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Approaches to Quantum Gravity (now in Cambridge Press catalog)

  1. May 26, 2008 #1


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    the publication date for the book is given as May 2009

    the list of contributors is
    C. Rovelli, G ‘t Hooft, R. Sorkin, J. Stachel, N. Savvidou, L. Crane, O. Dreyer, R. Percacci, F. Markopoulou, G. Horowitz, J. Polchinski, T. Banks, W. Taylor, T. Thiemann, E. Livine, A. Perez, L. Freidel, D. Oriti, J. Ambjørn, J. Jurkiewicz, R. Loll, R. Williams, R. Gambini, J. Pullin, J. Henson, G. Amelino-Camelia, C. Burgess, S. Majid, J. Kowalski-Glikman, F. Girelli, J. Collins, D. Sudarsky, L. Smolin

    there are 34 contributors including some very noteworthy ones

    Dan Oriti is the editor. He gets a lot of credit for bringing together people from highly diverse approaches to QG.

    Horowitz, Polchinski, Banks and Taylor are STRING
    Loll, Ambjorn, Jurkiewicz and Williams are TRIANGULATIONS (CDT and Regge calculus)
    Sorkin and Henson are CAUSAL SETS
    Rovelli, Thiemann, Livine, Perez, Freidel, Gambini and Pullin are LOOP

    that is just a sample, I know I am missing some.

    The title is
    Approaches to Quantum Gravity:
    Toward a New Understanding of Space, Time and Matter

    Now that Cambridge University Press has it in the catalog and has set a definite publication date we can be pretty confident the book is going to come out. I was wondering when I didn't hear anything about it for a while.
    Last edited: May 26, 2008
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  3. May 26, 2008 #2


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    This thread is not intended to be a review of the book. What I plan to do is to assemble a list of links of the chapters which have already appeared in the arxiv so that we can access the available chapters and get a sense of the PHYSIC CONTENT which will be assembled in the book. Please bear with me. This is not a book review.

    The book will have 630 pages.

    Here is the catalog summary and the Table of Contents

    The theory of quantum gravity promises a revolutionary new understanding of gravity and spacetime, valid from microscopic to cosmological distances. Research in this field involves an exciting blend of rigorous mathematics and bold speculations, foundational questions and technical issues. Containing contributions from leading researchers in this field, this book presents the fundamental issues involved in the construction of a quantum theory of gravity and building up a quantum picture of space and time. It introduces the most current approaches to this problem, and reviews their main achievements. Each part ends in questions and answers, in which the contributors explore the merits and problems of the various approaches. This book provides a complete overview of this field from the frontiers of theoretical physics research for graduate students and researchers.

    • Presents the fundamental issues involved in the construction of a quantum theory of gravity and building up a quantum picture of space and time
    • Contains question and answer sections, in which the contributors explore the merits and problems of the various approaches
    • A complete overview of this field from leading researchers at the frontiers of theoretical physics research


    Part I. Fundamental Ideas and General Formalisms:
    1. Unfinished revolution C. Rovelli;
    2. The fundamental nature of space and time G. ‘t Hooft;
    3. Does locality fail at intermediate length scales R. Sorkin;
    4. Prolegomena to any future quantum gravity J. Stachel;
    5. Spacetime symmetries in histories canonical gravity N. Savvidou;
    6. Categorical geometry and the mathematical foundations of quantum gravity L. Crane;
    7. Emergent relativity O. Dreyer;
    8. Asymptotic safety R. Percacci;
    9. New directions in background independent quantum gravity F. Markopoulou;
    Questions and answers;

    Part II:
    10. Gauge/gravity duality G. Horowitz and J. Polchinski;
    11. String theory, holography and quantum gravity T. Banks;
    12. String field theory W. Taylor;
    Questions and answers;

    Part III:
    13. Loop Quantum Gravity T. Thiemann;
    14. Covariant loop quantum gravity? E. Livine;
    15. The spin foam representation of loop quantum gravity A. Perez;
    16. 3-dimensional spin foam quantum gravity L. Freidel;
    17. The group field theory approach to quantum gravity D. Oriti;
    Questions and answers;

    Part IV. Discrete Quantum Gravity:
    18. Quantum gravity: the art of building spacetime J. Ambjørn, J. Jurkiewicz and R. Loll;
    19. Quantum Regge calculations R. Williams;
    20. Consistent discretizations as a road to quantum gravity R. Gambini and J. Pullin;
    21. The causal set approach to quantum gravity J. Henson;
    Questions and answers;

    Part V. Effective Models and Quantum Gravity Phenomenology:
    22. Quantum gravity phenomenology G. Amelino-Camelia;
    23. Quantum gravity and precision tests C. Burgess;
    24. Algebraic approach to quantum gravity II: non-commutative spacetime F. Girelli;
    25. Doubly special relativity J. Kowalski-Glikman;
    26. From quantum reference frames to deformed special relativity F. Girelli;
    27. Lorentz invariance violation and its role in quantum gravity phenomenology J. Collins, A. Perez and D. Sudarsky;
    28. Generic predictions of quantum theories of gravity L. Smolin;
    Questions and answers;


    My comment: the Questions and Answers section at the end of every part is a good idea. It allows each author to be questioned by all the other authors, and to reply to their questions. This gives the book an aspect of dialog. I have seen the questions and answers connected to Roberto Percacci's chapter on Asymptotic Safety. They are highly informative. It is an efficient means of getting across the ideas.
    Last edited: May 26, 2008
  4. May 26, 2008 #3


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    OK now let's see what chapters are available free online, that we can look at.

    I know that Roberto Percacci's chapter is, together with the Q and A.
    this is a really good source on Asymptotic Safety so it is a good link to have
    Asymptotic Safety
    R. Percacci

    one way to dig up these things is to use the arxiv search engine

    that search gives 10 articles all of which have been contributed to the book:

    1. http://arxiv.org/abs/0709.3851 [ps, pdf, other]
    Title: Asymptotic Safety
    Authors: R. Percacci
    Comments: To appear in "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter", ed. D. Oriti, Cambridge University Press
    Subjects: High Energy Physics - Theory (hep-th)

    2. http://arxiv.org/abs/gr-qc/0703097 [ps, pdf, other]
    Title: New directions in Background Independent Quantum Gravity
    Authors: Fotini Markopoulou
    Comments: Comments 26 pages. Contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    3. http://arxiv.org/abs/gr-qc/0608135 [ps, pdf, other]
    Title: Towards a Covariant Loop Quantum Gravity
    Authors: Etera R. Livine
    Comments: 13 pages, review, draft chapter for the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press, comments welcome
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

    4. http://arxiv.org/abs/gr-qc/0607032 [ps, pdf, other]
    Title: The group field theory approach to quantum gravity
    Authors: Daniele Oriti
    Comments: 17 pages, 2 figures; expanded version of a contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

    5. http://arxiv.org/abs/hep-th/0605052 [ps, pdf, other]
    Title: Generic predictions of quantum theories of gravity
    Authors: Lee Smolin
    Comments: For inclusion in "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
    Subjects: High Energy Physics - Theory (hep-th)

    6. http://arxiv.org/abs/hep-th/0604212 [ps, pdf, other]
    Title: Quantum Gravity, or The Art of Building Spacetime
    Authors: J. Ambjorn, J. Jurkiewicz, R. Loll
    Comments: 22 pages, 6 figures. Contribution to the book "Approaches to Quantum Gravity", ed. D. Oriti, Cambridge University Press
    Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)

    7. http://arxiv.org/abs/gr-qc/0604075 [ps, pdf, other]
    Title: Emergent General Relativity
    Authors: Olaf Dreyer
    Comments: Contribution to Towards Quantum Gravity, a collection of essays on the different approaches to quantum gravity edited by Daniele Oriti. To be published by Cambridge University Press
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    8. http://arxiv.org/abs/gr-qc/0603022 [ps, pdf, other]
    Title: Doubly Special Relativity: facts and prospects
    Authors: Jerzy Kowalski-Glikman
    Comments: Submitted to the volume "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", D. Oriti ed
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    9. http://arxiv.org/abs/gr-qc/0601121 [ps, pdf, other]
    Title: The causal set approach to quantum gravity
    Authors: Joe Henson
    Comments: 22 pages, 4 figures, Latex. Extended version of a review to be published in "Approaches to Quantum Gravity - Towards a new understanding of space and time" (ed. D. Oriti), Cambridge University Press, 2006. Ref added. Dedicated to Rafael Sorkin on the occasion of his 60th birthday
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    10. http://arxiv.org/abs/gr-qc/0512065 [ps, pdf, other]
    Title: Consistent discretizations as a road to quantum gravity
    Authors: Rodolfo Gambini, Jorge Pullin
    Comments: 17 Pages, Draft chapter contributed to the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    results of a further search:
    4. arXiv:gr-qc/0606108 [ps, pdf, other]
    Title: Quantum Gravity and Precision Tests
    Authors: C.P. Burgess
    Comments: Contribution to `Towards Quantum Gravity,' edited by D. Oriti, Cambridge University Press, 2006; 18 pages, no figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc)

    5. arXiv:hep-th/0605202 [ps, pdf, other]
    Title: String Field Theory
    Authors: Washington Taylor (MIT, Stanford)
    Comments: To appear in "Towards Quantum Gravity", ed. Daniele Oriti, Cambridge University Press; 22 pages latex; v2: updated references
    Subjects: High Energy Physics - Theory (hep-th)

    9. arXiv:hep-th/0603002 [ps, pdf, other]
    Title: Lorentz Invariance Violation and its Role in Quantum Gravity Phenomenology
    Authors: John Collins, Alejandro Perez, Daniel Sudarsky
    Comments: Draft chapter contributed to the book "Towards quantum gravity", being prepared by Daniele Oriti for Cambridge University Press
    Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)

    10. arXiv:gr-qc/0602037 [ps, pdf, other]
    Title: Gauge/gravity duality
    Authors: Gary T. Horowitz, Joseph Polchinski
    Comments: To appear in "Towards quantum gravity", ed. Daniele Oriti, Cambridge University Press. 20pgs; v2, v3: references updated
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

    12. arXiv:gr-qc/0601095 [ps, pdf, other]
    Title: The spin-foam-representation of loop quantum gravity
    Authors: Alejandro Perez
    Comments: Draft chapter contributed to the book "Towards quantum gravity", being prepared by Daniele Oriti for Cambridge University Press. 19pgs
    Subjects: General Relativit
    Last edited: May 26, 2008
  5. May 26, 2008 #4


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    Great! so we have 15 chapters available online.
    We even have the (string) chapter by Horowitz and Polchinski
    and the string field theory chapter by Taylor

    most of these lack the Q and A dialog that goes with them, an informative part of the book
    and many of them may not be final drafts, so what is published may be improved considerably over the preprints that we can see.

    but we've got a resource, and can begin to see what Oriti has put together.

    Christine Dantas also has a list, but I don't have a link to it. She may well have these very same 15 chapters, and even more.

    many people have asked what it would look like to try to put these rival approaches together
    rather than having one trying to tear down the other
    roughly speaking I think this is what it looks like, for starters at least.

    so take a look and see what you think
    Last edited: May 26, 2008
  6. May 26, 2008 #5


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    Now we come to the meat course :biggrin: if anyone has questions and wants to discuss the physics content of some of these chapters.

    Personally I've read some of them and would be happy to try to answer questions.
    These include Asymptotic Safety, Generic Predictions, The Art of Building Space Time,
    and a few others I'm more or less familiar with.
    There may be posters besides myself who are familiar with some of these papers, as well, and prepared to comment or respond.

    Let's see if there is any interest.

    BTW links still coming in, here is another
    Unfinished Revolution
    Carlo Rovelli

    and another
    Prolegomena to any future quantum gravity
    John Stachel

    18 so far, by my count

    and two more, making twenty so far
    Categorical Geometry and the Mathematical Foundations of Quantum General Relativity
    Louis Crane

    Does Locality Fail at Intermediate Length-Scales?
    Rafael D. Sorkin

    have to go back later
    Last edited: May 26, 2008
  7. May 27, 2008 #6
    Hi Marcus,

    You have a nice updated list (thanks for building it); I stopped following this for some time. But on searching my old archives, I noticed the following two missing from your list:

    Title: Towards Gravity from the Quantum
    Authors: Fotini Markopoulou
    Comments: Expanded version of the contribution to
    "Towards Quantum Gravity", edited by D.Oriti, to be published by C.U.P

    Title: Algebraic approach to quantum gravity II: noncommutative spacetime
    Authors: S. Majid
    Comments: 26 pages, 2 .eps figures; book chapter to appear in D. Oriti,
    ed., Cambridge Univ. Press

    I'm looking forward to purchase the book anyway, but it's excellent to have many of the articles freely available, so that we can have an idea of the contents of the book. The sections on questions and answers were a very good idea, and I'm curious about it.

    All the best,
  8. May 27, 2008 #7
    I don't know if Fotini replaced her article of my previous post by the one you link, which is more recent, or if she has two contributions to the book, which seems unlikely.


    Edit: Indeed, from the table of contents, she seems to have disregarded or expanded the 2006 article to the one that you link.
    Last edited: May 27, 2008
  9. May 27, 2008 #8


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    reflection of rovelli's key question

    I just skimmed this.

    I like this note from the paper.

    "Conceptually, the key question is whether or not it is logically possible to understand the world in the absence of fundamental notions of time and time evolution, and whether or not this is consistent with our experience of the world."

    To me the question of wether "it is logically possible to..." brings the associations to the notion and dynamics of the theory itself. Maybe looking for "a universal theory" is not consistent because no matter what we come up with, this something will always keep changing. Perhaps this would suggest that instead of looking or a classical style "theory" we should be looking for a strategy to evolve theories? If there is a non-trivial feedback at each level, this also prevents this strategy to be an old style "theory of theories". How about if the constant improvement, is the part of the reflections that might help solve the problem of time?

    About consistency with experiences. Is our experience really to relying on fundamental notions? As far as I see it, this is not so. We are relying on highly subjective references. So I see no issue of the breakdown of realism and fundamental objectivity contradicting intuitive experiences.

    Maybe this seemingly unavoidable self-references is why it's so hard to make a definitive conclusion? could it be a little easier if we stop looking for a destination and instead try to focous on the journey? (ie the dynamics and physics of reasoning rather sets of universal truths - such a focus would be truly relational indeed).

  10. May 27, 2008 #9


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    Christine, thank for the Majid link. The Majid article becomes chapter 24, in Part V. At least in draft form, we now have 21 of the chapters! I think what you suggest is right about Fotini's chapter.
    Judging from the Oriti TOC in the catalog, her 2007 contribution does indeed replace the 2006 one.


    Fra, I'm glad you brought up the Rovelli essay for discussion! I will take a look at it and offer some comments.

    BTW I think it is nice that his first reference, [1], is a quote from the eminent string theorist and nobelist David Gross. It is a short essay with a broad historical perspective. Here Rovelli tries to illuminate the whole picture and not narrow down to one or two particular approaches.

    Maybe I will quote a couple of paragraphs at the beginning. (In a way, the 8-page Rovelli essay, coming at the beginning, provides an introduction to the whole collection of articles.)
    Last edited: May 27, 2008
  11. May 27, 2008 #10


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    One hundred and forty-four years elapsed between the publication of Copernicus’s De Revolutionibus, which opened the great scientific revolution of the XVII century, and the publication of Newton’s Principia, the final synthesis that brought that revolution to a spectacularly successful end. During those hundred and forty-four years, the basic grammar for understanding the physical world changed and the old picture of reality was reshaped in depth.

    At the beginning of the XX century, General Relativity (GR) and Quantum Mechanics (QM) once again began reshaping our basic understanding of space and time and, respectively, matter, energy and causality —arguably to a no lesser extent. But we have not been able to combine these new insights into a novel coherent synthesis, yet. The XX century scientific revolution opened by GR and QM is therefore still wide open. We are in the middle of an unfinished scientific revolution. Quantum Gravity is the tentative name we give to the “synthesis to be found”.

    His reference [1] quoting David Gross, comes soon after that, in this context:

    ...Others, on the other hand, and in particular some hard–core particle physicists, do not accept the lesson of GR. They read GR as a field theory that can be consistently formulated in full on a fixed metric background, and treated within conventional QFT methods. They motivate this refusal by insisting than GR’s insight should not be taken too seriously, because GR is just a low–energy limit of a more fundamental theory. In doing so, they confuse the details of the Einstein’s equations (which might well be modified at high energy), with the new understanding of space and time brought by GR. This is coded in the background independence of the fundamental theory and expresses Einstein’s discovery that spacetime is not a fixed background, as it was assumed in special relativistic physics, but rather a dynamical field.

    Nowadays this fact is finally being recognized even by those who have long refused to admit that GR forces a revolution in the way to think about space and time, such as some of the leading voices in string theory. In a recent interview [1], for instance, Nobel laureate David Gross says: “ [...] this revolution will likely change the way we think about space and time, maybe even eliminate them completely as a basis for our description of reality”.

    I think it is apt that he has David Gross summarize exactly the unfinished revolution that is the subject of the essay. Indeed the goal would seem to be finding degrees of freedom more fundamental than those living in the spacetime continuum provided by metric differential and Euclidean geometries. As Gross said, to eliminate space and time from the most basic description of reality. (Let them come back as appearances, emerging from something more basic.)

    This is an ambitious program whether it is Carlo Rovelli or David Gross talking about it, and I suppose it is one of the main jobs of these essays to offer some substance to it. Some provisional scaffolding to climb out on, instead of leaping into thin air. Let's see if any of the essays do this for us, and to what extent.
    Last edited: May 27, 2008
  12. May 27, 2008 #11


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    Extra info on Rafael D. Sorkin
    Does Locality Fail at Intermediate Length-Scales
    Authors: Rafael D. Sorkin (Perimeter Institute and Syracuse University)
    (Submitted on 20 Mar 2007)
    Most current version is available at (or wherever my home-page may be) http://www.physics.syr.edu/~sorkin/some.papers/
    Latest paper
    Is the cosmological "constant" a nonlocal quantum residue of discreteness of the causal set type?
    Authors: Rafael D. Sorkin (Perimeter Institute and Syracuse University)
    (Submitted on 9 Oct 2007)
    To appear in the proceedings of the PASCOS-07 Conference, held July, 2007, London, England
    PASCOS-07 Conference
    The causal sets programme is an approach to quantum gravity.
    Voronoi diagram
  13. May 27, 2008 #12


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    I personally do not have an overview of all the approaches referenced in the catalog, but I'm temporarily focusing on the somewhat related "spin network ideas" of Rovelli, Penrose and Smolin and I am trying to understand the spin networks from the conceptual point and try to figure out if they really reflect the intent spelled out. That is still in progress.

    IMHO, I think one difficulty if one tries to reduce spacetime to something more "fundamental" - like David Gross puts it "maybe even eliminate them completely as a basis for our description of reality” - is to not make the mistake to replace the (spacetime) reference, with another reference that has the same principle flaws, which the only exception of increasing the complexity of the model. In particular when one is talking about the space of spacetimes, this seems to be a difficulty. It's also related to the equiprobability hypothesis for microstates. Sometimes a choice seems innocent and free of arbitrary priors while this is not so.

  14. May 27, 2008 #13


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    There is a relevant quote from Einstein in a letter written 1915 mentioning the principle of general covariance where he says

    "time and space thereby lose the last vestige of physical reality"

    see for example this book on the history of General Relativity
    this brings up a page with the quote about halfway down the page

    about your concern, Fra. A common way to understand this is simply that whatever replaces the metric background geometry must be BACKGROUND INDEPENDENT. the essential thing that any background independent approach should do is respond to your concern about not getting stuck in the same trap as the thing you are trying to replace.

    So the name of the game is to avoid starting off by specifying Euclidean space, or Minkowski space, or more generally some metric continuum a la differential geometry. Einstein GR shows one way to do that. It starts off with no particular metric on the manifold. GR is background independent.

    so in a sense what Rovelli is talking about (and maybe also David Gross) is simply continuing in the tradition of General Relativity and taking the requirement of background independence seriously.

    It sounds easy, after all Einstein did it in 1915. But apparently it gets thorny when you try to include quantum mechanics.

    I found the quotes in German
    for an online source see page 43 of
    www.tc.umn.edu/~janss011/pdf files/Besso-memo.pdf

    " 117 In the introduction of the paper on the perihelion motion presented on 18 November 1915, Einstein
    wrote about the assumption of general covariance “by which time and space are robbed of the last
    trace of objective reality
    ” (“durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt
    Einstein 1915b, 831). In a letter to Schlick, he again wrote about general covariance that
    thereby time and space lose the last vestige of physical reality” (“Dadurch verlieren Zeit & Raum
    den letzter Rest von physikalischer Realität.”
    Einstein to Moritz Schlick, 14 December 1915 [CPAE 8,
    Doc. 165]).

    So both quotes are from Nov-Dec 1915
    One quote is from a published paper on perihelion motion. and the other is from a letter to Moritz Schlick a few weeks later.
    Last edited: May 27, 2008
  15. May 27, 2008 #14


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    background information

    I have a strong feeling that there are still different ways to think about this and maybe subtle differences here are reflected in the strategy. I think the notion of background indepedence, taken seriously, goes beyond spacetime only.

    As always, I can only guess what anybody means but in "The case for background independence", smoling argues about many things and in particular makes this comment...

    "Does the relational view have implications broader than the nature of space and time? I will argue that it does."
    -- http://arxiv.org/PS_cache/hep-th/pdf/0507/0507235v1.pdf

    For me personally the most intuitive abstraction is in terms of information, and to me space is just in a sense a kind of "indexing" of distinguishable events. Two points that can't be distinguished, are the same IMO. So the coordinates themselves has no physical content. There are merely labels serving hte purpose of an index. Here I agree with the relational sentiment of classical GR. But space indexes are not in a fundamental way different than indexing of any events. And this entire notion makes no sense unless there is an observer. The distinction between spacetime and either degrees of freedom is ambigous - or at least I don't understand why not. Consistency of the information view, suggest unification of information. The ad hoc separation of degrees of freedom should have a deeper justification.

    So for me "background" generically means any kind of _background information_, since that's my way of thinking. Perhaps less common, but anyway.

    Classical GR is not formulated in a clear information abstraction, so interpretations is hard. What bothers me is the notion of the 4D-manifold to start with. This has a clear information content. So it's not background independent in the deeper sense I seek.

    Somehow, I think "including" QM, means recasting the entire thinking in information terms. I guess I here reveal my interpretation of QM, but to me the essence of QM is the information concepts, and we are dealing with the physics of information. The problem with QM I see is that if fails to realise that realistic information, have capacity bounds - here I think GR comes with good clues indeed.

    As I see it, neither classical GR nor standard QM as it stands is satisfactory. So at least from my point, none will escape the new synthesis without modification. So I don't think we need to choose between starting with GR and add QM, or start with QM and add GR. Neither such extremes way is plausible to my amateur view.

  16. May 27, 2008 #15


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    In any case, this book is going to open us some deep questions. I will bet that you will like Gerard 't Hooft's chapter. He faces fundamental issues squarely (as you seem to try always to do).
    I haven't seen any draft or preprint of 't Hooft's contribution to the book

    BTW I see that Amazon has a page already. And they give the date it is to be available as February 2009.

    The publisher catalog now also says February 2009, if ordered from the US distributor---the date has been advanced since the last time I looked:

    That is interesting, a couple of days ago they said available May 2009, now they say February 2009, and for the UK market they also give a price (in pounds)
    so things are shaping up.
    Damned expensive book though!
    Last edited: May 27, 2008
  17. May 27, 2008 #16


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    In the Table of Contents 't Hooft's contribution comes right after Rovelli
    "2. The fundamental nature of space and time G. ‘t Hooft"

    He doesn't beat around the bush, does he? :biggrin:
    Goes right to the heart of the matter without hesitating a moment.

    Here is the TOC in case anyone is coming in new to the discussion
    Last edited: May 28, 2008
  18. May 28, 2008 #17


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    > I haven't seen any draft or preprint of 't Hooft's contribution to the book
    > [...]
    > "2. The fundamental nature of space and time G. ‘t Hooft

    I couldn't find ′t Hooft's paper anywhere either. The book sure is a bit expensive but if it's good I wouldn't mind buying it, but february next year is a long time to wait :|

    It would be interesting to know if anyone is aware of any kind of preprint of 't hooft's chapter.

  19. May 28, 2008 #18


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    I found two older Papers of 't hooft that may or many not help expose his thinking.

    -- http://arxiv.org/abs/gr-qc/9903084

    I skimmed this too quick, but it seems he makes use of the notion of equivalence classes of states. And I suspect it's related to what I think of as distinguishable states, which by construction is the equivalance class of all states that are indistinguishable from any other state in the class - and this entire construct is relative to an observer which provides the context and logic of what it means to distinguish two states. But I need to read again where he puts the observers... and how we pictures the details, I didn't get it while skimming. Maybe another thing to get back to later.

    The class thinking is interesting, and in particular the dynamics transition between different microstructures of classes. That is in line with my thinking. the main headache is the massive self reference that seems hard to get away from. How to break circular reasoning, and instead turn it into a constructive of evolutionary reasoning where more or less the only background we have, is the part of the our history that we have retained in some coded form.

    "The mathematical basis for deterministic quantum mechanics"
    -- http://arxiv.org/abs/quant-ph/0604008

    This I haven't read yet, just found it.

  20. May 31, 2008 #19


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    Fra I think we are both guessing what previous 't Hooft writing will seem to us closest to the topics and reasoning in his new essay The fundamental nature of space and time which we have not seen.

    I will tell you my guess. Just a quick guess---I did not spend so much time on it. I think the previous writing to read and think about is a very EASY TO READ essay from December 2005 which he wrote for a magazine Physicsworld

    (from my little knowledge of you, Fra, I suspect you may already know this essay well.)

    I think this contains the essential 't Hooft message which he will put in this book. But since it is 3 years later, he will carry it further. Tell me if my guess turns out wrong:wink:

    I will highlight a later part, but give some of the reasoning with which he leads up to it.

    Just like all other successful theories of nature, the Standard Model obeys the notions of locality and causality, which makes this theory completely comprehensible. In other words, the physical laws of this theory describe in a meaningful way what happens under all conceivable circumstances. The standard theory of general relativity, which describes the gravitational forces in the macroscopic world, approaches a similar degree of perfection. Einstein’s field equations are local, and here, cause also precedes effect in a local fashion. These laws, too, are completely unambiguous.

    But how can we combine the Standard Model with general relativity? Many theorists appear to think that this is just a technical problem. But if I say something like "quantum general relativity is not renormalizable", this is much more than just a technicality. Renormalizability has made the Standard Model possible, because it lets us answer the question of what happens at extremely tiny distance scales. Or, more precisely, how can we see that cause precedes effect there? If cause did not precede effect, we would have no causality or locality - and no theory at all.

    [Fra, he is talking about the chaotic small-scale geometry that Renate Loll experiences in the computer models, where even dimensionality seems poorly defined]

    Asking both questions in quantum gravity does not appear to make sense. At distance scales small compared with the Planck scale, some 10-33 cm, there seems to be no such thing as a space-time continuum. That is because gravity causes space-time to be highly curved at very small distances. And at small distance scales, this curvature exceeds all bounds. But what exactly does this mean? Are space and time discrete? What then do concepts such as causality and locality mean? Without proper answers to such questions, there is no logically consistent formalism, not even a quantum-mechanical one.

    One ambitious attempt to combine quantum mechanics with general relativity is superstring theory. However, I am unhappy with the answers that this theory seems to suggest to us. String theory seems to be telling us to believe in "magic": it is claimed that "duality theorems", which are not properly understood, will allow us to predict features without reference to locality or causality. To me such magic is synonymous with "deceit". People only rely on magic if they do not understand what is really going on. This is not acceptable in physics.

    In thinking about these matters, I have reached a conclusion that few other researchers have adopted: the problem lies with quantum mechanics, possibly with general relativity, or conceivably with both.

    Quantum mechanics could well relate to micro-physics the same way that thermodynamics relates to molecular physics: it is formally correct, but it may well be possible to devise deterministic laws at the micro scale. However, many researchers say that the mathematical nature of quantum mechanics does not allow this - a claim deduced from what are known as "Bell inequalities". In 1964 John Bell showed that a deterministic theory should, under all circumstances, obey mathematical inequalities that are actually violated by the quantum laws.

    This contradiction, however, arises if one assumes that the particles we talk about, and their properties, are real, existing entities. But if we assume that objects are only real if they have been precisely defined, including all oscillations as small as the Planck scale - and that only our measurements of the properties of particles are real - then there is no blatant contradiction. One might assume that all macroscopic phenomena, such as particle positions, momenta, spins and energies, relate to microscopic variables in the same way thermodynamic concepts such as entropy and temperature relate to local, mechanical variables. Particles, and their properties, are not (or not entirely) real in the ontological sense. The only realities in this theory are the things that happen at the Planck scale. The things we call particles are chaotic oscillations of these Planckian quantities. What exactly these Planckian degrees of freedom are, however, remains a mystery.

    [Fra, chaotic oscillations of geometry sounds to me a bit like Yidun Wan and Song He "braid-matter". particles are tangles in the web of geometric relationships]

    This leads me to an even more daring proposition. Perhaps general relativity does not appear in the formalism of the ultimate equations of nature. In making the transition from a deterministic theory to a statistical - i.e. quantum mechanical - treatment, one may find that the quantum description develops many more symmetries than the deeper deterministic description.

    Let me try to clarify what I mean. If, according to the deterministic theory, two different states evolve into the same final state, then quantum mechanically these states will be indistinguishable. We call such a feature "information loss". In quantum field theories such as the Standard Model, we often work with fields that are not directly observable, because of "gauge invariance", which is a symmetry. Now, I propose to turn this around. In a deterministic theory with information loss, certain states are unobservable (because information about them has disappeared). When one uses a quantum-mechanical language to describe such a situation, gauge symmetries naturally arise. These symmetries are not present in the initial laws. The "general co-ordinate covariance" of general relativity could be just such a symmetry. This is indeed an unusual view on the concept of symmetries in nature.

    Nature provides us with one indication that perhaps points in this direction: the unnatural, tiny value of the cosmological constant Λ. It indicates that the universe has a propensity to stay flat. Why this happens is a mystery that cannot be explained in any theory in which gravitation is subject to quantum mechanics. If, however, an underlying, deterministic description naturally features some preferred flat co-ordinate frame, the puzzle will cease to perplex us. There might be another example, which is the preservation of the symmetry between the quarks in the subatomic world, called charge-parity (CP) symmetry - a symmetry that one would have expected to be destroyed by their strong interactions.

    The problem of the cosmological constant has always been a problem of quantum gravity. I am convinced that the small value of Λ cannot be reconciled with the standard paradigms of quantized fields and general relativity. It is obvious that drastic modifications in our way of thinking, such as the ones hinted at in this text, are required to solve the problems addressed here.


    Fra, this could almost have been written for Oriti's book because it is a collection of approaches many of which are "drastic modifications" of our way of thinking about space and time. Where he says "such as the ones hinted at in this text" he could well have instead said "such as the ones explicitly discussed in this book edited by Oriti"

    So this is the flow of thinking which I think the new essay of 't Hooft might follow.
    What do you think?
    Last edited: May 31, 2008
  21. Jun 1, 2008 #20
    I am quite fond of consistent histories (article 5, Savvidou); one can find an idea of the content at http://arxiv.org/find/gr-qc/1/au:+Savvidou/0/1/0/all/0/1 . This started out as an interpretation of QM (Gell-Mann & Hartle, Omnès), but was then turned into a calculational framework by Isham an collaborators; I think Ntina Savvidou might be a former Isham student. The idea, at least in my related work like http://arxiv.org/abs/0709.2540 , is to regard dynamics as a constraint in the space of arbitrary histories, and quantize first before imposing the dynamics constraint a la BRST.

    As for the title of Rovelli's piece, the revolution is as unfinished today as it was incomplete in 1999, see http://arxiv.org/abs/hep-th/9910131 .
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