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

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

That's gonna 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 Schwarzchild 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

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 cancelled 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:
http://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 Schwarzchild 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 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 Takayanagi and Ugajin's ideas are consistent with fuzzball ideas.

tom.stoer

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

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....

This is what is claimed.

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.