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BMS symmetries and black hole horizons

  1. Aug 28, 2015 #1

    Physics Monkey

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    I would like to discuss a bit this paper (http://arxiv.org/abs/1508.06577):

    BMS invariance and the membrane paradigm
    Robert F. Penna
    (Submitted on 26 Aug 2015)
    We reinterpret the BMS invariance of gravitational scattering using the membrane paradigm. BMS symmetries imply an infinite number of conserved quantities. Energy conservation at every angle is equivalent to the fluid energy equation on the membrane (a conservation law at each point in the fluid). Momentum conservation at every angle is equivalent to the Damour-Navier-Stokes equation on the membrane. Soft gravitons are encoded in the membrane's mass-energy density, Σ(z,z¯). Fluid dynamics is governed by infinite dimensional reparametrization invariance, which corresponds to the group of volume preserving diffeomorphisms. This coincides with the generalized BMS group, so there is a connection between the fluid and gravity pictures at the level of symmetries. The existence of membrane fluid conservation laws at event horizons implies BMS symmetries also act on event horizons. This may be relevant for the information problem because it implies infalling information can be stored in Σ(z,z¯) at the horizon. The teleological nature of the membrane at the horizon may be related to the black hole final state proposal.

    I think it is related to the recent idea sketch reported by Hawking-Perry-Strominger.

    To me it appears that the paper has two parts. First, a technical discussion of BMS-like symmetries on black hole horizons in flat space. Second, a brief speculation about how the technical part might relate to the black hole information paradox. My impression is that the HPS idea is in roughly the same state, but we haven't really seen a proper presentation of the HPS idea so I'll focus on Penna's work.

    Some questions I would like to understand:

    1. Since Penna is proposing that there is a conservation law for every angle, it seems to me that any information stored in this BMS-like data would not scramble much or at all (see e.g. http://arxiv.org/abs/0708.4025 and http://arxiv.org/abs/0808.2096). Is this right?

    2. It doesn't seem quantum enough. How can the quantum state of an infalling qubit be encoded in the classical BMS data? There also doesn't seem to be enough of it.

    3. In AdS things should work slightly differently. Penna observes that the fluid on the horizon doesn't conduct heat which is why one has an infinite number of conservation laws. But in AdS/CFT the dual field theory hydrodynamics does conduct heat, so are these extra symmetries not present in that case? Is the problem the cosmological constant?
  2. jcsd
  3. Aug 28, 2015 #2


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    Not to distract, just a little background. Hossenfelder "tweeted" about this paper yesterday (2AM pacific time 27 August):
    ==quote https://twitter.com/skdh/status/636826091823472640 ==
    https://pbs.twimg.com/profile_images/378800000054721985/d5389553dbb2514fbbd282fd373e1429_bigger.jpeg [Broken]Sabine Hossenfelder‏@skdh
    Academic twist: New paper with an argument very similar to Hawking's *unpublished* proposal http://arxiv.org/abs/1508.06577
    I think the "academic twist" is that by traditional standards Penna has priority on the idea because he published first, on arXiv, and the proposal by the other three (H,M,and S) has only been talked about in vague terms at a hastily convened conference, not published. If Hossenfelder is right, I can see that this would constitute an academic twist to the story.

    Laura Mersini-Houghton, who organized the Stockholm gathering, was quoted as saying that when she called Hawking with the idea of a conference he immediately took her up on it and set the dates himself.
    Last edited by a moderator: May 7, 2017
  4. Aug 28, 2015 #3


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    Reference [12] of the paper has the same idea, but less detail.
  5. Aug 28, 2015 #4

    Physics Monkey

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    It is definitely closely related but I think Penna does have a new twist in that he considers hair at the horizon in addition to null infinty. SZ seem only to talk about hair at null infinity.

    EDIT: It's not really clear to me how these two sets of symmetries are related.
  6. Oct 8, 2015 #5
    I red this article today. In my opinion it is strictly related to the Idea of Hawking, Perry and Strominger.

    For the questions, imho
    3) AdS or other non asymptotically flat spacetime is entirely a new story. The BMS group is an asymptotic symmetry group only for asymptotically flat spacetimes, like Schwarzschild. I think in spaces asymptotically AdS or dS the asymptotic symmetry group is different. But I didn't find anything about that.

    2) I think this point is analogous to the 't Hooft S-matrix idea: The leading term is the classical BMS Horizon hair, then there are other terms also.

    1) I haven't read those papers yet, but only the shenker and stanford Idea http://arxiv.org/abs/1306.0622 but I don't know how to answer.

    What is your opinion?
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