Approaches to Quantum Gravity (now in Cambridge Press catalog)

[marcus]

the publication date for the book is given as May 2009
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521860451

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)
Percacci is ASYMPTOTIC SAFETY
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.

 

[marcus]

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

SUMMARY:
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

TABLE OF CONTENTS

Preface;
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;

Index.

===============================
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.

 

[marcus]

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
http://arxiv.org/abs/0709.3851
Asymptotic Safety
R. Percacci

one way to dig up these things is to use the arxiv search engine
http://arxiv.org/find/grp_physics/1/co:+AND+Oriti+AND+gravity+AND+approaches+quantum/0/1/0/all/0/1

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

 

[marcus]

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

 

[marcus]

Now we come to the meat course 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
http://arxiv.org/abs/gr-qc/0604045
Unfinished Revolution
Carlo Rovelli

and another
http://arxiv.org/abs/gr-qc/0609108
Prolegomena to any future quantum gravity
John Stachel

18 so far, by my count

and two more, making twenty so far
http://arxiv.org/abs/gr-qc/0602120
Categorical Geometry and the Mathematical Foundations of Quantum General Relativity
Louis Crane

http://arxiv.org/abs/gr-qc/0703099
Does Locality Fail at Intermediate Length-Scales?
Rafael D. Sorkin

have to go back later

 

[ccdantas]

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:


[hep-th/0604120]
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

[hep-th/0604130]
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,
Christine

 

[ccdantas]

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.Christine

Edit: Indeed, from the table of contents, she seems to have disregarded or expanded the 2006 article to the one that you link.

 

[Fra]

reflection of rovelli's key question

BTW links still coming in, here is another
http://arxiv.org/abs/gr-qc/0604045
Unfinished Revolution
Carlo Rovelli

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).

/Fredrik

 

[marcus]

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.

I don't know if Fotini replaced her article of my previous post by the one you link, which is more recent,..

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.)

 

[marcus]

==quote==
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”.
==endquote==

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

==quote==
...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”.
==endquote==

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.

 

[jal]

Extra info on Rafael D. Sorkin
http://arxiv.org/abs/gr-qc/0703099
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
http://arxiv.org/abs/0710.1675
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
-------
http://www.pascos07.org/programme/
PASCOS-07 Conference
---------
Compare
http://en.wikipedia.org/wiki/Causal_sets
The causal sets programme is an approach to quantum gravity.
WITH
http://en.wikipedia.org/wiki/Voronoi_diagram
Voronoi diagram
------------

 

[Fra]

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.

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.

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.

/Fredrik

 

[marcus]

I
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,

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
http://books.google.com/books?id=vD...AibQM3W&sig=AqrLcrvuIEGOlH8J-_1IAohyW4E&hl=en
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%20files/Besso-memo.pdf[/URL]

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

 

[Fra]

background information

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.

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.

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

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 realize 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.

/Fredrik

 

[marcus]

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 bookBTW I see that Amazon has a page already. And they give the date it is to be available as February 2009.
https://www.amazon.com/dp/0521860458/?tag=pfamazon01-20

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:
http://www.cambridge.org/us/catalogue/catalogue.asp?isbn=9780521860451

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)
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521860451
so things are shaping up.
Damned expensive book though!

 

[marcus]

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?
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
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521860451

 

[Fra]

> 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.

/Fredrik

 

[Fra]

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

"QUANTUM GRAVITY AS A DISSIPATIVE DETERMINISTIC SYSTEM"
-- 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.

/Fredrik

 

[marcus]

> 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.

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

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

http://physicsworld.com/cws/article/print/23668
(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

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

==exerpt==
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 let's 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.


==endquote==

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?

 

[Thomas Larsson]

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 .

 

[Fra]

There are many interesting reflections put forward by various people in http://physicsworld.com/cws/article/print/23668, but predicting what their next move is, seems hard still :)

't Hooft has, as I understand him, the idea the QM could related to micro-physics like classical thermodynamics relates to classical deterministic micro-physics. He envisions many macroscopic notions such as position, spin, energy to relate to such microscopic variables. And that spacetime notions and GR may emerge from some micro-dynamics.

That is partly appealing but still makes me suspect. I don't directly connect to this. I fear is that he microstructure required to make the connection, requires a more massive observer - which means we are trying to ask a question that doesn't fit in our limited complexity (here associate to Paul Davies comments). The way I can appreciate it, is if the micro-dynamics takes place in the observers microstructure, and in that sense I guess the degrees of freedom corresponds to the distinguishability limit of the observer. But I don't understand how a sensible determinism can be established at this level. I am not sure if this is what Hooft means. I haven't had time to read his other papers.

Reading what the other authors said in http://physicsworld.com/cws/article/print/23668 I think there may be a common exploit to the
1) a sort of history approach (extension of feynmanns path integrals)
2) holographic principle
3) the complexity issues Paul davies mentions

In my thinking, a synthesis of these three ideas would be interesting. A sort of history approach that respects the complexity bound. This is a flaw in the normal path integral as I see it, and most certainly the reason for infinites. If the construct is made at the correct level of observation from start, I don't see that special renormalisation schemes should be necessary. The holographic principles and complexity bounds are IMO possible keys to make better sense of the history approaches. But to make sense out of it the construction must probably take off prior to introducing notions of space and time. Hopefully the system will on it's own develop the distinguishable microstructures that can later be identified as space and time. Maybe the actions of standard model also will be found as emergent properties. If there is a simplest possible action (I mean logic of reasoning) then one would expect that this represents the properties of the simplest possible systems (ie. "elementary particles"). So that the logic implemented in the selected microstructures would in a sense represent the "logic of the elemetary systems" and thus determine how it's interaction properties are. I would expect also gravity here to emerge as a complexity phenomenon, relating to complexity, which explains it universality. If there is a utility for any system to take control of complexity from it's environment (ie. to learn; if you can predict the environment, it means you can also control it! - and in effect use it for data storage), and that should result in a sort of attractive force.

I am going to reflect over these things in the nearest future and perhaps read up on 't Hooft. I must admit I do not like the sound of inventing deterministic microphysics. I think it's going the wrong way.

/Fredrik

 

[friend]

I am going to reflect over these things in the nearest future and perhaps read up on 't Hooft. I must admit I do not like the sound of inventing deterministic microphysics. I think it's going the wrong way.

It might be that at the most basic level all events determine all other events in a deterministic way. But propbability may be unavoidable simply because WE cannot measure every event at the same time. QM may come from having to take into account events we can't measure. But on the scale of everyday life, we can actually observe (measure) all the things that effect our life so that a deterministic relationship seems to be in effect.

PS. Is this a good thread to ask if the various approaches to QG comply with my recent understanding of what it should be?

 

[marcus]

... 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 ...

this morning you started me looking at Savvidou's work. Yes her 1999 PhD thesis on arxiv has Chris Isham as her advisor. Until 2002 she used her full first name Konstantina and then changed to the short form Ntina, so at first I had trouble finding all her papers on Spires.
I am impressed by her February 2006 one and even hopeful.

She's good and she seems to be on to something. I will copy the 2006 one's abstract here, for definiteness. Even though the title is different I strongly suspect that besides reviewing her consistent histories approach to Quantum Gravity, her chapter in Oriti's book will parallel what is in this article

http://arxiv.org/abs/gr-qc/0602021
The Barbero connection and its relation to the histories connection formalism without gauge fixing
Ntina Savvidou
23 pages
(Submitted on 7 Feb 2006)

"We present a histories version of the connection formalism of general relativity. Such an approach introduces a spacetime description--a characteristic feature of the histories approach--and we discuss the extent to which the usual loop variables are compatible with a spacetime description. In particular, we discuss the definability of the Barbero connection without any gauge fixing. Although it is not the pullback of a spacetime connection onto the three-surface and it does not have a natural spacetime interpretation, this does not mean that the Barbero connection is not suitable variable for quantisation; it appears naturally in the formalism even in absence of gauge fixing. It may be employed therefore, to define loop variables similar to those employed in loop quantum gravity. However, the loop algebra would have to be augmented by the introduction of additional variables."

 

[Fra]

It might be that at the most basic level all events determine all other events in a deterministic way. But propbability may be unavoidable simply because WE cannot measure every event at the same time. QM may come from having to take into account events we can't measure.

I see the logic you advocate here, but IMHO I don't think it's the best one. My objection is along the lines that the complexity constraints that apply to formulating a question, also apply to finding the answer.

I have seen that a logic of reasoning that seems very popular that in order to answer a question is that additional information is added, an in this richer setting the answer is easy, then the answer is "projected" back to the original context. But that kind of reasoning comes out as very strange to me.

One example is to imagine the state of the entire universe, and use that to explain why a system behaves the way it does. But that makes little sense to me. Because that is an answer the little people like me can not distiniguish. And conclusion based on information I do not have is taking place beyond me, and this fundamentally fails to answer _my_ questions.

But it's hard to tell what 't Hooft means. I could misunderstand him. In one way I can share the idea that microphysics can explain things in a better way. But instead of talking about hidden information, I'd talk about subjective information. Information that is hidden from say one side of the screen, is not from the other side. For example, I can imagine that there is a logic to the emitted radiation from a black hole. If you see if from the point of view of the black hole. But the reasoning taking place inside makes no sense to the outside. So I think the logic living on the inside does not make sense to the outside.

I think excessive inconsistencies are bound to appear if we mix logic from different observers.

/Fredrik

 

[marcus]

Two months ago, when I started this thread, the target date for publication was May 2009.
And no price for the book had been set.
Often times with academic books, the publication date SLIPS because the authors need to revise and there is delay stemming from the complexity of the material. lots of mathematics, footnotes, bibliography, addenda.

But in the past two months, for what it's worth, this book's publication date has ADVANCED and is now earlier.
Now it is listed as March 2009. In this case the opposite of the slippage that one expects.
And the price in the Europe market is listed as 60 pounds UK. that is expensive but at least they are far enough along to quote a price.
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521860451
to get information about regional price and availability, you go to this page and click on the Europe market, or whatever other, like the US market.

What I think about this is that the QUESTION AND ANSWER chapters at the end of each section could be interesting. That is where leading experts from different competing approaches get to question each other and put each other on the spot about possible difficulties with the other guy's approach. Or if they don't understand something, they can ask for clarification. It could be constructive if the editor, Oriti, can just get it to happen with enough of the authors.

Here's a corrected version of the TOC:
TABLE OF CONTENTS

Preface;
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 S. Majid;
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;

Index.

 

[Chronos]

No secret I'm on board with Smolin, et. al. ST might work if you are willing to take the long road, but, non-pertubative gravity is the most efficient 'road' for now. At worst, it is an excellent approximation.

 

[marcus]

Chronos, thanks for the comment. One thing I notice about this book is that String is included among the various approaches---and prominently represented: Tom Banks and Joe Polchinski are famous stringers, top flight. Gary Horowitz and Washington Taylor are highly regarded as well.
So whatever else the book accomplishes, it will help to put an end to the "camp mentality" where one crowd refuses to discuss with the other and makes a show of looking down their noses.

I think you can see the end of the camp mentality period in little things like this: Loops 05 invited Robbert Dijkgraaf (great guy, great speaker, a top string theorist) to speak in plenary session, and likewise Moshe Rozali at Loops 07. Now this year Strings 08 has invited Carlo Rovelli. Unprecedented for the annual Strings conference to do this. And look at the lineup at the April 2008 Bad Honnef workshop on Quantum Gravity. It is a real mix. People are talking to each other and (seemingly) listening to each other, at least in the European context.

Bad Honnef is in Germany. String 08 is at CERN. Oriti, this book's editor, is at Utrecht.

Whereever, and at whatever pace this is happening I welcome it.

the trouble with this book is the slowness of academic book publication. it takes so long to get a 600 page scholarly book assembled and edited and printed

I will get the speakers list for the Bad Honnef QG meeting, so you can see the non-campy mix. It's an eye-opener
Here's a PF thread about it:
https://www.physicsforums.com/showthread.php?t=241873
Here's a conference webpage:
http://quantumgravity.aei.mpg.de/program
Here is the list of speakers where I have bolded the names of non-string QG theorists whose work I normally follow, so that the non-string folks show up.

I. Adam: Superstring perturbation theory
A. Ashtekar: Loop quantum cosmology: a status report
R. Blumenhagen: The landscape of string vacua
B. de Wit: Supergravity and M theory
L. Freidel: Status of spin foam models
S. Giddings: Black Holes, high energy scattering, and locality
H. Hamber: Lattice quantum gravity
M. Henneaux: Cosmological billiards and hidden symmetries of gravity
C. Kiefer: Quantum Geometrodynamics: whence, whither?
D. Marolf: Black Holes, AdS, and CFTs
K. Meissner: Background independence: a particle physicist's view
M. Reuter: Asymptotically safe quantum gravity
K. Stelle: Finiteness (or not) of N=8 supergravity
T. Thiemann: Loop quantum gravity

Discussion/Summary: Ashtekar, Blau, Giddings, Reuter, Stelle

The PDF slides for many of the talks are available for download.

I think I see maybe 4 specifically string talks here, maybe other people can point out others. I would say
the talks by Adam, Blumenhagen, de Wit, and Henneaux. Would have to check Henneaux since the title is not
a clear giveaway. Glad if anyone wants to help out.

 

[marcus]

I want to remind readers about post #19
https://www.physicsforums.com/showthread.php?p=1750840#post1750840
and invite anyone coming fresh to this thread to read it.

We have a lot of the chapters of this book in preprint draft form but one notable chapter we don't have is Gerard 't Hooft The Fundamental Nature of Space and Time.

I think that basically this essay will set the bar high, for what a theory of QG has to do.

I located an earlier essay of 't Hooft which I suspect provides a basis for what we can expect his chapter to say. It is his vision of what QG has to be, and it is a remarkable statement.

I quoted the earlier 't Hooft essay at length in #19.

There's good logic for why the chapters by Rovelli and 't Hooft come right at the beginning of the book.

I'd be glad of any help locating some other writing by 't Hooft (on that theme, namely what could be the fundamental nature of space and time) which could give us some foretaste of what he is likely to say in that chapter.

 

[Emanuel]

This may be an odd suggestion, but do you suppose we could ask him? (Gerard 't Hooft) From his website he seems approachable enough. (I also happen to study at Utrecht University, though my major is CS, not Physics) This book seems very exciting :)

 

[Fra]

Yesterday I decided to try to read up more on hooft's reasoning in general, by looking at what he's publised. If I find anything relevant to this I'll get back with a post.

/Fredrik

 

[marcus]

This may be an odd suggestion, but do you suppose we could ask him? (Gerard 't Hooft) From his website he seems approachable enough. (I also happen to study at Utrecht University, though my major is CS, not Physics) This book seems very exciting :)

I think that is an excellent idea. He may have a draft of the chapter which he would be willing to share with others. Or he might reply that his contribution to the book is along the same lines as his essay in the December 2005 Physicsworld:
http://physicsworld.com/cws/article/print/23668
Any response would be helpful.

As a student at his university, you seem like a good person to write him an email asking about this. I think you are right about his website---it shows him wanting to educate and encourage students. I expect he would like to hear from a young person who is interested in what he has to say on that general topic (approaches to quantum gravity, investigating the fundamental nature of space and time.)

I don't believe he would be offended by being asked. Whether he says yes or no, I think he would be glad to be asked.

 

[Fra]

't Hooft ideas

I skimmed 't Hooft's

(1) The Free-Will Postulate in Quantum Mechanics
-- http://arxiv.org/abs/quant-ph/0701097

(2) The mathematical basis for deterministic quantum mechanics
-- http://arxiv.org/abs/quant-ph/0604008v1

(3) Emergent Quantum Mechanics and Emergent Symmetries
-- http://arxiv.org/abs/0707.4568

Unlike many others, 't Hooft sticks out by acknowledging some fundamental issues in QM.

From source (1) he writes

"An essential element of superstring theory is the assertion that it should be an allembracing
theory, a “theory of everything”. If so, then it should also, in a natural manner, answer some of the questions concerning quantum mechanics itself. What is Quantum Mechanics? Should a “theory of everything” not also explain (at least in principle) how all those fluctuations in space and time could have emerged, what their origin might be, in terms of rigorous equations of motion, how exactly galaxies, stars and planets came into being?..."

"...Superstring theorists thus-far have turned their backs to the issues raised by those who try to understand Quantum Mechanics at a deeper level. It is time to face these questions."

Regardless of the answers I like the way he doesn't hide from this issues! Considering that he seems to be among the exceptions to ask such questions, that alone motivates me to learn more about his thinking.

Further down in his papers I find some interesting things but I'm not sure I understand exactly what he has in mind. It seems he has some desire to restore a higher level of determinisim in QM. I am not sure how to interpret that, but after skimming those papers again, I suspect that he mainly means that there is a yet uncovered logic that will - not predict each outcome instead of probabilities - but that will EXPLAIN the emergence of the QM formalism. If that is so, I think it is great and I'm all in on that.

He also expresses in (1), reflecting over things like quantum logic, that

"We insist that there exists only one kind of logic, even if the observed phenomena are difficult to interpret."

I am not sure I know what he means with that. I guess he means that there should be a unified logical framework in which the two are related?? Ie. there should be a logical connection between classical and quantum? That makes sense.

He is also talking about the notion of unconstrained initial state and wether there are any constraines on which measurements a given observer "chooses" to do. Maybe I didn't understand it but there seems to be a lot left to do. But I got some good associations to this and it could relate to the general learning problem of "given our latest answers, what questions to ask next"

In a certain sense, we are "free to ask" any question we can come up with, but that doesn't mean that we, as seen from a third observer, will fire random questions. How can both be true?

So we have the "choice" to ask any question, but not all questions are of equal utility. Some choices are more beneficial than others. So it is somehow related to the question of "we have the choice to live or die", but most of us choose to live, why? ;-)

This might fit well into a evolutionary perspective. To me this might be keys to a "constructive logic".

I'm curious too see if you guys can get any drafts from Hooft that would be great.
I will stay tuned.

/Fredrik

 

[marcus]

A closer look at part of the 't Hooft Physicsworld essay

I skimmed 't Hooft's
...

(3) Emergent Quantum Mechanics and Emergent Symmetries
-- http://arxiv.org/abs/0707.4568

Unlike many others, 't Hooft sticks out by acknowledging some fundamental issues in QM.
...

I appreciate that you are trying to see what 't Hooft's essential train of thought is---that will probably come out in his chapter of Oriti's book about the fundamental nature of space and time. And so you are looking at past writing and abstracting ideas from it. I will try to do this too. Here is part of the essential message of that Physicsworld essay I quoted earlier.

...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.

... Just a quick guess...essay from December 2005 which he wrote for a magazine Physicsworld

http://physicsworld.com/cws/article/print/23668==exerpt==

...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.
...
...[THESIS]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.

[ANTITHESIS]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.

[SYNTHESIS]This contradiction, however, arises if one assumes that the particles ...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.
...
...
==endquote==

This essay contains a number of different exciting ideas. One of them is what I have exerpted here. It is a simple "hegel dialectic" type argument.

A. because of Heisenberg, spacetime is uncontrolled chaotic at small scale so it isn't even a smooth continuum. quantum mechanics and gen rel are only regular APPEARANCES that arise by averaging out, at larger scale. there are probably underlying micro degrees of freedom obeying deterministic laws-----like the deterministic micro picture that underlies thermodynamics.

B. but Bell-type arguments appear to say that there cannot be deterministic micro dynamics underlying quantum mechanics

C. however the Bell-type reasoning only works if particles exist at fundamental level.
suppose that when you get down to Planck scale, particles are just tangles or oscillations in what is really there. then we get around the Bell objection and we can try to construct some micro deterministic degrees of freedom from which the appearance of space time and matter arise by smoothing and averaging at larger scale (like the gas law arises from molecular dynamics, like pressure arises from collisions)

 

[Fra]

About the the part of 't Hooft's writing that Marucs exerpted. For the sake of discussion, let's assume that his line of reasoning so far are good. What's the next step?

He says himself
"What exactly these Planckian degrees of freedom are, however, remains a mystery."

So what is the physical basis for and the logic of these degrees of freedom and what is their observational connection? And how does the answer to that merge with the general information theoretic and relational information picture with the holographic ideas?

I don't know what you think but I always associate a sensible notion of degree of freedom with the notion of distinguishability. The degrees of freedom are associated to the number of - relative to a particular observer or logic - distinguishable (micro)states.

This on one hand, seems to suggest that if we consider all arbitrary observers, the notion of degree of freedom would be different?

But then what is the microstructure whose elementa correspond to these Planckian degrees of freedom, which would provide an (at least effective) universal microstructure?

Considering hooft's idea on the holographic principle one might suspect that he has interesting ideas on this. Maybe, Planck degrees of freedom somehow refers to communication channels, so that it's "communicated" degrees of freedom so that spacetime is more to be seen as a communication channel which develops self-organising structures.

Then what is it that is beeing communicated? And why?

But as I read Hooft there is no clear observer or relational notions in his reasoning. But he talkes about local determinism, and I wonder how to interpret local in this context.

Could he mean local in some space of observers? So that this could be interpreted in terms of relational logic? So that to a local observer, there is a possibly deterministic logic to the actions, but that this logic (the mecahnics) is not distinguishable to a remote observer.

Edit: what I mean here is like the assoication of a network of players trying to win, where the rules of the game are always in moution and the strategy of the other players aren't known. Yet it may seem like the best rational choice, to suspec that the other players also behave rationally. But then the rationality may be hidden since this isn't known. So that each player may "act rationally", but the actions of all other player "appear" irrational to a single player. Could Hooft be after something like that? so that

local determinism ~ local logic ~ local rationality

?

Because if space is a communication network, then it seems natural to suggest that local objects are like transceivers, or communicating observers? And that from each local point of view, there is a clear logic, but it is not possible to see the clear logic of a remote information processing node?

It might suggest that for a realistic simulation, each node would have to be intelligent, and somehow independently running simulation??

This could be a bit of a stretched interpretation of his writing though.

What other possibilities are there to the Planckian degrees of freedom? And WHO is supposed to answer that question? ( I don't mean which of us formum members :) I mean relative to what system/observer is the question formulated, or is that not a relevant question)

/Fredrik

 

[Emanuel]

Sorry about the delay guys, here's the e-mail I just sent to Gerard 't Hooft:
(Dutch)
"Nu bekend is dat dit boek (Approaches to Quantum Gravity) in maart volgend jaar uit komt en de definitieve index online beschikbaar is, loopt op Physics Forums een discussie over de bijdragen van de verschillende wetenschappers die eraan meegewerkt hebben. We hebben al een ruwe versie van zo'n twintig hoofdstukken gevonden, maar we zijn erg benieuwd naar wat u te zeggen heeft in 'The fundamental nature of space and time'.
Aangezien ik toch aan de UU studeer (hoewel in een andere faculteit) leek het mij een goed idee om het u gewoon direct te vragen: kunt u een samenvatting geven van wat u in uw hoofdstuk bespreekt? (of anders een artikel of publicatie waarin u hetzelfde overbrengt)
Hoe dan ook ben ik zeer benieuwd naar dit boek in zijn uiteindelijke vorm."

(English)
"Now that we know that this book (Approaches to Quantum Gravity) will be released in March next year and the definitive table of contents is available online, there's a discussion on Physics Forums about the contributions of the various scientists involved. Already we've gathered drafts of some twenty chapters, but we're quite curious about what you have to say in 'The fundamental nature of space and time'.
As I study at the University of Utrecht - though in a different faculty - I thought it would be a good idea to simply ask you directly: could you give a summary of what you discuss in your chapter? (alternatively perhaps an article or a paper in which you convey the same)
Either way, I'm greatly looking forward to seeing this book in published form."

If the first line seems awkward, the subject line was 'Approaches to Quantum Gravity' :) (for other oddities, blame difficulty in translating from Dutch to English)