Black Hole Core Mystery

[EWyatt]

The conversational theory of what constitutes a black hole core has resulted in many non-answers, some even dealing with an "infinitesimal singularity" which I find ridiculous. Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star! That would also keep current laws of physics intact, without all those hypothetical black hole hyperboles. Or has this neutron star thing been rebuffed already? Thanks.

Edited September 13, 2011 by EWyatt

[Realitycheck]

However, black holes are most likely dead and very cold while neutron stars still radiate.

Edited September 13, 2011 by Realitycheck

[questionposter]

The conversational theory of what constitutes a black hole core has resulted in many non-answers, some even dealing with an "infinitesimal singularity" which I find ridiculous. Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star! That would also keep current laws of physics intact, without all those hypothetical black hole hyperboles. Or has this neutron star thing been rebuffed already? Thanks.

 

While I don't think singularities are actually infinitely small (which means they would have infinite density too, but would that also mean infinite gravity since density can make gravity bigger?), I don't think it's a neutron star or anything like a neutron star because the gravity is so strong that the elementary particles of matter than we know can no longer function as we know them.

 

However, black holes are most likely dead and very cold while neutron stars still radiate.

 

How could a black hole be dead and cold with so much force putting strain on matter and all the random light it absorbs?

[ajb]

The truthful answer is that we do not understand physics in such extreme gravitational fields. No one really known what lies in the middle of a black hole. But for sure, most physicists believe that the classical singularity is not physically realised and that quantum gravity will regulate or smear this out on the scale of the Planck length. There maybe quite exotic physics here like a noncommutative nature to space-time, something akin to the phase space of quantum mechanics.

 

Rght now quantum gravity is an active area of research.

[csmyth3025]

The conversational theory of what constitutes a black hole core has resulted in many non-answers, some even dealing with an "infinitesimal singularity" which I find ridiculous. Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star! That would also keep current laws of physics intact, without all those hypothetical black hole hyperboles. Or has this neutron star thing been rebuffed already? Thanks.

Because of theoretical considerations, the "neutron star thing" has been rebuffed already:

 

In general, compact stars of less than 1.44 solar masses – the Chandrasekhar limit – are white dwarfs, and above 2 to 3 solar masses (the Tolman–Oppenheimer–Volkoff limit), a quark star might be created; however, this is uncertain. Gravitational collapse will usually occur on any compact star between 10 and 25 solar masses and produce a black hole.

(ref. http://en.wikipedia....ki/Neutron_star )

 

How could a black hole be dead and cold with so much force putting strain on matter and all the random light it absorbs?

 

Because of its event horizon, a black hole cannot emit thermal radiation. It is theorized, however, that it may produce Hawking radiation. The amount of radiation thus produced is inversely proportional to the mass of the black hole and, for stellar-mass black holes or larger, the "temperature" of the Hawking radiation is measured in billionths of a degree Kelvin or less:

 

As an example, a black hole of one solar mass has a temperature of only 60 nanokelvins; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits.

(ref. http://en.wikipedia....iation#Overview )

 

Chris

[BJC]

The truthful answer is that we do not understand physics in such extreme gravitational fields. No one really known what lies in the middle of a black hole. But for sure, most physicists believe that the classical singularity is not physically realised and that quantum gravity will regulate or smear this out on the scale of the Planck length. There maybe quite exotic physics here like a noncommutative nature to space-time, something akin to the phase space of quantum mechanics.

 

Rght now quantum gravity is an active area of research.

The statement in the original topic "Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star!" deserves a better answer than "we need quantum gravity". While it is true that no one knows "for certain" what occurs behind the event horizon of a black hole surely what happens some distance from the event horizon must be gravitationally similar to what happens to any object at the same distance (and equal mass).

 

Say we are a distance 200 million km. from a mass of about 3 solar masses. Would it matter gravitationally whether that mass was a gas, star, neutron star or a black hole? Now consider the mass to be a black hole and we slowly decrease our distance to about the same distance as the radius of a similar mass neutron star. Wouldn't we experience the same gravitational forces as an object on the surface of a same mass neutron star?

 

Is there a distance from the black hole where the protons and electrons form neutrons?

Why is a Black Hole considered "cold" when in-falling objects must be heated by gravitational tidal forces? Wouldn't those same forces cause the temperature to be very high very close to the black hole?

[ajb]

The statement in the original topic "Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star!" deserves a better answer than "we need quantum gravity".

 

Well it is not logical. Neutron stars are supported by the Fermi pressure. If the compact object is sufficiently massive, the gravitational force can overcome the Fermi pressure and a black hole is formed. Unless there is something we do not know about stopping this final collapse.

 

So to understand the classical singularity, or why it does not truly exist requires physics we do not have a great handle on.

[questionposter]

Because of its event horizon, a black hole cannot emit thermal radiation.

 

 

 

Exactly, it can only absorb radiation, not emit it, except in a really really really really really really slow evaporation. If the Earth could keep all the heat it started out with, it would still be boiling and have flowing lava everywhere.

Edited September 14, 2011 by questionposter

[csmyth3025]

Exactly, it can only absorb radiation, not emit it, except in a really really really really really really slow evaporation. If the Earth could keep all the heat it started out with, it would still be boiling and have flowing lava everywhere.

I think Realitycheck was referring to the perception of an observer outside of the event horizon. As ajb commented, no one really knows what lies in the middle of a black hole.

 

If it was possible to stand next to a black hole, you wouldn't feel any heat coming from it like you would standing next to a fireplace - no matter how "hot" it is inside the event horizon.

 

Chris

[BJC]

Well it is not logical. Neutron stars are supported by the Fermi pressure. If the compact object is sufficiently massive, the gravitational force can overcome the Fermi pressure and a black hole is formed. Unless there is something we do not know about stopping this final collapse.

 

So to understand the classical singularity, or why it does not truly exist requires physics we do not have a great handle on.

Yes - every lecture i have heard or read emphasizes your point regarding the "singularity"

 

However, I was asking if there could be similarities outside the event horizon of a black hole.

[ajb]

However, I was asking if there could be similarities outside the event horizon of a black hole.

 

You can examine the physics behind the horizon. You could use Kruskal–Szekeres coordinates as these are well behaved everywhere apart from the singularity. There are other coordinate systems that may also be convenient.

 

As long as one is not very close to the singularity one can use classical or semi-classical theory.

[EWyatt]

Thank You to all for an interesting discussion. The fact that no definitive answer is provided (or even possible yet!) regarding my original question just shows how much we have yet to learn. Again, I don't believe in singularities, anything infinitesimally small or dense, or one-solar-mass black holes (really?) because some things just don't pass the "there is logic in physics" test. Nevertheless, I'm sticking to my original point: the possibility that a very, very dense, i.e., large, neutron star would have the gravitational effect exhibited by a black hole, Fermi pressure or not. Anyway, this was fun!

Edited September 15, 2011 by EWyatt

[Realitycheck]

The neutron star has a gravitational effect very close to that of a black hole. The black hole just takes it a step further, increased gravity which prevents most radiation from escaping, cessation of atomic activity, it might even reach absolute zero. It's just that stuff zinging around the disk at high speed that creates Hawking radiation.

Edited September 15, 2011 by Realitycheck

[ajb]

The neutron star has a gravitational effect very close to that of a black hole.

 

 

All massive spherically symmetric non-rotating objects are described by the Schwarzschild solution. The only difference is that black holes have an event horizon. For other massive objects this horizon would lie inside the objects, but the Schwarzchild metric does not describe interior solutions, it is a vacuum solution.

 

For slowly rotating objects the Schwarzschild solution would be a very good approximation. Otherwise the Kerr metric describes a massive rotating object.

[questionposter]

The event horizon I just the point where light can't escape. Sure you can't see past it, but what's so special about that? Why does time have to stop there? Why does stuff have to infinitely stretch or get frozen there? It's just another random distance form the singularity like any other distance, which if you went past the event horizon would have an even greater escape velocity.

 

Also if a singularity is infinitely small, how can there even be distance from it? It's sort of like trying to measure the exact surface area of something like an eroded rock and running into that fractal problem where there's an infinitely smaller level of area to measure from since like a crack would keep having cracks inside it, and then cracks inside those, and etc.

Edited September 16, 2011 by questionposter

[ajb]

The event horizon I just the point where light can't escape. Sure you can't see past it, but what's so special about that? Why does time have to stop there? Why does stuff have to infinitely stretch or get frozen there? It's just another random distance form the singularity like any other distance, which if you went past the event horizon would have an even greater escape velocity.

 

 

These problems are due to a poor choice in coordinates. There are coordinates that have better behaviour at the event horizon for the Schwarzschild black hole. I have already given one set of such coordinates.

[imatfaal]

AJB - glad you mentioned that, I am arguing this in another place; am I right in thinking that the mathematical singularity (ie getting an undefined/infinte answer) that you must get by using schwarzchild coordinates can be avoided by using Kruskal–Szekeres coordinates or Eddington–Finkelstein coordinates - and that when these other systems are used you no longer end up with infinite time dilation

[ajb]

AJB - glad you mentioned that, I am arguing this in another place; am I right in thinking that the mathematical singularity (ie getting an undefined/infinte answer) that you must get by using schwarzchild coordinates can be avoided by using Kruskal–Szekeres coordinates or Eddington–Finkelstein coordinates - and that when these other systems are used you no longer end up with infinite time dilation

 

 

You still have the singularity at the centre, this is a real singularity and is an intrinsic property of the space-time. The singularity at the horizon as found in Schwarzschild coordinates is not "real", it is an artefact of these coordinates.

[Schrödinger's hat]

You can examine the physics behind the horizon. You could use Kruskal–Szekeres coordinates as these are well behaved everywhere apart from the singularity. There are other coordinate systems that may also be convenient.

 

As long as one is not very close to the singularity one can use classical or semi-classical theory.

 

Wait, I thought they were also ill defined at the event horizon. Or is it enough that the limit exists from one side for each function?

If that's the case wouldn't it make events on the event horizon degenerate?

Can they be distinguished in other coordinate systems?

Edited September 16, 2011 by Schrödinger's hat

[imatfaal]

thanks - it was the mathematical artefact at the horizon that I meant.

 

Hey - if we could get rid of the singularity at middle I would be booking tickets for oslo!

[ajb]

Wait, I thought they were also ill defined at the event horizon.

 

I am fairly sure that the coordinates can be extended to cover the entire space-time, apart from the singularity at the centre. It is well known that the Schwarzschild radius is nothing special and that the singular nature there is due to the choice of coordinates.

Edited September 16, 2011 by ajb

[Schrödinger's hat]

I am fairly sure that the coordinates can be extended to cover the entire space-time, apart from the singularity at the centre. It is well known that the Schwarzschild radius is nothing special and that the singular nature there is due to the choice of coordinates.

 

Wait, silly me. I was looking at the animation on the wiki page of lines at constant r and thinking that the ones on the light cones looked like they represented something indeterminately smeared around the entire event horizon -- without actually realising that was the concept which that part of the diagram was trying to get across (ie. not realizing that a line of constant r at the event horizon is a line of constant r at the event horizon ).

My confusion was furthered by looking at the closed form conversion from Schwarzchild to Kruskal, the badly defined bit there comes from the Schwarschild coordinates.

Edited September 16, 2011 by Schrödinger's hat

[baric]

The conversational theory of what constitutes a black hole core has resulted in many non-answers, some even dealing with an "infinitesimal singularity" which I find ridiculous. Why not just take a logical step back and conclude that a black hole, and its core, is simply a very large, dense neutron star! That would also keep current laws of physics intact, without all those hypothetical black hole hyperboles. Or has this neutron star thing been rebuffed already? Thanks.

 

Because we already understand the force sustaining the neutron star core and it does not produce densities sufficient to create an event horizon.

 

Now, if are suggesting that neutron star cores are somehow compressible so that, at a certain mass level, the density of the core is sufficient to create an event horizon then you still need to explain why crossing this threshold does not constitute a transition to a new state.

 

Remember, the pressure at the center of a star is much greater than at the mantle. If the core is compressible, then once the core becomes sufficiently dense to generate an event horizon, it will immediately trigger a collapse of the entire star as core material falls into the event horizon and expands it.

 

But mainly you need to provide the math supporting your state model for a compressible core.

[DrRocket]

Wait, I thought they were also ill defined at the event horizon. Or is it enough that the limit exists from one side for each function?

If that's the case wouldn't it make events on the event horizon degenerate?

Can they be distinguished in other coordinate systems?

 

Nothing is ill-defined at the event horizon. In fact if you were crossing the event horizon you would note nothing remarkable.

 

All sorts of odd things may appear to happen in some coordinate system, including the event horizon appearing to be singular (as in Schwarzchild coordinates) but that is illusory, an artifact of an arbitrary choice of coordinates.

 

The singular nature in the interior, as proved by Penrose, is independent of any coordinate system.

 

Note: The whole point of "general covariance" in general relativity is the formulation of physics without reference to any coordinate system. That is how Riemannian geometry works.

[Schrödinger's hat]

Nothing is ill-defined at the event horizon. In fact if you were crossing the event horizon you would note nothing remarkable.

 

All sorts of odd things may appear to happen in some coordinate system, including the event horizon appearing to be singular (as in Schwarzchild coordinates) but that is illusory, an artifact of an arbitrary choice of coordinates.

 

Yeah, I saw my mistake now. I thought some of those odd things also happened in Kruskal coordinates (forgetting that was exactly why they were derived), and then spent a little while mis-understanding the wiki page on them before understanding what abj said.

 

Thanks