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Crossing The Desert-without h-bar

  1. Mar 2, 2007 #1

    jal

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    Kirill Krasnov _
    01 March 2007
    http://arxiv.org/PS_cache/gr-qc/pdf/0703/0703002.pdf
    Non-Metric Gravity I: Field Equations

    Can it be done?
    What will the slim down versions of h-bar look on this side of the desert?
    This is another great step.
    This is an approach to keep your eye on.
    Jal
     
  2. jcsd
  3. Mar 2, 2007 #2

    marcus

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    hello jal,

    where you quote from Krasnov's paper he is using the symbol hbar
    and it doesnt come thru in your quote

    you would probably enjoy using TEX more so you can get those symbols
    there is a sticky about it somewhere at PF.

    there are two modes, ordinary tex on its own line
    and inline tex where you use the symbol amongst the rest of what you have written in that line. To get these delimiters to work remove the *

    [t*ex] and [/t*ex]

    put the delimiters around \hbar

    Maybe you know all this and have been using tex for years :-)

    Remove the asterisks in this:
    [t*ex] \hbar [/t*ex]

    that may not look so good INLINE, so to make sure it works inline use the word ITEX instead of TEX and write the delimiters [it*ex] and [/it*ex]

    Here is the ordinary version, where I removed the asterisks in
    [t*ex] \hbar [/t*ex]

    [tex] \hbar [/tex]
     
    Last edited: Mar 2, 2007
  4. Mar 2, 2007 #3

    marcus

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    I can understand why you might be interested in Krasnov's paper. I was reading it last night.

    the passage you quoted was the second paragraph of page 3.

    HE IS NOT HOWEVER TALKING ABOUT DOING AWAY WITH HBAR. What he means by passing to the classical limit as hbar -> 0 is just what people have been doing for 70 or 80 years in quantum mechanics.

    they set up some model with hbar in it and then they pass to the limit as hbar -> 0 to see what it looks like classically.
    =============

    Krasnov's paper is interesting and represents a new QG gambit.

    Since you like this paper, you might like to watch him talking about it to Lee Smolin and friends on video----the video explains things in a more introductory way, and people ask questions.

    Also Krasnov has an earlier paper about the same thing, from a month or two ago.

    I will get some links.
     
  5. Mar 2, 2007 #4

    marcus

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    To get the Krasnov video, go here
    http://www.perimeterinstitute.ca/index.php?option=com_content&task=view&id=113&Itemid=167
    check the box "quantum gravity" and press "do search"
    and scroll down to 0611 (November 2006)
    and you will see

    "PIRSA:06110041
    Title: Renormalizable Non-Metric Quantum Gravity? ( Windows Media , Macromedia Flash , MP3 Audio , PDF)
    Speaker(s): Kirill Krasnov - University of Nottingham
    ...
    ...
    Date: 30/11/2006 - 1:30 pm
    Series: Quantum Gravity
    Location: 405
    URL: http://pirsa.org/06110041/"

    And maybe you can avoid all the preliminaries and simply use this link

    http://pirsa.org/06110041/

    Anyway what I pasted just now has "windows media" which you can click and watch the seminar. And the lecture note slides are available by clicking "PDF"
    ==================

    I will get the arxiv preprint link too, for the earlier paper. It is the paper he is discussing in this seminar talk

    http://arxiv.org/abs/hep-th/0611182
    Renormalizable Non-Metric Quantum Gravity?
    Authors: Kirill Krasnov
    Comments: 5 pages, no figures

    We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological ``constant'' and may provide a mechanism for naturally small ``dark energy''.
     
    Last edited: Mar 2, 2007
  6. Mar 2, 2007 #5

    jal

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    Thanks Marcus!
    You're making it easier to learn.
    jal
     
  7. Mar 3, 2007 #6

    Chronos

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    This a really bad idea, IMO. Marcus is just trying to be nice. An anger management thing. The audacity is deafening.
     
  8. Mar 3, 2007 #7

    marcus

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    When Albert Einstein was 5 years old they put him in a Roman Catholic kindergarten. I can see how you might interpret the rest of his creative life as anger management. It's a theory. But you still have to look at each new idea as it comes along.

    Delighted by your image of two girls kissing under a waterfall. Gratuitously beautiful---inexplicable.
    https://www.physicsforums.com/showthread.php?p=1261855#post1261855
     
    Last edited: Mar 3, 2007
  9. Mar 3, 2007 #8

    jal

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    I'm alway looking for GOOD Ideas.
    Don't be shy ... point me to your simple idea .... maybe I can learn something from your idea.
    jal
     
  10. Mar 3, 2007 #9

    marcus

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    Hi Jal,
    two things to notice about Krasnov's paper.

    He's really psyched up about this new approach to gravity, but at the same time he doesn't argue that it is the final answer to QG problem. If you read carefully you see where he is finding ample reasons to motivate investigating this non-metric formulation whether or not it ultimately turns out right.

    he presents definite ideas of what can be learned by seeing how this re-formulation goes.

    (that part was, to me, quite convincing---it is an innovative approach just in how it goes after classical gravity already---it virtually obvious to me that things of value will be learned whether or not he actually reaches the South Pole :-))

    And here is the other thing to notice. On an average of about once per page, Krasnov mentions that this Roman Numeral I paper is part of a series of papers he is writing about this. The guy is heavy-duty motivated about this. It is a big project to un-earth this new version of classical gravity. We have to be prepared to be patient (unless one or two other researchers get interested and help speed it up, he is describing a several year project)

    ==========
    EDIT I am replying here, since I can still edit and I don't want to cap yours with another merely to say *yes*
    I strongly agree with what you say in the next post.
     
    Last edited: Mar 3, 2007
  11. Mar 3, 2007 #10

    jal

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    Yes, Marcus. That is my take on his approach.
    Once the others see the potential of this line of investigation, we could end up with a new path towards the understanding of the universe and how it works. Then again.... it could end up to be a dead end.
    It is just as important to point out the dead ends as it is to find new paths.
    jal
     
  12. Mar 4, 2007 #11

    Chronos

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    No disagreement there Jal. That was never my intent. We probably got off on the wrong foot. I do not doubt your sincerity, just concepts. They don't make sense to me. It is my nature to question every assumption from every angle. That approach leads to surprising conclusions at times.
     
  13. Mar 4, 2007 #12

    jal

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    Questioning everything??? Must lead to proposing an approach that should be investigated.
    How about Casual Set
    This paper does not have too much math. and should be understandable to the average readers.
    http://arxiv.org/PS_cache/gr-qc/pdf/9706/9706002.pdf
    FORKS IN THE ROAD, ON THE WAY TO QUANTUM GRAVITY*
    Rafael D. Sorkin
    01 June 1997
    Abstract
    In seeking to arrive at a theory of “quantum gravity”, one faces several choices among alternative approaches. I list some of these “forks in the road” and offer reasons for taking one alternative over the other. In particular, I advocate the following: the sum-over-histories framework for quantum dynamics over the “observable and state-vector” framework; relative probabilities over absolute ones; spacetime over space as the gravitational “substance” (4 over 3+1); a Lorentzian metric over a
    Riemannian (“Euclidean”) one; a dynamical topology over an absolute one; degenerate metrics over closed timelike curves to mediate topology-change; “unimodular gravity” over the unrestricted functional integral; and taking a discrete underlying structure (the causal set) rather than the differentiable manifold as the basis of the theory.
    In connection with these choices, I also mention some results from unimodular quantum cosmology, sketch an account of the origin of black hole entropy, summarize an argument that the quantum mechanical measurement scheme breaks down for quantum field theory, and offer a reason why the cosmological constant of the present epoch might have a magnitude of around 10−120 in natural units.

    Another approach that tries to find out how the universe is made and how it works.
    jal
     
  14. Mar 5, 2007 #13

    Chronos

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    Interesting you should mention that paper. While more recent observations have constrained some of Sorkin's 'forks', this paper remains one of my personal favorites on QG. The causal set approach is irresistably logical to me.
     
  15. Mar 5, 2007 #14

    jal

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    Hi Chronos!
    I have been working on trying to make a blog with Casual set that would show how QMLS would fit in with this approach.
    Now that h-bar has been set free...heheheh .... perhaps they will be able to combine the two approaches into one.
    I do not have any recent links of what they have been doing.
    Here is what I have that should interest others.

    http://arxiv.org/PS_cache/gr-qc/pdf/0507/0507078.pdf
    Structure, Individuality and Quantum Gravity
    John Stachel∗
    12 July 2005
    First I look at the effective field theory and asymptotic quantization approaches to general relativity, and then at string theory. Then a discussion of some issues common to all approaches to quantum gravity based on the full general theory of relativity argues that processes, rather than states should be taken as fundamental in any such theory.

    p. 20 Fay Dowker states: Most physicists believe that in any final theory of quantum gravity, space-time itself will be quantized and grainy in nature. .... So the smallest possible volume in four-dimensional space-time, the Planck volume, is 10-42 cubic centimeter seconds. If we assume that each of these volumes counts a single space-time quantum, this provides a direct quantification of the bulk ([15], pp. 38
    ---------------
    http://arxiv.org/PS_cache/gr-qc/pdf/0508/0508109.pdf
    Causal sets and the deep structure of spacetime
    Fay Dowker
    Blackett Laboratory, Imperial College, London, SW7 2AZ, UK.
    May 19, 2006
    --------------
    http://www.maths.lse.ac.uk/Personal/graham/research-appl.html
    Causal Sets
    The hope is that an appropriate model can be found, which on large scales approximates the classical structure of the universe, but on small scales gives rise to "quantum" effects. Of particular interest is the case where the causal set has a unique minimal element.
    ---------------
    http://uk.arxiv.org/PS_cache/gr-qc/pdf/0309/0309009.pdf
    Notes for the Valdivia Summer School, Jan. 2002
    Rafael D. Sorkin
    01 Sept 2003
    For the purposes Causal Sets: Discrete Gravity
    of quantum gravity, a causal set is, of course, meant to be the deep structure of spacetime
    . Or to say this another way, the basic hypothesis is that spacetime
    ceases to exist on sufficiently small scales and is superseded by an ordered discrete structure to which the continuum is only a coarse-grained, macroscopic approximation.

    … one might anticipate that the entropy of a black hole is effectively counting suitably defined “molecules” of its horizon. With this possibility in mind, one can ask whether any simply definable sub-structures of the causets associated with a given geometry could serve as candidates for such “horizon molecules” in the sense that counting them would approximately measure the “information content” of the black hole.
    ----------
    Do you have any link to recent papers that I could read?
    jal
     
  16. Mar 6, 2007 #15

    Chronos

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    Stefan Zohren has published a number of related papers in the past year
    [including one in collaboration with Ambjorn]. This most recent entry is very good, IMO:

    Counting entropy in causal set quantum gravity
    Authors: D. Rideout, S. Zohren
    Comments: 5 pages, 1 figure. Talk given by S. Zohren at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006

    The finiteness of black hole entropy suggest that spacetime is fundamentally discrete, and hints at an underlying relationship between geometry and "information". The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We review recent attempts to define a microscopic measure for black hole entropy and for the maximum entropy of spherically symmetric spacelike regions, within the causal set approach to quantum gravity.

    My partiality to the causal set approach is admittedly showing here. Any approach that does not include causal sets is, IMO, fundamentally incomplete. I'm not happy with the 'grainy space' hypothesis. There is observational evidence that does not set well with that proposition - an issue I find very troubling. High energy gamma ray photons should exhibit more scattering than is observed.
     
    Last edited: Mar 6, 2007
  17. Mar 6, 2007 #16

    jal

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    Hi Chronos!
    :smile:
    If different approaches yields similar results, then, it should be possible to said that "the results" arise from "First Principle".
    By examining the "unexplained" "taken for granted", "presumptions" and "assumptions" of the different approaches, It should be possible to arrive at some "First Principle".
    Therefore, all of the approaches which use "Minimum length", "Planck Scale", "grainy space", should end up with a "structure", "casual set".
    I would search, for the assumptions and presumptions that have been made, in process of going from 'grainy space' to 'observational evidence'.

    I think that the fun is in the search.
    The answer will probably be "in our face" and prove to be very dull.
    jal
     
    Last edited: Mar 6, 2007
  18. Mar 7, 2007 #17

    Chronos

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    Well [nudging pebble with toe], I'm not entirely happy with 'First Principle' concepts. They tend to let time dependency sneak into the backdoor without a pass. I'm not convinced this is necessarily valid.
     
  19. Mar 7, 2007 #18

    jal

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    Hi Chronos!
    hehehe…. We are all little pebbles …. Hehehe
    We are all questioning …. And doing … what if??? …. Some people are more structured in their approaches and have more skills to apply to their postulates.
    Example: Kirill Krasnov 01 March 2007
    http://arxiv.org/PS_cache/gr-qc/pdf/0703/0703002.pdf
    Non-Metric Gravity I: Field Equations
    That started this thread.

    (I don’t like to get into the para, meta domains since their postulates have not been advanced to being able to work with them.)
    The following paper (from another thread) will surely motivate some skillful individuals to attack this approach or to try to develop this approach.
    Like I’ve said, “It’s just as important to identify the dead ends as it is to forge new paths”.


    http://arxiv.org/PS_cache/quant-ph/pdf/0604/0604064.pdf
    Relational EPR
    Matteo Smerlak†, Carlo Rovelli‡
    March 4, 2007
    -----------------
    When I say, “First Principle”, I am thinking of the opposite of random and chaos.
    “Causal set” and Quantum Minimum Length Structure (QMLS)” are approaches that could end up as a dead end ..or …. Not.
    jal
     
  20. Mar 8, 2007 #19

    Chronos

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    Looks like we are at the same lunch stand. Wondering how fine the line is between reality and imagination is an interesting excercise. On the other hand, I sometimes question if the line actually exists. More often I wonder if it is meaningful to make the attempt to draw one. It is an extension of the human condition and nature does not always respect that distinction.

    My issues are with continuity between science and observation. I loathe unfalsifiable theories. I think that is an escape hatch. Did the universe have a 'beginning'? The question is inherently unfalsiable on the face of it. You can hardly justify its existence without a beginning. The only saving grace is causality ceases to be meaningful when you truly push the envelope. The chains of intellect are forged from a substance much tougher than steel - imagnination. That is what we lack. I suspect future historians will will characterize the 21st century as the 'gray age'.
     
    Last edited: Mar 8, 2007
  21. Mar 8, 2007 #20

    jal

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    Hi Chronos!
    I wonder if the "old guards" are paying attention to the work coming out of perimeter Institute. They cannot be accused of lack of imagination.
    I'm sure that if they can accomodate a new idea in their approaches they would. How else can someone get "headlines" and "funding".
    Do the "old guards" want to know about the dead ends and the new paths?
    Does it really matter to them if the "Big Bang" is right or wrong? The people at CERN do not need a new interpretation to be able to do their experiments.
    Even if SUSY is wrong, the experiments are capable of being performed.
    So with cynicism aside ....
    Marcus last link sort of answered you question about time coming in the back door.
    --------------
    http://arxiv.org/PS_cache/gr-qc/pdf/0703/0703027.pdf
    Conserved Quantities in Background Independent Theories
    Fotini Markopoulou
    05 march 2007
    Could someone explain the quoted phrases that I put in bold letters?
    -------------------------
    http://arxiv.org/PS_cache/gr-qc/pdf/0604/0604075.pdf
    Emergent General Relativity
    Olaf Dreyer
    18 April 2006
    ------------------
    http://arxiv.org/PS_cache/hep-th/pdf/0604/0604120.pdf
    Towards Gravity from the Quantum
    Fotini Markopoulou
    18 April 2006
    Jal
     
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