If no singularity, what’s inside a big black hole?

[suprised]

The interesting issue is what's inside the event horizon and if it can be confirmed by observation.

That's going to be the hard part... ;-)

 

[Bernie G]

"The interesting issue is what's inside the event horizon and if it can be confirmed by observation."

Not necessarily. Observing gamma ray bursts from other galaxies, which might be clearly identified as neutron star-BH mergers, might become routine in the future. Small BH-BH mergers will be rarer, perhaps only once a year. If two small black holes merge, and IF the internal object is 75% of the Schwarzschild radius, perhaps one solar mass will be ejected. Very observable.

 

[Bernie G]

The above post should have read: "Not necessarily that hard to observe."

 

[suprised]

The above post should have read: "Not necessarily that hard to observe."

Well you talked about _inside_ the event horizon... that's per def unobservable!

 

[Bernie G]

"Well you talked about _inside_ the event horizon... that's per def unobservable!"

Not when 2 small black holes merge. At the merger location the effect of gravity is canceled out.

 

[Constantin]

What cannot be observed does not exist.
For any observer outside the event horizon, there's nothing inside a black hole. The infalling matter remains frozen near the event horizon.

Discussions about the interior of black holes, observers inside a black hole, or observers falling into a black hole and passing through the event horizon, are just pleasant mind games.

 

[Bernie G]

"For any observer outside the event horizon, there's nothing inside a black hole. Discussions about the interior of black holes ... are just pleasant mind games."

Not so. If black holes merge, the gravity situation at the merger location changes dramatically. Are you saying small black holes don't merge?

 

[suprised]

I don't think that anything concrete is known about what happens quantum mechanically if 2 holes merge. It is not even known what precisely happens if two particles collide with sufficient energy as to form a black hole. As said, non-perturbative quantum gravity is relevant there, and AFIAK so far no existing formalis is able to capture that and eg compute the S-matrix.

And computing such an S-Matrix (say of formation and subsequent evaporation of a bh) is certainly not a mind game but of highest conceptional importance. Because eg violation of unitarity would, by virtual black hole loops, trickle down to low energies.

 

[Constantin]

I'm sure black holes do merge, but I don't think any information from inside the event horizon can escape during the merger, nor during any other event.

The explanations given by PAllen in his posts in this thread are quite interesting:
https://www.physicsforums.com/showthread.php?t=526367

They shed light on the behavior of the event horizon during a merger.

 

[Bernie G]

Yes. About BH mergers that thread says: "Thus whatever the details are at the point of collision are, they will quickly be shrouded behind the horizon." Yes. But if something exists inside a BH with 75% of the Schwarzschild radius, there will be a lot of stuff escaping during "quickly", perhaps roughly one solar mass of ejected radiation (for 2 merging 8 solar mass black holes). Thats quite an ejection.

 

[atyy]

I guess, though this seems controversial, that the infalling observer experiences a coherent superposition of fuzzball states to the effect that he notices nothing particular at the horizon. I understand, though, that he infalling observer problem seems to be the weakest point in this proposal.

I guess in the fuzzball proposal, the microscopic state is that there is actually no event horizon?

And the event horizon somehow appears by coarse graining to a macroscopic outside observer?

I'd also be interested in knowing whether http://arxiv.org/abs/1008.3439" 's ideas are consistent with fuzzball ideas.

 

[tom.stoer]

I guess in the fuzzball proposal, the microscopic state is that there is actually no event horizon?

What do you mean by that?

What is so special about event horizons? Classically black hole event horizons are nothing else but lightlike, closed, non-expanding 2-surfaces. The reason why a classical observer feels nothing special when crossing the horizon is simply due to the fact that the difference between an arbitrary lightlike surface and an event horizon cannot be defined locally. There are infinitly many lightlike surfaces the observer can cross. What's special about the horizon is that its closed and non-expanding. But the infalling observer can't detect that b/c it's a global property.

Does such a classical geometry emerge from fuzzballs?

 

[suprised]

I guess in the fuzzball proposal, the microscopic state is that there is actually no event horizon?

Right - the fuzzball microstates do not have horizons.

Incidentally, the fuzzball states really "require" the full compactified 10-dim (or corresponding non-geometric notion) string degrees of freedom. We knew that from state counting before, what is new here is the actual explicit construction of those microstates. And one really needs precisely all of those in order for this mechanism to work. This is a remark to those who believe that this problem can be solved from within pure gravity...

And the event horizon somehow appears by coarse graining to a macroscopic outside observer?

This is what is claimed.

I'd also be interested in knowing whether http://arxiv.org/abs/1008.3439" 's ideas are consistent with fuzzball ideas.

No idea...

 

[Physics Monkey]

I'm curious about the relationship, if there is one, between fuzzballs and black hole complementarity. I have favored the idea of complementarity for some time, but was never able to understand very completely how it relates to fuzzballs.

On a related note and in the spirit of complementarity, I have always found the nice slices used e.g. in Mathur's discussion to be rather disturbing since they include regions behind the horizon. This seems manifestly wrong to me.

 

[Bernie G]

Correction: Previously I said there may be some BHs with 10^12 solar masses. This is roughly 10X the mass of the Milky Way, not 1000X.

 

[tom.stoer]

This is a remark to those who believe that this problem can be solved from within pure gravity...

There is a proposal for black holes in LQG which defines horizons in terms of spin networks, i.e. with pure gravity ...

 

[suprised]

There is a proposal for black holes in LQG which defines horizons in terms of spin networks, i.e. with pure gravity ...

And how do they get the necessary states? Quite a few people doubt that it could ever work.

 

[tom.stoer]

They count spin network states forming the classical horizon area. The result reprocudes the Bekenstein-Hawking-entropy plus corrections. I bet marcus has a list of publications.

 

[suprised]

They count spin network states forming the classical horizon area. The result reprocudes the Bekenstein-Hawking-entropy plus corrections. I bet marcus has a list of publications.

AFAIK up to an arbitrary factor, which means that the result is meaningless?

 

[tom.stoer]

AFAIK up to an arbitrary factor, which means that the result is meaningless?

What is the proton mass according to string theory? Up to how many arbitrary factors?

So let's continue seriously? or polemically?

 

[Haelfix]

The difficult thing is not so much calculating the correct area scaling law (although that was difficult enough), but rather giving a precise microscopic story about what the two local observers see, and how details of their measurements must be somehow entangled and noncommuting.

Since this is very much about details of semiclassical states, afaik this is way beyond LQG's current technology and it is not even addressed yet.

In fact, the exact details is not even known in string theory or AdS/CFT, and the fuzzball proposal is the only one that even tries to address this incredibly difficult problem head on.

 

[suprised]

What is the proton mass according to string theory? Up to how many arbitrary factors?

So let's continue seriously? or polemically?

You start here polemics. Indeed has been shown since long that in string theory the factor comes out right, to every detail. And what contributes are states that do go beyond "pure gravity", ie., have, in a sense, an extra-dimensional origin. In LQG, as far as I know, the result is proportional to an abitrary constant, the Immirzi parameter. This ambiguity (in front of a log!) thus does not allow to decide whether the number of states contributing is correct or not. So this is meaningless for settling this question in LQG.

This string computation is undisputable. What is disputable, and is disputed, is whether Mathur et al's explicit construction of the microstates, which goes much beyond just counting the states, is correct or not. While it looks convincing, there has been criticism, like for example whether the nice slice argument is physically well-defined etc.

As said we have been running a workshop on Quantum Gravity right now, which discusses this kind of questions. Tomorrow is LQG day and we will see what the LQG persons have to tell.

 

[atyy]

As said we have been running a workshop on Quantum Gravity right now, which discusses this kind of questions. Tomorrow is LQG day and we will see what the LQG persons have to tell.

http://www.physics.ntua.gr/corfu2011/st.html ?

 

[qsa]

http://arxiv.org/PS_cache/gr-qc/pdf/9404/9404036v2.pdf


Wavefunction of a Black Hole and the
Dynamical Origin of Entropy

 

[tom.stoer]

You start here polemics.

Sorry about that, but you started this kind of reasoning.

Indeed has been shown since long that in string theory the factor comes out right, to every detail.

For extremal black holes with maximal SUSY, no Schwarzschild and no Kerr, right?

This ambiguity thus does not allow to decide whether the number of states contributing is correct or not. So this is meaningless for settling this question in LQG.

We know that all theories including quantum gravity (including string theory) are work in progress. So of course there are open questions. Everybody in the LQG community would agree that the Imirzi parameter os one of them.

All what I wanted to say is that there seems to be a very detailed description based on microscopic degrees of freedom which can be applied to "classical black holes". The Immirzi parameter has to be fixed, then the prediction is unambiguous. I do not see a problem to have one parameter in a theory w/o being able to derive it theoretically. You can't do that in other theories, either (QCD coupling constant / scale, GSW coupling / Fermi constant, ...)

As said we have been running a workshop on Quantum Gravity right now, which discusses this kind of questions. Tomorrow is LQG day and we will see what the LQG persons have to tell.

fine

 

[suprised]

Sorry about that, but you started this kind of reasoning.

not aware of...

For extremal black holes with maximal SUSY, no Schwarzschild and no Kerr, right?

Sure, that's the way non-perturbatively exact statements can be made without directly solving the theory.

We know that all theories including quantum gravity (including string theory) are work in progress. So of course there are open questions. Everybody in the LQG community would agree that the Imirzi parameter os one of them.

All what I wanted to say is that there seems to be a very detailed description based on microscopic degrees of freedom which can be applied to "classical black holes". The Immirzi parameter has to be fixed, then the prediction is unambiguous. I do not see a problem to have one parameter in a theory w/o being able to derive it theoretically. You can't do that in other theories, either (QCD coupling constant / scale, GSW coupling / Fermi constant, ...)

Maybe I didnt make the significance clear enough. This is not only just some parameter like the QCD coupling that needs to be fixed. This would indeed be a triviality and no reason to muck around. Rather, because it multiplies the entropy, it directly affects how you count the number of states of the theory. Since this parameter is arbitrary, AFIAK, it is impossible to tell whether the states provided by LQG are "enough" such as to account for the microstates of black holes. Tuning the parameter to the "right" value won't continuosuly change the number of states until it matches the correct count. Rather it should be seen as a prefactor multiplying an unknown state count.

Thus, this result does not shed light on the question whether LQG provides, or not, the correct degrees of freedom of QG. This in contrast to strings, where the state count (in toy model examples of black holes) comes out right on the nose, including subleading quantum corrections.

These facts are known to anybody working in the field, and this was also confirmed by today's discussions.

 

[phinds]

if matter and anti-matter annihilate,only the expansion of time can keep them apart.thus a singularity at the center of a black hole i suggest consists of anti-matter 13.7 billion years in the past(back to the big bang)and matter 13.7 billion years into the future.the matter in the surrounding galaxy is being sucked into the future this is my understanding of space time.

Welcome to the forum.

I don't know what your purpose is here but this kind of "personal opinion" doesn't fly well with the moderators, especially when it looks like nonsense. If you are asking a question (and I don't see one in the above post), I would suggest that it be "why is my understanding of space-time so totally at odds with accepted physics".

 

[tom.stoer]

... Rather, because it multiplies the entropy, it directly affects how you count the number of states of the theory. Since this parameter is arbitrary, ... it is impossible to tell whether the states provided by LQG are "enough" such as to account for the microstates of black holes. Tuning the parameter to the "right" value won't continuosuly change the number of states until it matches the correct count. Rather it should be seen as a prefactor multiplying an unknown state count.

...

These facts are known to anybody working in the field, and this was also confirmed by today's discussions.

I never understoof the Immirzi parameter as a multiplicative parameter for the number of states (for a given area) but always as a multiplicative constant for the (classical) area given a predefined state count. So there are two issues: is the counting correct? what's the value of the Immirzi parameter?

What was the result of the discussion with the LQG colleagues you mentioned.

 

[tom.stoer]

So there are two issues: is the counting correct? what's the value of the Immirzi parameter?

What was the result of the discussion with the LQG colleagues you mentioned.

I checked some papers (especially Sahlman, Agullo, Barbero) and I think they agree on the state counting. So this issue goes away.

I still have to look for recent results regarding the Immirzi parameter (which does not affect the entropy for a given spin network, but 'only' the area related to a given spin network; so as I said, the value must be fixed, but it does not affect the counting itself, only its relation to the 'classical area').

The picture within LQG is remarkable simple:
- the horizon is characterized by the 'isolated horizon condition'
- the state count is defined by spin network punctures of the horizon
- the microscopic degrees of freedom are spin networks (plus induced surface degrees of freedom)
- the calculation is known for realistic Schwarzschild black holes
- afaik the Kerr solution has not been studied so far
- afaik neither a dynamical collaps nor evaporation has been studied so far
- entropy is related to microstates but not yet to temperature

 

[suprised]

Well, we have had various discussions and the general consensus seems that this formula is inconclusive, with regard to the question whether the right number of states is counted. In fact the "right" value of the Immirzi parameter depends on the particular LQG model, and thus is non-universal. And for spin foam models, which seem to have replaced LQG, there are AFAIK few, if any, relevant entropy calculations, and the Immirzi parameter does not appear. So the general feeling seems that while there are encouraging signs, something still is wrong or at least not understood.

The whole issue seems always to boil down to the following two possibilities: either gravity can be made sense of out of itself (by regularizing/discretizing it, UV self-completing it, etc), or it needs to be embedded into a "larger" theory which UV-completes it. But this is getting off-topic.

 

[tom.stoer]

Well, we have had various discussions and the general consensus seems that this formula is inconclusive, with regard to the question whether the right number of states is counted. In fact the "right" value of the Immirzi parameter depends on the particular LQG model, and thus is non-universal.

I agree that there is no consensus regarding the Immirzi parameter. But we do not agree on the reasoning in general.

As far as I understand the models in LQG the idea is always to define a surface by isolated horizons and to "count states". Whether this counting is correct or not has nothing to do with the free parameter; the parameter is needed to define the area of the horizon. So as I said there are two issues: one is the counting itself, another one ios the value of the parameter.

I agree that something is still missing, but at the same time I would say that a lot is already correct, b/c up to a free parameter everything seems to fit nicely.

 

[suprised]

http://www.physics.ntua.gr/corfu2011/st.html ?

No, http://cern.ch/go/zRg9

 

[Bernie G]

If the radiation ball model described earlier is correct (0.75 Schwarzschild radius ), its possible for similar size merging black holes to partially or completely annihilate in a mini big bang.

 

[Bernie G]

I wonder if some of the merged galaxies that appear to have had an explosion in the center actually have had an explosion in the center. Its now accepted that when galaxies merge the super massive black holes in the center can also merge. If the radiation ball model is correct these super massive black hole mergers might result in a massive ejection.

 

[Bernie G]

Maybe a radiation star of R > 0.75 SR can exist in a black hole, as a partial radiation/quark mixture.