Back to the real world re Lorentz bending

[marcus]

As far as I know there is no firm prediction of any Lorentz bending derived from mainstream Loop.
The authorities on what Loop says or doesn't say are Ashtekar, Rovelli, and Freidel. These are the people that organizers of any major world conference on gravity and related science would normally invite to give the Loop plenary talk.

Examples of relevant conferences: The Marcel Grossmann meetings are held every three years. Most recent one had over 600 participants. Top committee includes Hermann Nicolai and Steven Weinberg.
The other main series is the GRG (General Relativity and Gravitation), which also happens every three years and gets about the same number of participants. The most recent one, GRG 18, was held in 2007.

At Marcel Eleven, in Berlin 2006, Ashtekar gave a Loop overview and FAQ to plenary session.
Marcel Twelve, in Paris 2009, had Freidel give the Loop plenary talk, while Ashtekar chaired the session.
Strings 2008, in Geneva, had Rovelli give a Loop overview in plenary session.
GRG 18, in Sydney 2007, had Freidel give the Loop plenary talk, and GRG 19, planned for next year, has invited Rovelli to do the same.

Major international conferences like this help to define and identify the mainstream. At the really big ones like Marcel and GRG there will be many Loop talks presented in several split-off parallel sessions, plus one or a small number of main talks in plenary session where all the participants are gathered in one hall.

None of these talks would have indicated that Lorentz bending had been derived from Loop as a prediction, because that is simply not the case. Humanino already cited Ashtekar's 2006 talk, and pointed this out. It's on line. Rovelli's talk to Strings 2008 is on line. Freidel's Marcel Twelve slides are on line. If anyone wants links please ask me.

Currently the authoritative review article on LQG is this one by Rovelli dated May 31, 2008.
http://relativity.livingreviews.org/Articles/lrr-2008-5/
There is no competing review that is equally current.

 

[marcus]

I would not expect what we have been hearing about on-time photons and delayed photons to have much effect on main trends in the quantum gravity research community. There was no firm prediction about energy-dependent slowing derived.

In fact Loop people tried hard around 2005-2006 to derive such a prediction. There were hopes that it might be possible---this uncertainty is reflected in Ashtekar's plenary talk at the Marcel Eleven Berlin conference. But by 2007 most had given up and were busy with other things (the new spinfoam model.)

When the 2007 MAGIC report of some delayed high-energy photons came out, it was stringy people (Ellis, Mavromatos, Nanopoulos) who said "our model predicted it!" The loop people said not a word. Their quietness struck me as strange so I went back over the papers and found no definite prediction. Only speculation as to the possibility/plausibility and nothing at all by central figures like Ashtekar and Rovelli.

So in the Loop case these gamma photon observations from Fermi-LAT or from MAGIC neither confirm or falsify. However they turn out, they might conceivably help researchers in the future by guiding exploration.

What I am saying is of the nature of a prediction itself. I would predict that the main research trends now in progress will continue unchanged.

I want us to be able to check, six months or a year from now, and see if this prediction was correct or not. So I'll try to say explicitly what the trends are, that characterize today's situation.

You are welcome to add your own view as well---regarding the main QG research currents.

 

[marcus]

One general trend I guess would be the tendency for top stringy people to moderate or shift their research interest. In some interesting cases prominent string or former string people are shifting into 4D gravity theories

An example is Petr Horava, who has his own 4D gravity idea now. Formerly a top string theorist, co-author of Witten's.

Another example is Steven Weinberg, who has a renewed research interest in AsymSafe QG, which is 4D.

There is also Hermann Nicolai, one of Europe's top string theorists, who is now working on 4D based QFT, with co-author Kris Meissner. See his talk at the Wroclaw Planck Scale conference video site.

The common theme which I see here is a strategy of relocating QFT and the standard model on a 4D quantum geometry.

Weinberg has stated the goal of a unified theory clearly in his 6 July Cern talk. String, he said, might just not be how the world is. The world might be better described by "good old" QFT based on a better version of 4D spacetime---the one he is currently working on under the AsymSafe rubric. And quite a few others will be joining him at Perimeter in November for a conference on this.

All these gambits are alike in pointing towards a unified theory based on 4D geometry (no extra dimensions) and being undertaken by former string people or those continuing with string part time.

Jan Ambjorn was an earlier example of this, formerly did string and still does now and then. Became a mainstay of Renate Loll's triangulations QG group--which of course is 4D.

My guess is that Steve Giddings' interest is shifting in the 4D direction.

Nima Arkani-Hamed's papers seem to get less and less stringy as time goes on. His talk at Strings 2009 conference this year illustrates this.
================

Anyway that is one trend---a tendency of the most talented people to moderate or shift interest out of (strictly defined) string theory.

Another trend that is very noticeable is the growth in Loop research. Increasing numbers want into the field, especially the Loop Cosmology area.
But this is not unique to Loop. It also is happening with AsymSafe QG. Also a Noncommutative approach to QG which I can't precisely identify but which is represented at next months QG School at Corfu. The school has funding from the European Science Foundation (ESF) and it is interesting to read the lineup. The majority is Loop (Ashtekar, Rovelli, Barrett) but several other approaches are strongly represented. All of them growth areas.
The reason for the school, which is the second such that the ESF has funded, is to enable more PhD students and postdocs to enter the field.
=================

Another trend is more 4D QG jobs. I keep noticing people getting hired. Rovelli started solo at Marseille, but now has two others (Perez and Speziale) and recently advertised a professorship. His group is growing. Renate Loll got that big grant to build up her network and the Utrecht group.
Among the younger, Hanno Sahlmann just got a position at Karlsruhe, Florian Girelli just got a position at Sydney Australia. Daniele Oriti and Bianca Dittrich each got large longterm grants enabling them to build groups of young researchers at the Albert Einstein Institute. Etera Livine got a position at Lyons. Noui at Tours. Laurent Freidel made faculty already 3 years ago. These people were just grad students and postdocs when I first began to pay attention to their work. If you want hard data look at the lists of former students at Rovelli's website and Renate Loll's. Somehow the field is growing.
==================

There are various other trends. Maybe I will think of some others later. But this is just a sample of what I think will not change.
I predict that the recent Gammaray ruckus is essentially irrelevant to Loop health and prosperity, and that whatever we learn about on-time photons or late photons is just icing on the cake and will not change the basics.

The main prospects for testing LQG (perhaps other 4D QG approaches as well) are in cosmology, according to what Rovelli said in his 2008 Loop review paper. And that means primarily in the CMB. So for example the Planck spacecraft that was launched this year is relevant.

So I expect an increasing number of papers deriving CMB features from Loop Cosmology---features that Planck and subsequent space missions can look for.
If my expectations are wrong you are welcome to laugh six months from now, or whenever

If anyone would like links to video lectures or papers that I mentioned here, please ask.

 

[Finbar]

My guess is that Steve Giddings' interest is shifting in the 4D direction.

Why do you think this?

 

[marcus]

Why do you think this?

Look at this school that is happening next month on Corfu.
It is aimed at introducing PhD students and postdocs to the main branches of QG research.
Very up-to-date with the most high power people. Ashtekar, Baez, Barrett, Rivasseau, Rovelli. It will have Loop in force. Barrett is giving the introduction to spinfoams. There will be talks about noncommutative field theory and noncommutative renormalization (which I know little about). There will also be Asymptotic Safety discussion. Martin Reuter will be giving a workshop talk.
http://www.physics.ntua.gr/corfu2009/qg.html

Look who is attending. Giddings. I think he has something to contribute to a primarily 4D QG school/workshop. It could be an important contribution, also he could be curious about some of the other stuff being presented. I can't guess but I respect something about him and his appearing at this unexpected place at this time.

 

[MTd2]

John Baez will focus on the use of categories on string theory. But, I don't think he is shifting to any kind of quantum gravity, not even to LQG. He seems more and more agnostic towards any theory.

 

[marcus]

John Baez will focus on the use of categories on string theory. But, I don't think he is shifting to any kind of quantum gravity,...

Neither do I. I did not mention the school to give a list of people who are shifting interest.

Barrett is not shifting interest that I know of, or Baez, or Rivasseau, or Rovelli, or Ashtekar. Those are the main lecturers at the school. So what is your point about Baez? We don't expect him to be on any particular trajectory, do we?

Please note what I said about shifting interest. I was talking about top string people whose interest in string seems to be moderating. Baez is not in the group I'm talking about. The reason I mentioned the Corfu school here is because I noticed that Steve Giddings is a participant.
Giddings is someone I respect. I noticed in 2007 that he seemed to be shifting out of string, or showing broader research interest. I may be wrong. But he and Don Marolf attended GRG 18 at Sydney in 2007 and my impression is that Giddings' talk was not string. He had something else he wanted to talk about.

Now here two years later he is showing up in Corfu (just three weeks from now!) at a primarily Loop and 4D QG school. Maybe there is some obvious explanation why your average string theorist would do that, and I just don't know the explanation. But it gets my attention.

What I'm seeing these days is specifically something in string theory that seems to be making the top people restless. There is Juan Maldacena as another possible example. He was invited to give a plenary talk at Marcel Twelve in Paris this summer. The talk very much downplayed string. What is happening with these guys?
At Marcel Eleven in Berlin 2006 there were at least six plenary talks that were conspicuously in-your-face string, pushing it and flaunting it. Now in 2009 there is Maldacena who acts like he doesn't even want to be identified string.

 

[Finbar]

Look at this school that is happening next month on Corfu.
It is aimed at introducing PhD students and postdocs to the main branches of QG research.
Very up-to-date with the most high power people. Ashtekar, Baez, Barrett, Rivasseau, Rovelli. It will have Loop in force. Barrett is giving the introduction to spinfoams. There will be talks about noncommutative field theory and noncommutative renormalization (which I know little about). There will also be Asymptotic Safety discussion. Martin Reuter will be giving a workshop talk.
http://www.physics.ntua.gr/corfu2009/qg.html

Look who is attending. Giddings. I think he has something to contribute to a primarily 4D QG school/workshop. It could be an important contribution, also he could be curious about some of the other stuff being presented. I can't guess but I respect something about him and his appearing at this unexpected place at this time.

Giddings does work in higher dimensional black holes at the LHC. So I think he's still very much into higher dimensions. That doesn't mean that he's not interested in non-string approaches to gravity though...

 

[marcus]

Giddings does work in higher dimensional black holes at the LHC. So I think he's still very much into higher dimensions. That doesn't mean that he's not interested in non-string approaches to gravity though...

I'm interested to see how it works out. Could there be a connection with Baez n-category higher-gauge theory? Don't know much about this.

The Corfu school website says that the talks will be posted online. I assume this means at least the lecture series by Ashtekar Baez Barrett Rivasseau Rovelli. I hope it also includes the workshop talks and that we can hear what Giddings is contributing. Such an exciting time! The assumptions in flux and field divisions being redrawn.

 

[marcus]

...Examples of relevant conferences: The Marcel Grossmann meetings are held every three years. Most recent one had over 600 participants. Top committee includes Hermann Nicolai and Steven Weinberg.
The other main series is the GRG (General Relativity and Gravitation), which also happens every three years and gets about the same number of participants. The most recent one, GRG 18, was held in 2007.
...

I checked and can be more accurate as to details. Marcel Twelve (Paris 2009) had 886 participants according to the official conference website.

GRG 18 (Sydney 2007) had over 600, as I recall. Don't have the precise figure.

These large international conferences are part of what defines the mainstream in research fields related to gravity.

 

[atyy]

Now in 2009 there is Maldacena who acts like he doesn't even want to be identified string.

I looked at Maldacena's slides, and it seems to be entirely based on AdS/CFT, which is one of the most amazing things to come out of string theory.

http://www.icra.it/MG/mg12/en/..\talks_plenary\Maldacena.pdf

 

[humanino]

I looked at Maldacena's slides, and it seems to be entirely based on AdS/CFT, which is one of the most amazing things to come out of string theory.

The string-gauge duality does not date from 1997, it has actually been around since the earliest days of string theory. One might even argue that string theory was created to describe hadronic strings. Generally, much historical material is recollected in Polyakov's "gauge field and strings" whose first edition was 1987. It's both a pro and a con for string theory : while Maldacena 97 is a remarkable consistency output, it was in from the inception, and whether it is about Nature (Nature is stringy) or whether it is about mathematics (gauge theories are effectively described by strings) remains a matter of personal interpretation.

edit
I would also like to add, on the gauge side (I guess I need not comment on the large N) the use of conformal symmetry has been developed greatly independently from Maldacena 97
The Uses of Conformal Symmetry in QCD

This subject attracted a lot of interest, enhanced in recent years in connection with the conjecture about AdS/CFT correspondence

 

[marcus]

What did either of you find that was new in Maldacena's Paris talk? Just wondering.
AdS/CFT conjecture has been around for quite a while, and did come out of string research.
A lot of people seem to be focusing on applying the conjecture to studying black holes, these days. Anything significant turning up?

 

[marcus]

One of the things I am commenting on, in connection with Maldacena at Marcel Twelve is the radical change in presentational tone.

Look at the lineup of invited string plenaries for Marcel Eleven (Berlin 2006)
https://www.physicsforums.com/showthread.php?p=2271713#post2271713

==quote==
If you go back to 2006, Marcel Eleven, http://www.icra.it/MG/mg11/ you see these among the invited plenary talks:

Thibault Damour
Cosmology and string theory

Sasha Polyakov
The structure beyond spacetime
(The abstract, though not the title, explicitly says strings.)

Eva Silverstein
Cosmological singularities in string theory

Igor Klebanov
Gauge Theories, Strings and Cosmology

Joe Polchinski
Cosmic superstrings

Dieter Luest
String theory and the standard model of particle physics
==endquote==

Also in the abstracts for these talks, the trumpets were playing. By contrast, 3 years later, only two string-related plenaries invited and Maldacena, in his, makes no explicit reference to string either in his title or in his abstract. In effect the style is toned down. I won't bother to describe the other invited string-related talk, by Veneziano.
Of course AdS/CFT is very interesting and it can be applied as a calculation tool to many different things. Even to things unrelated to fundamental physics and unification. Hermann Nicolai has recently been emphasizing these unrelated applications of the technique. But my point is partly just the new modesty of tone. The abrupt shift in tone certainly gets my attention, don't know about yours.

Here is Maldacena's rather discretely worded title and abstract:
==quote==
Juan Maldacena

Black holes as a source of information

"We will review the duality between gravity in negatively curved spacetimes and quantum field theories on the boundary. Black holes in these spacetimes represent finite temperature configurations for the theory on the boundary. The classical dynamics of these black holes can be used to get some insight into strongly coupled quantum systems."
==endquote==

 

[marcus]

To get back on topic, I will quote

...
Please note what I said about shifting interest. I was talking about top string people whose interest in string seems to be moderating. Baez is not in the group I'm talking about. The reason I mentioned the Corfu school here is because I noticed that Steve Giddings is a participant.
Giddings is someone I respect. I noticed in 2007 that he seemed to be shifting out of string, or showing broader research interest. I may be wrong. But he and Don Marolf attended GRG 18 at Sydney in 2007 and my impression is that Giddings' talk was not string. He had something else he wanted to talk about.

Now here two years later he is showing up in Corfu (just three weeks from now!) at a primarily Loop and 4D QG school. Maybe there is some obvious explanation why your average string theorist would do that, and I just don't know the explanation. But it gets my attention.

What I'm seeing these days is specifically something in string theory that seems to be making the top people restless...

I am going to make a guess as to the physical basis of this trend and conjecture that it is the negative Lambda trouble.
Lenny Susskind makes a big point in his most recent book that he does not like the term "string theorist" and that he does not want to be identified as a "string theorist". He wants to be recognized as doing theory outside that mathematical framework.

He has several times stated that string theory is in real trouble if the cosmological constant Lambda turns out to be positive. I have no idea how to take that, it could be flamboyant exaggeration. It might not be true--and just something Susskind says for dramatic effect. However it was the 1998 discovery by astronomers of what looks like positive Lambda that provoked the famous KKLT paper of 2003 (trying to find +Lambda vacuua) and led to the famous Landscape rumpus and the 10⁵⁰⁰ versions of physics. This in turn catapulted alternative, mostly 4D, approaches to the forefront. So there may be something in what Susskind says, some negative Lambda trouble inherent in string math (or in string physics if you believe it describes fundamental nature).

In any case Susskind did not attend Strings 2008 at CERN. Witten gave a talk about 3D gravity at Strings 2007, and then did not attend Strings 2008. Witten was invited to give a public lecture at Strings 2009 in Rome and chose to talk about all sorts of other things but not string. He did not give an in-conference talk of any sort. Witten has thus tacitly made clear that he is not to be identified merely as "string theorist". Like so many other of formerly top string people he has recently done a bunch of creative work outside string confines, e.g. in Geometric Langlands.

Those two are not the prime examples though. For me the most admirable responses to "the negative Lambda trouble" are shown by Petr Horava and Hermann Nicolai---both formerly top string theorists. I really like what each of them has done. Right or wrong! In science you can't tell in advance. Both have taken bold steps into new territory---and what they are doing is unquestionably intended as physics---by no stretch is it just some more abstract mathematics. Look up their papers and see. Watch Nicolai's talk at the Planck Scale conference.

So what I'm pointing to is a trend of top (former) string theorists to be restless. To be somewhat less comfortable with the "string theorist" label, to show forcefully (as Witten, Nicolai, and Horava have) that they can do other stuff and very creatively. To revitalize asymptotic safety, as Weinberg has. Hopefully to eventually invent a new 4D quantum geometry that can challenge Loop on its home turf.

This could be good for everybody. I think it may already be having a stimulating effect on Loop and Triangulations. Ambjorn and Loll will both be at the Asymptotic Safety conference at Perimeter this November. If Weinberg hadn't come out would there even have been that conference or would Loll have bothered to attend? I think the pressure of real competition is an important driving force for these people.

 

[Naty1]

Marcus posted this reference: http://relativity.livingreviews.org/...es/lrr-2008-5/
by Carlo Rovellie, 5/2008...

WHAT A GREAT CONCISE SUMMARY. Thanks Marcus, very enjoyable.

I would have preferred a few different words here and there but from a conceptual perspective Rovelli seems to have captured a LOT in little. I'll leave it to others to comment on the mathematical summary.

As I read I wondered "Is this a fair comparsion of LQG and string theory" as it seems LQG was given the edge...Marcus comments regarding 4D QG seem consistent so it appears reasonable. Maybe string theory, which is more comprehensive, is just too big a bite for current theorists?

There are a lot of quotables but I especially liked:

The best way to learn a theory is to read different presentations of the same theory and appreciate different points of views. This is particularly true for a theory, which is tentative and in evolution, like loop quantum gravity.

 

[marcus]

I'm glad you found that review article useful!
Atyy reminded me of it, in the other Lorentz thread:
https://www.physicsforums.com/showthread.php?p=2318095#post2318095

I knew the article and had used it, but I didn't remember its relevance to what we were discussing. Atyy pointed to exactly the passage in section 7.
Many of those who joined in the discussion were extremely helpful in clarifying that issue and deserve thanks.
Finbar, Humanino, Christine (of course!), Metron, Atyy, MTd2, Fra, Turbo, Eelco...I don't recall who else... many.