## How the Black Hole Firewall Paradox Was Resolved 118

Stephen Hawking's recent comments about the nature of black holes have bred uncertainty about physics concepts that were relatively well understood. This article from astrophysicist Ethan Siegel explains that yes, black holes still exist, and how a group of three academic papers answered the black hole 'firewall' paradox. Quoting:

*"... And so what these three papers, in tandem, have done, is demonstrate that there is no firewall and that the resolution to the firewall paradox is that the first assumption, that Hawking radiation is in a pure state, is the one that's flawed. You won't read about this in the popular write-ups because it doesn't have a catchy headline, it's complex, and it's not work by someone that's already very famous for other work. But it's right. Hawking radiation is not in a pure state, and without that pure state, there's no firewall, and no paradox. There is still an incredible amount to learn and understand about black holes, event horizons, and the behavior of quantum systems in strongly curved spacetime, to be sure, and there's lots of very interesting research ahead. These findings arguably raise more questions than they answer, although at least we know that black holes won't fry you when you fall in; it will still be death by spaghettification, not by incineration!"*
## Re:I always thought... (Score:5, Informative)

The problem with that is that black holes need the mass they suck in to exist.

The mass cannot both be in the black hole and shot out the other side into a new universe.

So unless you can come up with a theory that has black holes creating mass out of nothing, that is simply impossible.

## Re:I always thought... (Score:5, Informative)

No it isn't. A cosmological model is a foliation of maximally-symmetric spacelike hypersurfaces. A black hole is not maximally-symmetric. A black hole is a non-evolving system -- it possesses at least one timelike Killing vector -- and a cosmological system is the exact opposite, an evolving system that has no timelike Killing vector. The only real similarity is that a black hole (as you're doubtless meaning it; a Schwarzschild solution) is spherically symmetric, and the spacelike slices in a cosmological spacetime are also spherically symmetric. But so is flat Minkowski space, and so are Tolman-Bondi spaces.

## Re: I always thought... (Score:4, Informative)

There is some debate exactly what happens when you get enough mass together to tie space-time into knots. The most important part of a black hole is really the event horizon, which is the point X distance from the center of mass of a black hole where the gravitational pull is so strong that nothing can escape. The original idea was that is was a very static place, a perfect circle.

There is a bunch of different problems with this. Because in this universe we have the idea that mass, energy, and information cannot be lost. Something cannot just go into a black box and all knowledge of it is lost, because then information would then of been lost.

Stephen hawking's has just come forward with the idea that it is a far more stormy area with fluctuating gravity. Which would allow this information to escape. Previously the idea was Hawking radiation, which would allow things to escape from the black whole even without fluctuation gravity.

One of the problems of with the horizon, that someone just proposed, was that it would be so tumultuous at the edge that everything would be burned beyond recognition,. This article is about how stuff entering a BH would NOT in fact be burned beyond recognition.

## Re:I always thought... (Score:2, Informative)

"However, I'd like to point out that nothing in your analysis validates wisnoskij's contention that the mass of a black hole has to be considered as existing entirely within this universe, therefore preventing it from acting as a "wormhole" to another one."

A fair comment - I put my entire reply in parantheses because it was meant to pick at one of your statements, rather than the entire post, something I should have made a lot clearer.

With the wormhole thing, the idea basically comes from a maximal extension of a black hole. You can split the spacetime of a black hole into four sections: section I is our universe, asymptotically flat. (So, evidently, not actually our unvierse but let's ignore that for now.) Section II is the future inside the black hole, section III is the past inside the black hole, and section IV is another asymptotically flat region on the "other side" of the black hole. (See here [prime-spot.de].) This then obviously raises questions about what that other region is and how to get there -- perfectly valid questions, the answer to which is commonly called a wormhole.

Unfortunately it does have to be pointed out that this has arisen because we've maximally-extended the spacetime. In reality, we can already guarantee that section III does not exist, because the extension into the past cannot be made -- the black hole has not existed for all time. This basically screw us, and the realistic situation is more like this [ggpht.com].

Of course, this isn't the only way we can envisage getting a wormhole out of GR, but it's one of the best studied.

And also, of course, the singularity does render any speculations -- even using GR -- nothing more than speculations. We simply can't say what goes on there. I know that in loop quantum gravity the singularity issue is rather lessened but I don't actually know what happens there, since it's very much not my field.

You're totally right that Hawking's more recent statements have been conceptual arguments rather than mathematical models. That was the case when he talked about the information paradox, and it's the case now talking about the firewall. That doesn't mean the arguments are not worth listening to, but it does mean there's no reason to think that the issues are finished just because Hawking's deigned to talk about them.