Of Leaks and Gaskets
Though largely indifferent to actual science the public is obsessed with scientists; and while textbooks may warn against judging scientific truth from "authority" the public in fact does exactly that. Truth comes from the biggest name. Once a famous scientist pronounces on a subject then it must be true. So when Stephen Hawking recently declared that Black Holes didn't exist after decades of being famous for them the public did the natural thing. It reversed its beliefs. OK. Black Holes don't exist.
But as Brian Koberlin notes in the Universe Today that's an oversimplification at best. Hawking has acknowledged that Black Holes have 'leaked' for a long time, meaning they aren't really "Black". The 'leak' was required to reconcile two conflict mandates, one from quantum mechanics and the other from classical relativity.
On the one hand, classical relatively predicted that an inescapable Black Hole which possessed a gravitational field so large that nothing could escape from it. But on the other hand quantum mechanics decreed that information could never be destroyed. If Black Holes existed then whatever fell into them was lost forever and information was destroyed. This was called the Black Hole Information Paradox which was nothing less than a direct clash between the fundamental tenets of two foundational theories in the world of physics. So some method of escape had to be invented which observed the rules of each in order for them to both be valid.
The resolution to the problem was to posit a slow leakage due to quantum physics in which one entangled particle popped up on one side of the event horizon and its companion on the other. That allowed both relativity and quantum mechanics to call it a draw. But this notion created two problems in return. The proposed escape mechanism required a gasket-like firewall.
it would create a firewall of high energy particles near the surface of the event horizon. This is often called the firewall paradox because according to general relativity if you happen to be near the event horizon of a black hole you shouldn’t notice anything unusual. The fundamental idea of general relativity (the principle of equivalence) requires that if you are freely falling toward near the event horizon there shouldn’t be a raging firewall of high energy particles. In his paper, Hawking proposed a solution to this paradox by proposing that black holes don’t have event horizons. Instead they have apparent horizons that don’t require a firewall to obey thermodynamics. Hence the declaration of “no more black holes” in the popular press.
But that only created worse problems, for relativity decreed there could no observable Hollywood-style firewall. Nor did it propitiate the quantum gods. The new slowly evaporating Black Hole still destroyed information. Matt Ford writing for Ars Technica put it this way.
The black hole information paradox began life, so to speak, in 1975, when Stephen Hawking and Jacob Bekenstein proved that black holes were not really black, but that they emitted thermal radiation and eventually evaporated. This leads to a problem; other cosmological tenets (the no-hair theorem) suggest that the Hawking radiation that leaves a black hole should be independent of the material that goes in. This is problematic, because if one could have an initial quantum state where everything is known with exact certainty and send it into a black hole, then as the black hole evaporates and evolves, the final state of the system cannot be predicted. In this case, the best result is that a probable outcome can be computed. Here information has been lost: you knew exactly what went in, but you don't know exactly what may come out—quantum mechanics has been violated.
Physicist Sabine Hossenfelder actually claims that Hawking has said nothing new so far. That is to say the problem and paradox remains unsolved. Hossenfelder says: "The actual quote is: 'The absence of event horizons mean that there are no black holes – in the sense of regimes from which light can’t escape to infinity.'" which only means she says that the gasket doesn't last forever.
What Hawking is saying is essentially that he believes that a matter collapse only leads to a temporary apparent horizon but not to an eternal event horizon. That is an opinion which is shared by many of his colleagues (including me) and there is nothing new about this idea whatsoever....
Having said that, Hawking’s “paper” is really just a writeup of a talk he gave last year. It’s mostly a summary of his thoughts on the black hole firewall, none of which I found very exciting or remarkable. Had this paper been posted by anybody else, nobody would have paid attention to it.
In summary, nothing has changed in our understanding of black holes due to Hawking’s paper. Move on, there’s nothing to see here.
And we have a headline without news. We still don't know how reality does what it does, only that it does. And though we see stuff happening we can't figure out how it works.
That doesn't mean we never will. Ethan Siegel notes that there are mathematical possibilities (some developed by Sabine Hossenfelder) which allow the problem of the firewall to be sidestepped and which taken together suggest "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 [entangled in a particular way], is the one that’s flawed."
Pardoxes hold a special place in human knowledge. They are markers not just for the things we do not know but for problems we do not know how to think about. As BG Sidharth once observed, "the greatest breakthroughs in our concepts of physics, and science in general, have been counterintuitive." Progress occurs when we learn how to think about a problem. And in that respect it is the unsolved problem, the outlier, rather than in the well trodden mean where new knowledge is to be found.
In this age of "settled science" and "scientific consensus" is often important to remember that in the end there's the Truth and there's us. And we learn from Truth -- and not from men. Then for better or for worse, however crazy it may seem, the Truth shall make us free.
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