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Black Hole Firewalls

  1. Jul 19, 2012 #1
  2. jcsd
  3. Jul 19, 2012 #2


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    Interesting stuff!

    I've only skimmed the two papers, and lack the background needed in order to understand them deeply (even though the authors have made admirable efforts to make their writing accessible to a non-specialist).

    My gut reaction is to question whether there is, even in principle, any way to test the existence of a firewall against experiment, if it exists only behind an event horizon. If not, then this is really philosophy and not science. Maybe it's testable if you have access to a black hole that you are able to observe through the process of evaporation...? On the other hand, it would be interesting if there was theoretical evidence that quantum gravity effects can manifest themselves at scales far bigger than the Planck scale, because that would suggest that q.g. could be testable with foreseeable technology.
  4. Jul 19, 2012 #3


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    both LQG and string theory (fuzzball proposal) suggest that QG effects are relevant not at Planck scale but at horizon scale!
  5. Jul 20, 2012 #4

    Physics Monkey

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    This is certainly a provocative set of thoughts, but I'm skeptical of their interpretation.

    One thing I was thinking about was the eternal ads black where I don't see any sign of a firewall. Granted the physical setup is a bit different, but its not clear how essential that is for their argument.
  6. Jul 28, 2012 #5
    Actually I prefer Bousso's statement that the cloning paradox and entanglement paradox don't exist at all(http://arxiv.org/abs/1207.5192). No observer can see both of the qubits, so it does not contradict no cloning principle. This is the 'observer complementarity'.
  7. Jul 30, 2012 #6
    Bousso gave a talk at Strings 2012 on this topic, the slides are available on the conference website [direct link here]. The video is also available.
  8. Jul 30, 2012 #7
    I think that Bousso's response to these papers helps clarify the beautiful subtleties of BHC.

    In particular that inconsistencies between what two observers see but cannot communicate with each other are not physically inconsistent since they have no operational meaning. Instead they are complementary.
  9. Jul 31, 2012 #8


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    In quantum mechanics, complementarity can be expressed in a clear and simple mathematical way, as the fact that the same state can be expanded in different bases, corresponding to eigenstates of different observables. The nice thing about this is that the state itself, viewed as an object which does not depend on the choice of basis, does NOT DEPEND ON THE OBSERVER.

    However, as far as I know, nothing similar exists for black hole complementarity. As far as this is the case, I cannot take black hole complementarity seriously.
  10. Jul 31, 2012 #9
    I share your reservations. BHC is not standard QM. But I think what QM taught us is that nature can be very weird to the point that there almost seems to be a contradiction (light is both a wave and particle). But it was found that in-fact it can be this way without leading to any paradoxes.

    BHC is inspired by complementarity(in QM) but should not be confused with it. The point of BHC is to remove the paradoxes of black hole physics. To do this it makes the rather drastic step of saying that the states become observer dependent with huge inconsistencies between the two observers. The crux being that aslong as these two observers cannot communicate then there is no paradox. At leasts this is my understanding.

    Not sure I buy it. But I remain open minded.
  11. Aug 1, 2012 #10


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    I think it is extremely important in physics to have a description which does NOT depend on the observer. I have already explained what it is in QM. In classical GR we also have it - a covariant and even a coordinate-independent formalism. In particular, in classical GR the universe behind the horizon exists irrespective of the observer.

    Otherwise, it seems to me that ANY PARADOX in any inconsistent theory can be "resolved" by introducing a new kind of "complementarity". For example, if some inconsistent theory predicts that x=1 AND (not OR) x=2, you can always say it is consistent because it corresponds to two complementary observers who use the theory in two different ways.
  12. Aug 4, 2012 #11
    To Demystifier:

    A theory of physics, with complementarity, should not allow inconsistencies for *each* observer i.e. if one is making a measurement in a specific reference frame, then there should be no inconsistencies in the laws of physics. However this does not mean that the same measurements, performed in a different reference frame, will yield the exact same results as those in the original frame. This is one of the principles of complementarity.

    Furthermore, one should not be so hesitant in discussing observer dependent theories. For example, in quantum gravity it is impossible to define a preferred vacuum state and therefore different observers will see drastically different physics (see Unruh radiation).
  13. Aug 4, 2012 #12


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    But there is a possibility to construct a map between the different vacuum states via a Bogoliubov transformation. So whenever I can introduce a class of states with an explicit map I wouldn't call that theory observer-dependent (of course the theory allowes to extract observer dependent physics)
  14. Aug 4, 2012 #13
    Yes it is true that there is a mapping via Bogoliubov transformations between two different observer frames. But this is also the case for BHC. Any information carried into the black hole in a falling frame should in principle be mapped to states in the Hawking cloud.

    I guess we should really say that interpretation of the physics is observer dependent. This is of course true in BHC and in the non-unique vacua of quantum gravity.
  15. Aug 4, 2012 #14


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    Sorry to say that but if that is all what there is to say then it's rather trivial!

    We know this since Galilei invariance of Newtonian physics. I mean a Bogoljubov transformation between vacuum states of QFT on curved background is more complicated that just x' = x-vt, but conceptionally is identical. You have the same underlying physics, but observer dependend observations.

    Do I miss something?


    There is a interpretation problem regarding the "reality" of the observed particles.
    Last edited: Aug 4, 2012
  16. Aug 4, 2012 #15
    In some sense BHC is a statement that there is only observer dependent observations. What makes BHC interesting is that one cannot do quantum mechanics in a *global* space-time picture. That is to say one cannot count independent degrees of freedom (DOF) throughout all of space-time.

    This is different from standard field theory where one assigns to each point a DOF everywhere in space.
  17. Aug 5, 2012 #16
    Isn't a Bogoljubov transformation conceptually the same as a transformation between non-inertial observers rather than intertial ones?

    But I agree with Jarod I think BHC is conceptually different from this.
  18. Aug 5, 2012 #17


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    with x' = x-vt I didn't want to say that the Bogoljubav trf. applies to inertial frames. I only wanted to claim that observations are frame dependent and that the Unruh effect is nothing else but an effect due to this frame-dependence. The big difference is that it acts on the Fock space and "creates particle states from the vacuum".

    But as said it's slightly more complicated than that: there is an interpretation problem regarding the "reality" of the observed particles i.e. regarding a real event"; and there seems to be a lack of "global definition" of states or d.o.f.
    Last edited: Aug 5, 2012
  19. Aug 5, 2012 #18
    Yes but if the observer measures anything but the Minkowski vacuum this implies that there is some force (be it gravity or any other force) acting on them right? This is rather different from an inertial transformation. Also to measure the Unruh effect each observer must detect particles. So each observer is conducting an experiment. But these experiments are different! So i am not so sure that it's simply frame dependence.
  20. Aug 5, 2012 #19


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  21. Aug 6, 2012 #20


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    In the case of Unruh effect, all observers agree that the state is |0_Minkowski>. In Susskind black hole complementarity there is no such a universal object on which all observers agree.
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