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QG status paper-explication only thread

  1. Apr 18, 2006 #1

    marcus

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    QG status paper---explication only thread

    If you wish to critique or challenge the Rovelli paper about current QG status, please start your own separate thread to do that.

    It is a short paper with a lot in it and it need first of all to be EXPLICATED which means paraphrased and unpacked--the way you do a suitcase---to see what is in it.

    French kids learn this in school called "explication de texte" and you should really do it with everything before you start criticising and arguing---make a detailed paraphrase of the main points and assemble some references to other work.

    So this is a FRIENDLY INTERPRETATION and paraphrase thread. And if you have critique/challenge type stuff that is material for another thread---please start one, as controversy will surely prove useful.
    ==============

    OK, so I will start quietly unpacking the paper, which is

    Unfinished Revolution
    Carlo Rovelli
    http://arxiv.org/gr-qc/0604045
     
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  3. Apr 18, 2006 #2

    marcus

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    I am not going to go top down (most important ideas first) or in any particular order.

    the first thing that comes to mind is the footnote on page 1
    and a relevant paper is
    http://arxiv.org/abs/astro-ph/0602266
    MOND habitats within the solar system
    Jacob Bekenstein, Joao Magueijo

    and also the Perimeter talk online video
    by Joao about the same thing

    busy now, I will explain as time permits
     
  4. Apr 18, 2006 #3

    marcus

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    another point in the paper is that GR has been something of a sleeper untll 1980 or so. this helps to understand the history of QG.

    For the first 70 years or so GR was not taken as seriously as today by a substantial part of physicists.

    this helps to understand how, in 2005, David Gross could say something as if it were news (Gross quoted by C.R. on page 2) which WAS ALREADY KNOWN 70 YEARS AGO to someone who took GR seriously.

    As C.R. says:
    "This is of course something that has been known since the 1930’s [2] by anybody who has taken seriously the problem of the implications of GR and QM."

    Understanding this helps to appreciate the history, which otherwise does not make very good sense. Because GR has been a sleeper until the last 20 years or so, it is having an effect somewhat like a new theory!

    C.R. summarizes this point on page 5 in the first paragraph of section 2.

    "Research in quantum gravity has developed slowly for several decades during the XX century, because GR had little impact on the rest of physics and the interest of many theoreticians was concentrated on the development of quantum theory and particle physics. In the last twenty years, the explosion of empirical confirmations and concrete astrophysical, cosmological and even technological applications of GR on the one hand, and the satisfactory solution of most of the particle physics puzzles in the context of the SM on the other, have led to a strong concentration of interest in quantum gravity, and the progress has become rapid. Quantum gravity is viewed today by many as the big open challenge in fundamental physics."

    he is talking about things like the binary pulsar measurements and GPS. the point is not that binary pulsars and the GPS obey SPECIAL relativity. Special was assimilated quickly and has been used by several generations already. The point is that the binary pulsars and the GPS obey GENERAL relativity. As does mainstream cosmology and so on. New instruments, satellites, and greater accuracy have brought these things to the forefront.

    It is maybe not such a big point but it helps to understand that the advance of physics has been patchy---not an orderly synchronized march, and lately GR has been catching up to where by some logical order it perhaps should have been a long time ago.

    This makes it easier to understand how David Gross could have announced on NOVA as if it were news that we may need to eliminate the ideas of space and time.
     
    Last edited: Apr 18, 2006
  5. Apr 18, 2006 #4

    marcus

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    Here is a key paragraph on page 4
    "In general relativity, when we describe the dynamics of the gravitational field (not to be confused with the dynamics of matter in a given gravitational field), there is no external time variable that can play the role of observable independent evolution variable. The field equations are written in terms of an evolution parameter, which is the time coordinate x_0, but this coordinate, does not correspond to anything directly observable. The proper time ? along spacetime trajectories cannot be used as an independent variable either, as ? is a complicated non-local function of the gravitational field itself. Therefore, properly speaking, GR does not admit a description as a system evolving in terms of an observable time variable. This does not mean that GR lacks predictivity. Simply put, what GR predicts are relations between (partial) observables, which in general cannot be represented as the evolution of dependent variables on a preferred independent time variable."

    So already in the 1915 GR theory THERE IS NO TIME at least as usually understood.
    You can take a specific SOLUTION like deSitter, or a Friedman solution, and that will be a fixed spacetime. and that will have a time coordinate. But that specific solution is not REALISTIC. It is not bumpy with clumps of matter and it does not woggle with clumps of matter moving around. It is a fixed clean static solution, which cosmologists can treat as approximately right and calculate usefully with. but that is not how Nature does.

    Here on page 3:
    "Indeed, classical GR as well can be entirely formulated in the Hamilton-Jacobi formalism, where no time variable appears either. "

    Some people call this, in certain contexts, "the problem of time". But the view of this paper is NOT that it is a PROBLEM.

    In the past 20 years we see CLASSICAL GR work amazingly well and come to the forefront. To excellent approx. this is how NATURE works. But CLASSICAL GR DOES NOT HAVE TIME. (only specific solutions, some which roughly approximate the world at very large supergalactic scale, have time and that only approximates realistic time) THEREFORE NATURE DOES NOT HAVE TIME. Time is not fundamental in nature, rather it is an appearence or epiphenomenon which "emerges" for us macro animals. Fortunately:tongue2:

    Well, that is a bit of a shocker, but I think that is what C.R. is driving at
    in this paper.

    There is a lot more-----plenty of other things to get out----but I think I will pause here for a while.
     
  6. Apr 18, 2006 #5
    Clearly you're asking other's for explanations that will somehow improve your personal understanding of these issues. But how is it possible that you still don't understand the very simple ideas in this totally nontechnical paper since they've been hashed and re-hashed ad nauseum in threads that have been largely started by you. If you can't understand them now you'll never understand them. As far as I'm concerned, these threads of yours are basically spam.
     
  7. Apr 18, 2006 #6

    marcus

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    Thanks for your comment! I'm busy now but will do some more explicating when I get back.
     
  8. Apr 18, 2006 #7
    Hey josh! I was just complaining that all people in the US are so awfully polite that it makes me want to throw up. I miss all the unfriendly Russians at my home institution who told me their opinion about all and everything, whether I wanted to hear it or not! Glad to hear you are an authentic a*****e, I suddenly feel like home :biggrin:

    Anyway, I don't know how marcus comes to understand things or not, but I have spent quite some time on trying to teach people. The most important thing is to repeat. Repeat. Repeat. And to be patient. And then repeat again. Maybe marcus is just not such a genuis as you are :tongue2: Neither am I.

    Not to mention that there will always be people around who have not read ancient threads and are to lazy too look them up (as I am).

    But while we are at criticising, I would kind of appreciate it if a thread comes with a less messy introduction. Like, what *aehem* do you actually want to talk about? Is QG "unfinished"? A "revolution"? Both? Or do we analyze Rovelli's psychology? Or what? I mean, I am squeezing in PF at working time (offline at home), and don't want to read 3 pages before I know whether I am interested at all.

    Best,

    B.
     
  9. Apr 18, 2006 #8

    marcus

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    Hi Biene,
    you dont have to worry about this thread. Just do your research! And if you want to look at Rovelli's paper, just look at it yourself. I wont know what lessons to take from it until I have gone through it. When I am through assimilating I will make a SUMMARY ---a short list of what I get from it. And then if you want you can read the summary, or not.

    I don't know what the message will be. the banal message that the reorganization of how physics treats space and time and matter (as a whole) is not FINISHED yet----that is a familiar truism: nothing new.

    But there may be other stuff besides that truism

    I am just starting. So far I found what I think refers to a DARK MATTER PHENOMENOLOGY issue, and so to the recent paper of Bekenstein and Magueijo. If GR needs correcting, he indicates, it might be here.
     
  10. Apr 18, 2006 #9

    marcus

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    Anybody reading this paper now should please get the 2nd version 13 April. It is a bit different from 1st version that I had printed out and was marking up.

    Here is a heads-up about dark matter phenomenology

    "This set of fundamental theories has obtained an empirical success nearly unique in the history of science: so far there isn’t any clear evidence of observed phenomena that clearly escape or contradict this set of theories —or a minor modification of the same, such as a neutrino mass or a cosmological constant. [footnote 1]

    Footnote 1: Dark matter (not dark energy) might perhaps be contrary evidence."

    One thing that is nice about these short overview articles is that each detail is weighed. He sees NOTHING that contradicts GR in the domain of applicability of GR except possibly dark matter.

    So it would be important to have some tests of dark matter, and Magueijo has one. I will get a link to his perimeter talk.
    It is on page 4 of the Perimeter streamer seminar listing
    http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False

    It was a perimeter seminar talk dated 21 February 2006. MOND habitats within the Solar System
    he did not show bias either way about MOND---he gave balanced pro and con discussion of MOND strong and weak points versus Dark Matter strong and weak. then in second half of talk he said if you want to test MOND here is a way.

    (in all humility Biene, you KNOW all this. I am just trying to be thorough! It has paid off in the past with Rovelli. You can do your work. I will tell you when I am finished looking over the paper)

    ============================
    here is another thing re Phenomenology, not limited to dark matter or mond, but specific to prediction by string or non-string QUANTUM gravities.

    -----quote Rovelli page 6----
    Within these frameworks, classical and long intractable, physical, astrophysical and cosmological quantum gravity problems can finally be concretely treated. Among these: black hole’s entropy and fate, the physics of the big–bang singularity and the way it has affected the currently observable universe, and many others. Tentative predictions are being developed, and the attention to the concrete possibility of testing these predictions with observations that could probe the Planck scale is very alive. All this was unthinkable only a few years ago.

    ---endquote---
     
    Last edited: Apr 18, 2006
  11. Apr 18, 2006 #10
    Hey hossi!!

    Here's a shocker: I'm canadian :surprised

    Gee whiz hossi, is the purpose of the thread entitled “Hossi to Perimeter in September!” started by marcus clear enough for you? Personally I don’t really see what it has to do with the kinds of things this forum was set up to discuss and I'm going to go out on a limb and guess that neither do you.
     
  12. Apr 18, 2006 #11
    Why is that a shocker?

    Well, I find it kind of interesting to hear whether the intention PI was founded with is promising, or successful in other people's opinion - even yours. I mean, they are trying to do something different, and it's worth discussion whether it works so far.
    You might want to let me know your thoughts in that thread...

    But, no, the purpose why marcus titled the thread as he did is not clear to me. Maybe you could explain it. Do you think he wants to marry me?

    marcus, I have printed the paper anyway and take it to the beach :smile: Weather is too nice today to stay in my office. Best,

    B.
     
    Last edited: Apr 18, 2006
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