Black holes and firewalls

Astrophysics: Fire in the hole! : Nature News

Here's a lengthy and interesting article about some relatively new ideas and problems with the physics of black holes.

It's pretty remarkable that working out the physics on the boundary of a black hole is still the subject of such controversy, so many decades after physicists started to think about it.  Still, I guess it's not as if they have one nearby to actually toy with.  In fact, now that I think about it, how close is the nearest likely black hole to Earth?  Seems it might be 1,500 light years - not so far if you're considering the galaxy as a whole.  From a 2012 article:

This beautiful photo from the Hubble Legacy Archive offers a striking look at the Trapezium, four closely packed stars found inside the Orion nebula, some 1,500 light-years away. Lurking inside that image might be our nearest black hole neighbor.

The question of which black hole is the closest to Earth is surprisingly tricky to answer. V4641 Sgr might be just 1,600 light-years away, or it might equally possibly be more like 24,000 light-years away. We've got a better sense of the location of V404 Cygni, which is just 7,800 light-years away. Considering we're a little under 30,000 light-years from the supermassive black hole at the center of the Milky Way, those black holes are certainly in the cosmic vicinity, but they're not exactly super close.

That's why the Trapezium is so intriguing. Something about the stars' movements just isn't right, and the most likely explanation is a hidden black hole.

 Anyway, back to the Nature article.  It's mainly about the idea that the boundary of a black hole might, or might not, have a particularly dangerous aspect to it:

Hawking had shown that the quantum state of any one particle escaping from the black hole is random, so the particle cannot be carrying any useful information. But in the mid-1990s, Susskind and others realized that information could be encoded in the quantum state of the radiation as a whole if the particles could somehow have their states ‘entangled’ — intertwined in such a way that measurements carried out on one will immediately influence its partner, no matter how far apart they are.

But how could that be, wondered the Polchinski’s team? For a particle to be emitted at all, it has to be entangled with the twin that is sacrificed to the black hole. And if Susskind and others were right, it also had to be entangled with all the Hawking radiation emitted before it. Yet a rigorous result of quantum mechanics dubbed ‘the monogamy of entanglement’ says that one quantum system cannot be fully entangled with two independent systems at once.

To escape this paradox, Polchinski and his co-workers realized, one of the entanglement relationships had to be severed. Reluctant to abandon the one required to encode information in the Hawking radiation, they decided to snip the link binding an escaping Hawking particle to its infalling twin. But there was a cost. “It’s a violent process, like breaking the bonds of a molecule, and it releases energy,” says Polchinski. The energy generated by severing lots of twins would be enormous. “The event horizon would literally be a ring of fire that burns anyone falling through,” he says. And that, in turn, violates the equivalence principle and its assertion that free-fall should feel the same as floating in empty space — impossible when the former ends in incineration. So they posted a paper on the preprint server, arXiv, presenting physicists with a stark choice: either accept that firewalls exist and that general relativity breaks down, or accept that information is lost in black holes and quantum mechanics is wrong¹. “For us, firewalls seem like the least crazy option, given that choice,” says Marolf.

 There's lots more, and it is well worth reading.