Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Ashtekar's big one restructures Black Hole study

  1. Jul 15, 2004 #1

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Stingray mentioned this in an earlier post---he is at Penn State where Ashtekar is and goes to talks. Ashtekar has put the study of black holes on a new track, using a new kind of system of horizons.

    Instead of the old notion of Event Horizon which is awkward to use because requires looking infinitely far into the future-----that is, the EH is not operationally defined----instead of EH he and his co-workers say use
    dynamical and isolated horizons.

    I've seen smaller papers by Ashtekar about this (e.g. "How do black holes grow?") over the past two or three years. Now the big paper is out
    ----exerpt from http://arxiv.org/gr-qc/0407042 ------

    " Global nature of event horizons.

    An event horizon is defined as the future boundary of the causal past of future null infinity. While this definition neatly encodes the idea that an outside observer can not ‘look in to’ a black hole, it is too global for many applications. First, since it refers to null infinity, it can not be used in spatially compact space-times. Surely, one should be able to analyze black hole dynamics also in these space-times. More importantly, the notion is teleological; it lets us speak of a black hole only after we have constructed the entire space-time. Thus, for example, an event horizon may well be developing in the room you are now sitting in anticipation of a gravitational collapse that may occur in this region of our galaxy a million years from now. When astrophysicists say that they have discovered a black hole in the center of our galaxy, they are referring to something much more concrete and quasi-local than an event horizon. Is there a satisfactory notion that captures what they are referring to? ..."

    and indeed Ashtekar and Badri Krishnan have a satisfactory answer to that question
     
    Last edited: Jul 15, 2004
  2. jcsd
  3. Jul 15, 2004 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    My guess is that the mechanics and thermodynamics of black holes will get recalculated using the new horizons.

    that is, this paper changes the rules of the game

    interesting linguistic angle: They refer to classical General Relativity as
    "exact" General Relativity

    so "exact" is in contrast to quantum, and is precursor to quantum.
    oftentimes people say "classical" to mean that

    Here is some information on the paper:
    gr-qc/0407042
    Isolated and dynamical horizons and their applications
    Abhay Ashtekar, Badri Krishnan
    77 pages, 12 figures

    Here's some of the abstract:

    "... Over the last five years a new, quasi-local framework was introduced to analyze diverse facets of black holes in an unified manner. In this framework, evolving black holes are modeled by dynamical horizons and black holes in equilibrium by isolated horizons.


    ... it has introduced a more physical setting for black hole thermodynamics and for black hole entropy calculations in quantum gravity; suggested a phenomenological model for hairy black holes; provided novel techniques to extract physics from numerical simulations; and led to new laws governing the dynamics of black holes in exact general relativity."
     
  4. Jul 15, 2004 #3

    jeff

    User Avatar
    Science Advisor

    Not true. By "exact GR" I believe is meant GR as given by the hilbert action. By using this term, ashtekar is indicating his agreement with the consensus that we can't assume that hilbert isn't just a low energy effective theory with the full theory including higher order corrections. As a consequence of this view, because lqg needs to assume that hilbert receives no corrections, we must conclude that lqg is wrong, as all serious people know.
     
    Last edited: Jul 15, 2004
  5. Jul 15, 2004 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    You could be right about the terminology :smile:

    I was talking about how it is used in the last sentence of the abstract

    "...physics from numerical simulations; and led to new laws governing the dynamics of black holes in exact general relativity."

    It is clear he means non-quantum GR but he may also mean, as you suggest, non-quantum GR using the precise Hilbert action and no corrections.

    I will find out better what is meant as i peruse the paper.

    BTW my spies tell me that you are studying at Toronto---if this is true then I wish you good luck with your studies.

    -----------------
    the other stuff in your post, about "all serious people" thinking LQG is wrong,
    well...
     
  6. Jul 15, 2004 #5
    :uhh: :zzz:

    whovever these "serious people" are- we at least know they AREN'T physicists [or at least physicists that are acredited and known]- as quantum geometry is exploding everywhere and even injecting new life into strings-


    ___________________________

    /:set\AI transmedia laboratories

    http://setai-transmedia.com
     
  7. Jul 15, 2004 #6

    jeff

    User Avatar
    Science Advisor

    Respectfully setAI, these remarks are utterly wrong.
     
  8. Jul 15, 2004 #7

    jeff

    User Avatar
    Science Advisor

    But maybe I'm not.:smile:


    Good. If I'm wrong let me know and I'll edit your correction into my post


    Well, if the sentiment is genuine, thanks.


    I don't think so, but I admit I didn't have to say it like that.
     
    Last edited: Jul 15, 2004
  9. Jul 15, 2004 #8

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Here is the relevant quote from the abstract:
    We see that he is contrasting quantum gravity with "exact general relativity" as two theories that are helped by the new definition. Your interpretation requires him to be such a dope that he contradicts himself without even realizing it. A more conservative idea is that by exact he means classical.
     
  10. Jul 15, 2004 #9

    jeff

    User Avatar
    Science Advisor

    Well, I did say that I could be wrong. But after taking a look at the paper, I still think what he meant was what I said: GR based on the hilbert action.

    There are six other passages in which the phrase "exact general relativity" occurs. The first one is:

    "For fully dynamical black holes, there has essentially been only one major result in exact general relativity. This is the celebrated area theorem proved by Hawking in the early seventies [93, 95]: if matter satisfies the dominant energy condition, the area of the black hole event horizon can never decrease. This theorem has been extremely influential because of its similarity with the second law of thermodynamics. However, it is a qualitative result; it does not provide an explicit formula for the amount by which the area increases in physical processes.
     
  11. Jul 15, 2004 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I think "classical" is probably the safest bet about the meaning, from context.
    selfAdjoint here is a sign that PF is working nicely: here is Stingray's post #5 in wolram's thread called "naked singularity" in the Relativity part of Astronomy Forum.

    He essentially gave us background on the Ashtekar/Krishnam paper a few days before it appeared.
    I caught it and posted the link as soon as it appeared. here is what Stingray said---because it gives knowledgeable perspective on the paper:

    -----exerpt from Stingray post---

    "... Penrose called this hypothesis cosmic censorship. Nobody has proven it to be correct, and some people think it has been proven wrong. It depends on what you consider a realistic situation.

    There is a joke that astrophysicists, mathematical physicists, string theorists, and numerical relativists all mean different things when they say 'black hole.'

    As Marcus implied, the astrophysicists' version of a black hole is an object so dense that it doesn't seem like it can be anything else. There is no measurement of an event horizon, and in a sense, there never can be. It would require knowing the future.

    For this reason and others, the more mathematically minded relativists have tried to remove the concept of event horizon from black hole definitions. The results are Ashtekar, Krishnan, et al's isolated and dynamical horizon frameworks. These give measurable ways of determining whether you're at a (dynamical or isolated) horizon. This is not the same as an event horizon, but is actually much more useful...."
    ----end quote---

    https://www.physicsforums.com/showthread.php?p=252425#post252425

    Lots going on these days :smile:
     
    Last edited: Jul 15, 2004
  12. Jul 15, 2004 #11

    jeff

    User Avatar
    Science Advisor

    For me it seems that way. Athough there's yet no reason to believe that the whole de sitter space problem in strings is fatal, it's definitely making me a bit nervous, and I'm definitely going to look at that meissner paper: I'm starting to experience those old "quasinormal" feelings again. :smile:

    It's important to note that if lqg is correct, holography is wrong. For me, this is the proverbial ball you should keep your eye on.
     
  13. Jul 15, 2004 #12

    Stingray

    User Avatar
    Science Advisor

    Actually, you're both wrong about what's meant by "exact." The results he's referring to are "exact" in the sense that they involve no approximations within the boundaries of classical GR. He's contrasting with the vast majority of results in (classical) GR which come from perturbation theory.
     
  14. Jul 15, 2004 #13

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    even better! thanks for the clarification Stingray.
     
  15. Jul 20, 2004 #14

    jeff

    User Avatar
    Science Advisor

    I emailed ashtekar and his reply makes it clear that stingray is right:

    "Exact" means full, non-linear GR, without approximations. Much of the
    available literature assumes weak field perturbations around a Kerr black
    hole. These analyses do not capture the important strong field, fully
    non-linear effects of general relativity.

    ...Abhay Ashtekar
     
  16. Jul 20, 2004 #15

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    splendid, thanks to jeff and Ashtekar for the clarification!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Ashtekar's big one restructures Black Hole study
  1. Black Holes (Replies: 1)

Loading...