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Black holes and quantum gravity

  1. Aug 1, 2006 #1


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    http://xxx.unizar.es/PS_cache/gr-qc/pdf/0607/0607130.pdf [Broken]

    Title: Quantum Geometry and its Implications for Black Holes
    Authors: Martin Bojowald
    Comments: 16 pages, Plenary talk at ``Einstein's Legacy in the New Millenium,'' Puri, India, December 2005

    General relativity successfully describes space-times at scales that we can observe and probe today, but it cannot be complete as a consequence of singularity theorems. For a long time there have been indications that quantum gravity will provide a more complete, non-singular extension which, however, was difficult to verify in the absence of a quantum theory of gravity. By now there are several candidates which show essential hints as to what a quantum theory of gravity may look like. In particular, loop quantum gravity is a non-perturbative formulation which is background independent, two properties which are essential close to a classical singularity with strong fields and a degenerate metric. In cosmological and black hole settings one can indeed see explicitly how classical singularities are removed by quantum geometry: there is a well-defined evolution all the way down to, and across, the smallest scales. As for black holes, their horizon dynamics can be studied showing characteristic modifications to the classical behavior. Conceptual and physical issues can also be addressed in this context, providing lessons for quantum gravity in general. Here, we conclude with some comments on the uniqueness issue often linked to quantum gravity in some form or another.

    I could not see a mention on this in the forum.
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  3. Aug 2, 2006 #2


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    Hi wolram, I'm glad you called attention to this one. Actually it was noted a couple of days ago in post #505 of the bibliography thread, but that is more of just a store-room---no place for discussion.

    To me, this paper has the look of a survey. I didnt see anything especially new---he was just summarizing the current research picture for the people at the conference. But when I get that impression it is often wrong and it often pays to go back and take a closer look.

    I will do that, and see what, if any, new details come to light.
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  4. Aug 2, 2006 #3


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    I still find it difficult to understand how gravity reverses sign at small scalles,
    i am sure the maths works, but inutitivly it seems wrong, but then often times the things i think should work do not.
  5. Aug 2, 2006 #4


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    instead of "at small scales" I think of it as happening "at very high density" which is related but different. I mean it happens when there is very much matter or energy concentrated on a "per litre" basis.

    I am not trying to explain it, just describe. Can't explain. Can only go over the story and paraphrase it and imagine it.

    It is a HIGH COMPRESSION effect

    you take a cylinder like of a motor, say it is a one litre cylinder
    and you put some matter in (of whatever kind, iron, water)
    and then you begin to compress (you are impossibly strong, so you can do it no matter what)

    various forces fight against you (the atoms want to remain atoms and not have their electrons shoved down into their nuclei)

    some important forces resist compression but so far at least gravity is always helping (because it pulls things together)

    and after a while you compress it so much that all that is left is "neutron matter" in the cylinder---and gravity is still your friend and helping you by pulling it together---and you keep on jamming down the piston and compressing more and more

    until you reach a compression where the density is so high that EVEN GRAVITY TURNS AGAINST YOU and fights back and tries to keep you from compressing further.

    I don't understand this. You say you don't understand it. It apparently was a surprise to the QG people when they saw this coming out of their equation.
    It apparently is just something that occurs in the equation for quantum gravity. There is a critical density. It is calculated to be 80 percent of the usual Planck density (one planck mass per planck volume)

    at this density the quantum equation that Bojowald and Ashtekar and the others are using to model gravity shows that instead of the usual attraction effect there is a repulsion.

    maybe their equation is wrong, maybe it is right. still have to test it.

    BTW thanks for flagging this Bojowald article. I started reading it last night and found it is more interesting than I thought when I first took a look a couple of days ago.
    Last edited: Aug 2, 2006
  6. Aug 2, 2006 #5


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    It's interesting that B.F.L. Ward, in his graviton paper hep-ph/0607198, also finds repulsive gravity at short ranges; in his model it appears as the self-energy of the graviton. He atrributes to it both the finiteness of his own results and the asymptotic safety of GR. In fact he all but announces future papers on this subject.
  7. Aug 2, 2006 #6


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    Please exuse an ill put question , does a graviton (see) or sence mass ,
    if a mass is transiting space ,are the gravitons switching on and off in the path of the mass?
  8. Aug 2, 2006 #7
    Well, i think you are missin some points here. Well, i thionk everybody is doing in fact (so possibly it is me who is doin a mistake).

    First, the notiion of gravity as an exclusively atractive force rise an important question in the very moment that we live in an expanding universe wich is described by the Einstein equations.

    If you go to the calculations you see that these character is intrnisicc aand not related to the matter content. Ieman, you get an expandin universe even if you have in the energy-momentun source a perfect fluid composed of non interacting (and so no repulsive) matter.

    How is these possible?

    to try to understand i return to the newtonian viewpoint.I begin with a shell of dust of particles with an initial velocity pointing outward radially. Independently of the initial speed the shell goes under a expansion phase, later depending on the speed it recolapse or it doesn´t.Trivial, of course. What does this has to dod with repulsive gravity you may answer.

    Well, what keeps particles going outwark if the only force acting is gravity wich is atractive? Easy, it is the kinetical energy. And kinetical energy depends on mass, to be exact, in inertial mass. The atractive character of gravity is related to , well gravitational mass.

    But one of the foundations of RG is the equality of inertial and "gravitatory" mass. So somewhere inside the einstein equations there must be something wich support the inertial aspect of matter wich allows particles in the imaginary shell experiment i described before to expand.

    And it would be these inertial aspect of gravity wich would allow, in some ocasions, to describe repulsive behaviours.

    But, yes, i am conscient that i am in classical relativity and that these repulsive effects seems to be described as classical effects related to quantum fluctuations (from the viewpoint of the bojowald black-hole midisuperspace models) or to large "euclidean momentum". Inthe last case we are talking about "virtual" gravitons wich are of-sehll (in the sense of quantum field theory) so we would be in non classical scenaries and i amtriying to relate a repulsive behaviour to classical relativity. Soonce againi amposibly missing some important points. But well, may be you could say me what is bad in my argument.
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