this black hole conference in Stockholm, and at his public lecture yesterday evening, Stephen Hawking announced that he has figured out how information escapes from black holes, and he will tell us today at the conference at 11am.
As your blogger at location I feel a certain duty to leak information ;)
Extrapolating from the previous paper and some rumors, it’s something with AdS/CFT and work with Andrew Strominger, so likely to have some strings attached.
30 minutes to 11, and the press has arrived. They're clustering in my back, so they're going to watch me type away, fun.
10 minutes to 11, some more information emerges. There's a third person involved in this work, besides Andrew Strominger also Malcom Perry who is sitting in the row in front of me. They started their collaboration at a workshop in Hereforshire Easter 2015.
10 past 11. The Awaited is late. We're told it will be another 10 minutes.
11 past 11. Here he comes.
He says that he has solved a problem that has bothered people since 40 years, and so on. He now understands that information is stored on the black hole horizon in form of "supertranslations," which were introduced in the mid 1960s by Bondi and Metzner. This makes much sense because Strominger has been onto this recently. It occurred to Hawking in April, when listening to a talk by Strominger, that black hole horizons also have supertranslations. The supertranslations are caused by the ingoing particles.
That's it. Time for questions. Rovelli asking: Do supertranslations change the quantum state?
Just for the record, I don't know anything about supertranslations, so don't ask.
It's taking a long time for Hawking to compose a reply. People start mumbling. Everybody trying to guess what he meant. I can see that you can use supertranslations to store information, but don't understand how the information from the initial matter gets moved into other degrees of freedom. The only way I can see how this works is that the information was there twice to begin with.
Oh, we're now seeing Hawking's desktop projected by beamer. He is patching together a reply to Rovelli. Everybody seems confused.
Malcom Perry mumbling he'll give a talk this afternoon and explain everything. Good.
Hawking is saying (typing) that the supertranslations are a hologram of the ingoing particles.
It's painful to watch actually, seeing that I'm easily typing two paragraphs in the time he needs for one word :(
Yes, I figure he is saying the information was there twice to begin with. It's stored on the horizon in form of supertranslations, which can make a tiny delay for the emission of Hawking particles. Which presumably can encode information in the radiation.
Paul Davies asking if the argument goes through for de Sitter space or only asymptotically flat space. Hawking saying it applies to black holes in any background.
Somebody else asks if quantum fluctuations of the background will be relevant. 't Hooft answering with yes, but they have no microphone, I can't understand them very well.
I'm being told there will be an arxiv paper some time end of September probably.
Ok, so Hawking is saying in reply to Rovelli that it's an effect caused by the classical gravitational field. Now I am confused because the gravitational field doesn't uniquely encode quantum states. It's something I myself have tried to use before. The gravitational field of the ingoing particles does always affect the outgoing radation, in principle. The effect is exceedingly weak of course, but it's there. If the classical gravitational field of the ingoing particles could encode all the information about the ingoing radiation then this alone would do away with the information loss problem. But it doesn't work.You can have two bosons of energy E on top of each other and arrange it so they have the same classical gravitational field as one of twice this energy.
Rovelli nodding to my question (I think he meant the same thing). 't Hooft saying in reply that not all field configurations would be allowed. Somebody else saying there are no states that cannot be distinguished by their metric. This doesn't make sense to me because then the information was always present twice, already classically and then what would one need the supertranslations for?
Ok, so, end of discussion session, lunch break. We'll all await Malcom Perry's talk this afternoon.
Update: After Malcom Perry's talk, some more details have emerged. Yes, it is a purely classical picture, at least for now. The BMS group essentially provides classical black hole hair in form of an infinite amount of charges. Of course you don't really want an infinite amount, you want a finite amount that fits the Bekenstein-Hawking entropy. One would expect that this necessitates a quantized version (at least geometrically quantized, or with a finite phase-space volume). But there isn't one so far.
Neither is there, at this point, a clear picture for how the information gets into the outgoing radiation. I am somewhat concerned actually that once one looks at the quantum picture, the BMS charges at infinity will be entangled with charges falling into the black hole, thus essentially reinventing the black hole information problem.
Finally, to add some context to 't Hooft's remark, Perry said that since this doesn't work for all types of charges, not all models for particle content would be allowed, as for example information about baryon number couldn't be saved this way. He also said that you wouldn't have this problem in string theory, but I didn't really understand why.
Another Update: Here is a summary from Jacob Aron at New Scientist.
Another Update: A video of Hawking's talk is now available here.
Yet another update: Malcom Perry will give a second, longer, lecture on the topic tomorrow morning, which will be recorded and be made available on the Nordita website.