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

[tom.stoer]

OK, maybe there is a "horizon-issue", but only in the sense that there is an underlying microscopic structure to classical spacetime.

Or do you think that classical GR (to which I refer when I am talking about free fall, proper time etc.) will no longer be valid outside the horizon for large black holes? Of course we expect that the evaporation will change, but we do not expect any "quantum effects" for classical motian, do we?

 

[IttyBittyBit]

3. The asymptotic observer will see the infalling observer still there after the horizon evaporates.
4. Therefore from the asymptotic observer's point of view, she doesn't cross the horizon either, and will live to see it completely evaporate.

Actually, no. That is only true if the infalling astronaut is very far from the horizon. If the infalling astronaut is near the horizon, the asymptotic observer will see them go in, but only at the very last moment where the black hole vanishes in a blast of Hawking Radiation. Therefore, from the asymptotic point of view, the infalling astronaut never spends any time 'inside' the event horizon. This is explained in this page: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

This is wrong! (the blue shift is correct but the effect is tiny)

It seems you are right about this and I am wrong. I was thinking about a Schwarzschild reference frame. In such a reference frame you would see infinite blue-shift right before `going in'. However, a directly infalling observer would not see infinite blue-shift. Sorry for talking out of my a%$.

However, even though the blue-shift would not be infinite, it would still happen. It still remains that a theory of quantum gravity is needed before a definitive answer can be given. This paper talks about this subject at length: http://arxiv.org/abs/0806.0628

Have you ever made a single calculation in general relativity by yourself?

I've made many. If you know more about something than others, it is not appropriate to be rude about it.

 

[tom.stoer]

However, even though the blue-shift would not be infinite, it would still happen. It still remains that a theory of quantum gravity is needed before a definitive answer can be given. This paper talks about this subject at length: http://arxiv.org/abs/0806.0628

Thanks for the link.

I've made many. If you know more about something than others, it is not appropriate to be rude about it.

Sorry about that!

 

[Haelfix]

OK, maybe there is a "horizon-issue", but only in the sense that there is an underlying microscopic structure to classical spacetime.
Or do you think that classical GR (to which I refer when I am talking about free fall, proper time etc.) will no longer be valid outside the horizon for large black holes?

Hmm. The problem isn't in the details of the dynamics but rather that the horizon analysis leads to the seemingly inescapable clash between two cherished physical principles, namely unitarity and locality whereby only one of the two can remain true in our universe.

Details about the small corrections due to quantum gravity cannot change this conclusion, which is why the information loss paradox is one of the greatest unresolved problems in theoretical physics. It isn't some mere detail of quantum gravity to be determined by future generations but rather the type of clash (like the UV catastrophe) that signals a theoretical underpinning must be altered (which qg must thereafter explain in detail microscopically)

So in a sense the answer is yes, the classical theory most likely is incomplete (even macroscopically), or rather it appears necessary for there to be a complementarity between descriptions and/or a dual holographic formulation that rescues us from what would otherwise be an absurdity.

 

[suprised]

Details about the small corrections due to quantum gravity cannot change this conclusion, which is why the information loss paradox is one of the greatest unresolved problems in theoretical physics.

This is absolutely correct; see eg the arguments of Mathur (eg recent papers/reviews) why "small" quantum gravity effects cannot help, rather they need to be of order one near the horizon. How this actually works in detail is a highly controversial issue, the fuzzball proposal ist just one attempt, though quite explicit and at least for me, quite convincing.

At any rate, issues of singularity resolution at the center appear to be a red herring to this problem, it is not the relevant question to ask.

 

[atyy]

So how big roughly would the macroscopic effects be at the horizon? If say a 70 kg person fell through the event horizon of a large black hole, would he notice anything like a fuzzball?

 

[suprised]

So how big roughly would the macroscopic effects be at the horizon? If say a 70 kg person fell through the event horizon of a large black hole, would he notice anything like a fuzzball?

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.

 

[MTd2]

I know... but that transform is non-trivial! E.g. the part of the world line of the infalling observer which is inside the horizon is not even in the spacetime of the asymptotic observer, so the transform must have some singularities; my question is whether they are physical. Searching for literature on this gives remarkably thin results (i.e. none). I really would like to know the answer, but I don't think anyone has it --- I would be happy to be shown otherwise.

This is why I don't believe in black holes. To make hawking radiation compatible to an asymptotic observer, an infalling observer would receive an infinitely strong blast of radiation when crossing the horizon. This is why I think the fuzzball is better than the LQG solution, at least how it is interpreted. The infinite blast should be actually the leaking gas of hot sphere made by whatever entity a fundamental theory of quantum gravity regards as fundamental.

EDIT.: Just noticed what suprised said above. So, what I mean is a killer fuzball.

 

[Bernie G]

A lot of smart people decades ago questioned if black holes existed, and if they did exist doubted they were a point singularity. But black holes have now been confirmed by observation in our galaxy with a high degree of certainty. I think this talk about what happens at the horizon is too complicated; its different for smaller or bigger black holes anyway. The interesting issue is what's inside the event horizon and if it can be confirmed by observation.

 

[MTd2]

I do not doubt that the objects seen are BH, classically. For all practical purposes we are at an infinite distance from all of them.

 

[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?