to other space-times. My hunch is that they signal to a limitation of the description and that the interrelationship between gravity and gauge theory must be understood much better than it is today. Easier said than done, though.

[skydelph]

40 minutes ago, joigus said:

the description and that the interrelationship between gravity and gauge theory must be understood much better than it is today

Gravastar is a way to go away from singularity. Relying on hypothesis on quantum gravity, which has not been quantized yet whether it will ever be done. A gravastar is filled with dark energy. That may link major subjects of investigation of the universe today. LIGO might differentiate a gravastar and 'normal' black hole during the merge. So, there is another alive hypothesis related to the subject.

[joigus]

32 minutes ago, skydelph said:

Gravastar is a way to go away from singularity. Relying on hypothesis on quantum gravity, which has not been quantized yet whether it will ever be done. A gravastar is filled with dark energy. That may link major subjects of investigation of the universe today. LIGO might differentiate a gravastar and 'normal' black hole during the merge. So, there is another alive hypothesis related to the subject.

Interesting...

Thank you. I wasn't aware of gravastars.

[beecee]

7 hours ago, skydelph said:

Gravastar is a way to go away from singularity. Relying on hypothesis on quantum gravity, which has not been quantized yet whether it will ever be done. A gravastar is filled with dark energy. That may link major subjects of investigation of the universe today. LIGO might differentiate a gravastar and 'normal' black hole during the merge. So, there is another alive hypothesis related to the subject.

 

7 hours ago, joigus said:

Interesting...

Thank you. I wasn't aware of gravastars.

Yes, very!  https://en.wikipedia.org/wiki/Gravastar

I do remember reading about them, somewhere, sometime, perhaps in Martin Ree's and Mitch Begalman's book, "Gravity's Fatal Attraction" 

This maybe of interest......

https://arxiv.org/pdf/2010.05354.pdf

Black hole or Gravastar? The GW190521 case:  (Dated: August 31, 2021)

The existence of cosmological compact objects with very strong gravity is a prediction of General Relativity and an exact solution of the Einstein equations. These objects are called black holes and recently we had the first observations of them. However, the theory of black hole formation has some disadvantages. In order to avoid these, some scientists suggest the existence of gravastars (gravitation vacuum stars), an alternative stellar model which seems to solve the problems of the black hole theory. In this work we compare black holes and gravastars using a wide range of the literature and we emphasize the properties of gravastars, which are consistent with the current cosmological observations. Also, we propose gravastars as the solution of the ”pair-instability” effect and a possible explanation for the observed masses of the compact objects, before the collapse, from the gravitational signal GW190521, since in the formation of a gravastar there aren’t mass restrictions.

CONCLUSIONS:

Gravastar is a hypothetical stellar compact object alternative to a black hole. Until now, there is no observational way to distinguish a black hole from a gravastar. Thus, a possible observation of a black hole can be a possible gravastar. Recent observations of gravitational wave GW190521 pointed out that the theory of black holes formation is not consistent with observations, since there is a mass gap in their formation. From theoretical view, there is a way to avoid the pair instability effect/mass gap with gravastar formation. The GW190521 event could be an indication for the existence of gravastars, which might avoid the mass gap. This stellar configuration (gravastar) is a promising candidate to complete the puzzle or to replace black holes.

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[joigus]

15 minutes ago, beecee said:

This maybe of interest......

https://arxiv.org/pdf/2010.05354.pdf

Very much appreciated.

[J.C.MacSwell]

19 hours ago, geordief said:

I don't know.It was just my naive assumption that they would tend towards that state in the long run and that the more massive they were the more homogenous they would become

 

 

I thought all the matter inside a black hole was the same and even that matter no longer existed but that the black hole was considered as an extreme curvature in spacetime (without understanding what that or much else besides  actually  meant)

Generally speaking, the mass would tend toward the centre when it wasn't ingesting more mass from outside, and infinitely various states otherwise, but a homogenous state would be pretty unique/unlikely.

[geordief]

I am still watching this Cox series and towards the end of today's episode  ,in a description of a "creation story" he says  and I quote "In the beginning  there was an ocean of energy"

 

I think I have heard it said that energy is not considered to be a "thing" ,more a property of things or a system

 

Am I right to discount such a description or is he maybe just taking a shortcut to get some complicated ideas across to the public in a more direct way?

 

Or have I got it wrong and does that statement make sense in a scientific way?

 

Edit: found a link on youtube

https://youtu.be/G33j5Qi4rE8

 

Edited December 8, 2021 by geordief

[beecee]

2 hours ago, geordief said:

I am still watching this Cox series and towards the end of today's episode  ,in a description of a "creation story" he says  and I quote "In the beginning  there was an ocean of energy"

 

I think I have heard it said that energy is not considered to be a "thing" ,more a property of things or a system

 

Am I right to discount such a description or is he maybe just taking a shortcut to get some complicated ideas across to the public in a more direct way?

 

Or have I got it wrong and does that statement make sense in a scientific way?

 

Edit: found a link on youtube

https://youtu.be/G33j5Qi4rE8

 

The Ocean of energy was I believe the quantum foam...I am far more comfortable with something like the quantum foam existing for eternity, and the true definition of nothing, than anything anymore complicated.

[MigL]

I would assume by 'ocean of energy' that gave rise to 'matter' after the inflationary period, he means the radiation dominated era that gave rise to the matter dominated era.