## 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!"*
## Complementarity (Score:5, Interesting)

It all depends where you're observing from.

If you're alice, falling through the black hole horizon, you see no horizon, and no firewall. It's a harmless point of no return. In a particularly large black hole say with a horizon the size of the solar system (this would have to be a super-galactic beastie) you could potentially live out your life in there before getting crushed by tidal forces.

If you're bob on the outside, it looks like alice slows down and gets increasingly red shifted. Alice moves asymptotically towards the horizon but never quite reaches it. Just getting slower and redder. Of course the reverse is also true, if alice looked back at bob she'd see him all sped up like keystone cops.

Because the light coming to you from the regime around alice is so red shifted, you conclude that it must be incredibly high energy/frequency down where alice is (the firewall)

The funny part is, if you send photons at alice hoping to reflect them back to yourself (to see if she's alright) - the photons have to be so energetic to make the return trip that you end up vapourising alice just as the firewall would have done.

This is the impression I get from reading Leonard Susskind's stuff, broadly taken to be black hole complementarity. Neither view is objectively more 'correct' than the other. We've accepted wave/particle duality so I don't really see how we can't have two pictures of what happens in a black hole.

## Re:Complementarity (Score:2, Interesting)

"It's her movement relative to you that causes her time to slow down."

No, it's a lot more than that. There are more causes of time dilation than movement, and more causes of redshift than movement, too. What Maritz is referring to are gravitational time dilations and gravitational redshifts, which in this situation dominate those from motion.

If you bother looking at the equations, yes, as she nears the event horizon, to an external observer she'll be moving ever slower and getting ever dimmer. (Classically) you will *never* see something fall into a black hole. It will just get redder and redder until you can only detect it in microwave, then radio, and then not at all -- and at the point it seemed to vanish it will still not appear to be on the horizon.

From the point of view of the thing falling in, on a classical level nothing untoward happens at all, which is readily apparent if you swap from the rather misleading Schwarzschild coordinates to the Painleve-Gullstrand coordinates, where spacetime at the event horizon is locally flat. The argument Hawking has attempted to address in what the media laughably refer to as a "paper" -- a ridiculous statement Hawking has himself not made for what is nothing more than a two-page collection of unsubstantiated speculations and arguments -- is related to whether this picture is still true when one considers quantum effects.

## Re:I always thought... (Score:3, Interesting)

(I do have a doctorate in cosmology and I've a contention with what you've said: a black hole is not a singularity, whether by definition or otherwise. A "black hole" is simply a region in vacuum shrouded by an event horizon, and this situation occurs when a body is compressed enough that it lies entirely within its event horizon. In classical GR there are a few ways to get to this situation, with perhaps the most common being the collapse of a supermassive star. In classical GR there is also a singularity at the centre of the black hole, but a quantum theory of gravity would be expected to smear this out. What this does not imply is that a quantum theory of gravity would destroy the concept of a black hole entirely -- instead it seems very likely that in a quantum theory of gravity we would retain an event horizon, merely a somewhat "smeared" and non-absolute form of one (a distinction that would seem heartlessly academic to any poor sod falling into a hole). Hawking's conjecture, which is eerily similar to an equally unproven conjecture he advanced a few years back to "prove" that the information paradox was solved, is that ultimately there are no "black holes" because they are not an infinite state -- eventually they will dissipate, which immediately implies that their "event horizons" are actually apparent horizons. So far as this goes, it strikes me as eminently non-controversial.

Anyway, the concept of a singularity and a black hole are therefore rather distinct.)