Wednesday, November 21, 2012

Papers refuting black hole firewalls spread

Some research activity following the provoking black hole firewall claim by AMPS (Polchinski et al.) continues. The papers that say – or pretty much say – that black holes don't have any firewalls at/near the event horizon start do prevail.

It's interesting to mention that Samir Mathur's idea of fuzzballs used to look as an early predecessor of "firewalls". Many of us were slightly skeptical because it looked like Mathur was saying that the black hole interior never looks empty.

But he never did. While the interior has a complicated structure if you can measure all the degrees of freedom, macroscopic infalling observers will see the empty region that general relativity implies. Samir Mathur and David Turton made this point explicit in the August 2012 paper about fuzzball complementarity.

I have especially liked the October 2012 paper by Avery et al. that tried to check, present, and supplement this collection of ideas with some details. So the unlimited-resolution probes would see a non-empty black hole interior; but the macroscopic probes with typical wavelengths still much shorter than the black hole radius,\[

l_{\rm Planck} \ll \lambda_{\rm probe} \ll R_{\text{black hole}}

\] will see the empty black hole interior, in agreement with the equivalence principle in particular and general relativity in general. Only field modes with \[

\lambda_{\rm probe}\sim {\mathcal O}(R_{\text{black hole}})

\] (and be sure it makes no sense to talk about longer wavelengths than that which would be confined in the black hole) will see violations of the free fall. So the black holes have to reveal some nonlocal dynamics, to agree with all the conditions such as unitarity and the preservation of the information, but the only degrees of freedom that are "significantly" influenced by this nonlocality are the degrees of freedom whose wavelength is extremely long, comparable to the black hole radius.

What I love about the black hole fuzzball approach – yes, I have clearly become a fuzzball convert – is that it is much more positive, constructive, and specific. It may be viewed as a detailed extension of the well-known insights on the black hole entropy and black hole complementarity. It really tells you how the descriptions of the black holes look like. I am doing some research that has a similar spirit and could perhaps be formulated as research within the fuzzball paradigm. In some sense, I am amazed by the fact that people were able to find the infinite-parameter classes of fuzzball solutions. But this achievement became easier after the bubbling AdS space observation by LLM.

Today, there is a new hep-th paper on the arXiv that explicitly says that the firewall analysis by AMPS is flawed:
Black holes without firewalls
Klaus Larjo, David A. Lowe, and Larus Thorlacius say that the error of AMPS may be interpreted as a wrong exchange of the order of two limits. When AMPS exchange some limits, they erroneously conclude that the stretched horizon carries no degrees of freedom. In reality, there are degrees of freedom, they're able to retain the information for a time of order the black hole scrambling time, and they're able to restore the "smooth sensations" for the infalling observers.

Larjo et al. count Mathur et al. as "alternatives not only to firewalls but to the black hole complementarity", too. I don't think it's quite right. The fuzzball complementarity is a version of the black hole complementarity – it just specifies more accurately which probes will see that the principle is obeyed. In fact, the key role is played by the region between the event horizon and the stretched horizon in the Mathur-Turton paper as well.

So I would say that these folks don't seem to read papers by each other too much. Unfortunately. But despite the different language, their actual statements – their actual explanation why AMPS is wrong – are probably consistent with each other. They mostly differ in the amount of details that they try to determine about various related questions.

I view the black hole fuzzball proposal to a promising paradigm that may be answering absolutely all confusing issues related to the black hole information. It may be used to count the black hole entropy; one may probably derive why coarse-grained probes will see the empty interior; one should be even able to quantify the ways how the information propagates nonlocally (from the viewpoint of the effective, long-distance field modes) through the black hole interior; the proposal is free of any xeroxing and firewall paradoxes.

The precise links between the descriptions relevant for various observers as well as a more microscopic, stringy description of the processes involving the singularities is something I am trying to clarify, too (analytic continuations, averaging over many degrees of freedom, and other tools are rather important in this business), but I don't plan to publish anything because I am frustrated by the shortage of "genuine open-minded readers" of such papers. There are already some excellent papers in this business – I consider e.g. the recent paper by Avery et al. to be excellent – and they are earning almost no attention or citations as many people are dedicating their time and hype to some worthless and/or manifestly wrong research directions. These bright young folks are throwing pearls to the swines – it just sucks.


  1. Nice update on the firewall story :-)

    I like fuzzballs too, not least because the term "fuzzball" looks like a crazy "Neue Deutsche Rechtschreibung" version of the German word "Fussball" :-D

    Dear Lumo, by pointing their work out here on TRF, you maybe help your young bright colleagues getting the right attention they deserve ;-)

  2. You don't get it. Firewalls aren't about what freefalling observers notice subjectively.

    It's about what outside observers get as reports from freefalling observers just before they cross the horizon when measuring the near horizons modes at scales between the Planck scale and the radius R.

  3. No, this is not plausible. Firewalls, in the original AMPS paper, are about the behavior of the interior of "old" black holes, after the half-entropy-evaporated Page time. From the external observer's viewpoint, the observed physics is clearly indistinguishable from thermal radiation and this radiation can't be used to answer whether there is a firewall or not.

    Questions about the existence of firewalls make only sense when observations by observers inside the black hole are considered.