Light (not) escaping from black holes

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Neutron stars have a very high density, roughly the same density as nuclei but with a larger volume. .
Yet the neutron degeneracy pressure is unknown correct? Said size of neutron is a variable?

What if we view the pressure as a volume knob, why could not a BH be viewed as a NS with the volume turned up from higher pressure?
 

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  • #2
PeterDonis
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the neutron degeneracy pressure is unknown correct?
I suppose you could say it's "unknown" in the sense that nobody has actually put a pressure gauge inside a neutron star to measure it. But we have detailed theoretical models of neutron stars that include degeneracy pressure and predict the observable parameters of those objects with good accuracy. A good comprehensive (though advanced) reference is Shapiro and Teukolsky, Black Holes, White Dwarts, and Neutron Stars.

why could not a BH be viewed as a NS with the volume turned up from higher pressure?
Because a black hole is not a static object supported against gravity by internal pressure. It is a vacuum; it is "made" entirely of curved spacetime geometry.
 
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  • #3
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it is "made" entirely of curved spacetime geometry.
Thank you.


Cannot we rephrase "curved spacetime geometry" as large amounts of gravity ?

Does the gravity volume level not indicate dense matter is also present?
 
  • #4
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[without explaining the relationship of gravity and atomic particle density] If we took gravity and turned the level up to exceed the ability for light to escape from degeneracy pressure of atomic elements, that would not explain what we observe?
 
  • #5
PeterDonis
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If we took gravity and turned the level up to exceed the ability for light to escape from degeneracy pressure of atomic elements
I don't know what you mean by this. Pressure is not something that light can "escape" from.
 
  • #6
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Pressure is not something that light can "escape" from.
Plenty of light escapes the sun which internally has large amounts of pressure/heat ? Light can also escape a BH at times with luminosities and when quasars are turned on.

I don't know what you mean by this.
For a neutron star, its density raises gravity and degeneracy pressure increased, yet not enough to stop the light from escaping.

Think about adding quasars to the mix here, light is escaping and the guesses ive heard about it being caused by electrical forces seems like a weak hypothesis, I agree its a side effect, but not cause.

Think about why the luminosities are positioned where they are located on the BH. I cannot see them originating from the accretion disk, and see them originating from inside the BH itself.

Sorry nut busting your chops this topic is fun.
 
  • #7
PeterDonis
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Plenty of light escapes the sun which internally has large amounts of pressure/heat ?
Sure, but the pressure is not what the light is escaping.

Light can also escape a BH at times
No, it can't. The light you are referring to is not coming from inside the hole; it's coming from hot gas falling into the hole which emits the light while it is still outside the hole's horizon.

Think about why the luminosities are positioned where they are located on the BH. I cannot see them originating from the accretion disk, and see them originating from inside the BH itself.
Nope. See above.

It appears that much of your thinking might be based on the misconception that objects like quasars can emit light from inside a black hole; that is not the case, as I've explained. You need to correct this misconception if you want to understand what's going on.
 
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  • #8
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it's coming from hot gas falling into the hole which emits the light while it is still outside the hole's horizon.
I have heard some professors describe this as the leading hypothesis, or best guess, and understand your position quite well. I am just investigating the possibilities here.
 
  • #9
PeterDonis
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I have heard some professors describe this as the leading hypothesis, or best guess, and understand your position quite well. I am just investigating the possibilities here.
The light being emitted from inside the black hole is not possible. So it's not on the list of things to be investigated.
 
  • #10
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The light being emitted from inside the black hole is not possible..
Do we really know that?

I thought another name for a BH is "we don't know"

Maybe I should try asking a different question here. If one could turn the gravity volume knob on a BH down, at some point light would peak through the poles and mirror a quasar 100% as we see them today, luminosities included ?
 
  • #11
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I moved the posts to a new thread as it was off-topic in the old one.
I thought another name for a BH is "we don't know"
No. You underestimate what we know about black holes by a huge margin.
If one could turn the gravity volume knob on a BH down
You cannot. Asking what the laws of physics predict if the laws of physics don't apply is meaningless.

A black hole at the same mass with a lower gravitational constant would be smaller, but still a black hole. Light cannot escape a black hole, that is the definition of a black hole.

Quasars have matter orbiting a black hole outside the event horizon, this matter emits light we see. This is all understood well.
 
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  • #12
DrGreg
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The light being emitted from inside the black hole is not possible.
Do we really know that?
A black hole is something which by definition light cannot escape from within. So if light is escaping from within some object, that object cannot be a black hole.
 
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  • #13
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You underestimate what we know about black holes by a huge margin..
Please forgive my ignorance.

My view is that without a working model of gravity, and without a working model of the mechanics of a BH or gravity, I find it hard to see certainties on the mechanics.

I understand there is a great deal that can be observed and measured, but is not much of this still theoretical physics?

laws of physics
I think this all started when I was curious about the neutron degeneracy pressure and its relationship to gravity as seen with neutron stars, and if enough pressure was applied with enough matter, a BH would be formed if the mass was present.
 
  • #14
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My view is that without a working model of gravity, and without a working model of the mechanics of a BH or gravity, I find it hard to see certainties on the mechanics.
We have a working model of gravity and black holes. We have one that withstood thousands of tests. It is called General Relativity.
I understand there is a great deal that can be observed and measured, but is not much of this still theoretical physics?
No, there are countless experimental results testing it.
 
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  • #15
PeterDonis
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Do we really know that?
Yes.

I thought another name for a BH is "we don't know"
You thought incorrectly, as others have already commented.

I think this all started when I was curious about the neutron degeneracy pressure and its relationship to gravity as seen with neutron stars, and if enough pressure was applied with enough matter, a BH would be formed if the mass was present
There is a maximum mass that a neutron star can have, yes. However, the reason why is not quite what you appear to think. Pressure in an object like a neutron star is not applied inward; it's applied outward. It's what holds the neutron star up against its own gravity. But, as can be shown from the Einstein Field Equation, and as is confirmed by our observational data on neutron stars, pressure itself gravitates; it is part of the source of gravity. So as a neutron star gets more massive, and requires more pressure to hold itself up against its own gravity, its own gravity gets stronger, by more than you would expect just from the increased mass alone.

In addition, as the mass of a neutron star goes up and the pressure required to hold it up against its own gravity rises, the neutrons become relativistic. This reduces the ability of the neutron star matter to hold itself up against gravity with degeneracy pressure, compared to non-relativistic neutrons. (The technical terminology is that the adiabatic index of a Fermi gas--any body of matter made up of degenerate fermions--decreases from 5/3 in the non-relativistic regime to 4/3 in the relativistic regime.) The combination of these two factors--pressure gravitates, and neutrons become relativistic as degeneracy pressure rises--combine to determine a maximum mass for neutron stars, above which no amount of pressure can hold the star up against its own gravity, and it collapses to a black hole.
 
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  • #16
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Yes.



You thought incorrectly, as others have already commented.



There is a maximum mass that a neutron star can have, yes. However, the reason why is not quite what you appear to think. Pressure in an object like a neutron star is not applied inward; it's applied outward. It's what holds the neutron star up against its own gravity. But, as can be shown from the Einstein Field Equation, and as is confirmed by our observational data on neutron stars, pressure itself gravitates; it is part of the source of gravity. So as a neutron star gets more massive, and requires more pressure to hold itself up against its own gravity, its own gravity gets stronger, by more than you would expect just from the increased mass alone.

In addition, as the mass of a neutron star goes up and the pressure required to hold it up against its own gravity rises, the neutrons become relativistic. This reduces the ability of the neutron star matter to hold itself up against gravity with degeneracy pressure, compared to non-relativistic neutrons. (The technical terminology is that the adiabatic index of a Fermi gas--any body of matter made up of degenerate fermions--decreases from 5/3 in the non-relativistic regime to 4/3 in the relativistic regime.) The combination of these two factors--pressure gravitates, and neutrons become relativistic as degeneracy pressure rises--combine to determine a maximum mass for neutron stars, above which no amount of pressure can hold the star up against its own gravity, and it collapses to a black hole.
Could you please clarify what "pressure itself gravitates" means?
 
  • #17
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the gravity volume knob
What is the gravity volume knob? Please provide a reference for what you are referring to here because I have never heard of this.
 
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  • #18
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Could you please clarify what "pressure itself gravitates" means?
It means that the source of gravity is the stress energy tensor and pressure is part of the stress energy tensor.
 
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  • #19
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My view is that without a working model of gravity, and without a working model of the mechanics of a BH or gravity, I find it hard to see certainties on the mechanics.
We understand gravity very well, and have a working model as you call it that is very accurate. You may be thinking of a Quantum Theory Of Gravity - we don't have that yet - but our ignorance comes in below about the Plank scale - which is very very small. Then again we aren't sure of any of our theories at that scale since we cant experimentally probe it - yet.

General Relativity isn't that hard either. Particles move according to the Principle Of Maximal Time - which is just Newtons First Law in general coordinates. This leads to something called the metric guv (a 4X4 matrix) determining the motion of particles. This means guv acts like a field and we should be able to use field theory to see what equations it obeys. This is where a not very well known, but a truly amazing theorem, called Lovelocks Theorem comes in:
https://en.wikipedia.org/wiki/Lovelock's_theorem

This gives the Einstein Field Equations that, similar to Maxwell's Equations describing the electromagnetic field, describes the gravitational field guv. It is: Euv = Tuv. (I am ignoring the so called cosmological constant and units so that there is not a constant in front of Tuv). Euv, another 4X4 matrix, is called the Einstein tensor and depends on guv as Maxwell's equations depend on the electric and magnetic field, and Tuv is called the stress energy tensor:
https://en.wikipedia.org/wiki/Stress–energy_tensor.

The Einstein tensor, it turns out, is a measure of space-time curvature so we get that gravity is space-time curvature.

GR itself isn't that hard to get the gist of - it's the math like proving Lovelocks Theorem that is difficult. I have recently gone through the proof again (I did it many years ago but wanted to refresh my knowledge) and it took me a few days - it's very important but trivial to prove it is not.

Thanks
Bill
 
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  • #20
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There is a maximum mass that a neutron star can have, yes. However, the reason why is not quite what you appear to think. Pressure in an object like a neutron star is not applied inward; it's applied outward. It's what holds the neutron star up against its own gravity. But, as can be shown from the Einstein Field Equation, and as is confirmed by our observational data on neutron stars, pressure itself gravitates; it is part of the source of gravity. So as a neutron star gets more massive, and requires more pressure to hold itself up against its own gravity, its own gravity gets stronger, by more than you would expect just from the increased mass alone.

In addition, as the mass of a neutron star goes up and the pressure required to hold it up against its own gravity rises, the neutrons become relativistic. This reduces the ability of the neutron star matter to hold itself up against gravity with degeneracy pressure, compared to non-relativistic neutrons. (The technical terminology is that the adiabatic index of a Fermi gas--any body of matter made up of degenerate fermions--decreases from 5/3 in the non-relativistic regime to 4/3 in the relativistic regime.) The combination of these two factors--pressure gravitates, and neutrons become relativistic as degeneracy pressure rises--combine to determine a maximum mass for neutron stars, above which no amount of pressure can hold the star up against its own gravity, and it collapses to a black hole.
Thank you very much for spending to the time to show the mechanical explanation, that's what I was after.
 
  • #21
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have a working model as you call it that is very accurate
I thought we only measured its effects [Newton/Einstein] and had no idea how GR was actually formed, thank you for the links and reading material.

here was what I meant quoted from wiki [Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward (or gravitate toward) one another,]

Its how they classify it as a Phenomenon, not as a working hypothesis
 
  • #22
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What is the gravity volume knob? Please provide a reference for what you are referring to here because I have never heard of this.
Imagining applying more or less mass that increases or decreases degeneracy pressure on atomic elements. neutron, electron, proton
 
  • #23
russ_watters
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  • #24
russ_watters
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Imagining applying more or less mass that increases or decreases degeneracy pressure on atomic elements. neutron, electron, proton
Neutron degeneracy pressure is a threshold - a limit. It is not an operating pressure.

But yes, you can increase the pressure in a neutron star by adding mass.
 
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  • #25
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Imagining applying more or less mass that increases or decreases degeneracy pressure on atomic elements. neutron, electron, proton
Then next time say applying more or less mass. “Gravity volume knob” is confusing and highly non standard.
 
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