What do stringy people find interesting about LQG?

[marcus]

Tom.stoer recently started a thread to collect ideas on the topic of stringy dislike of LQG.
I thought that was a constructive initiative. At least for me it was helpful to sample what are the current rationales of dismissal, in part because this is a kind of folklore that has changed over time and probably will continue to shift. I think you can learn from watching it.

In any case, I'd like to complement that thread with another---what do stringy people (those that do) find valuable and interesting about LQG? And we should probably include all the main approaches to quantum geometry/gravity. Spinfoam, Triangulations, Loop Quantum Cosmology, Master Constraint LQG, maybe also Causal Sets... Or at least the first several of these.

For openers, I will point out that a Stringer like Hermann Nicolai has always shown an interest in getting string&loop folks together to compare and learn from each other. He set up the weeklong Strings Meet Loops conference at Potsdam in 2003. And I suspect he was instrumental in making sure that Carlo Rovelli was invited to give an intro to LQG at CERN to the Strings 2008 conference. Nicolai apparently believes there is something to be gained by discussion. So one can ask why?

In case someone imagines that Nicolai would be an isolated case, I will cite this up-coming conference in Cape Town:
http://www.mth.uct.ac.za/~jeff/Quantum_Gravity/Participants.html

It is a week long and participants include

string, SUGRA, branes cosmology, and related: Hermann Nicolai, Kelly Stelle, Cliff Burgess, Malcolm Perry, Brian Greene

master constraint: Thomas Thiemann

spinfoam/LQG: Carlo Rovelli, Daniele Oriti

triangulations: Renate Loll, Jan Ambjorn

quantum geometry/gravity freestyle: Roger Penrose, Steve Carlip

causal sets: Rafael Sorkin, Joe Henson

Now the question is, why did those 5 rather stringish people decide it was important enough for them to go to Cape Town South Africa to meet and talk with those 9 non-string quantum geometry people?

Or if you think it was just honor and sandwiches that brought them, then what motivated George Ellis to invite them in the first place?

 

[marcus]

Here's what I take to be George Ellis' statement of purpose in getting these QG and string people together for a weeklong meeting:

==quote==
Quantum Field Theory and General Relativity are among the most impressive of human achievements of the 20th century. Within its own domain, each theory has been remarkably successful, with agreement between theoretical predictions and experiments at the one-part-per-billion level in some cases. Yet, even after almost a century and a clear underlying theme - that of a fundamental length scale limiting the resolution of spacetime -- a clear unifying theoretical framework within which both theories might be reconciled is still sorely lacking.While several promising proposals - including String theory and Supergravity, Loop Quantum Gravity, Causal sets, Regge calculus and Dynamical triangulations -- have emerged with varying degrees of success, none can yet claim to be an unequivocal solution to the problem of quantum gravity complete with experimental predictions. The fundamental principles of the best solution are in dispute, and even the form of quantum theory that should be utilized in this context is not clear. One of the key issues is the foundational nature of space and time; and even here there is no agreement. It is becoming increasingly clear that the fundamental principles and essential progress of each research program are in need of review in a comparative context.While review meetings in quantum gravity are certainly not new, most are dominated by one specific research direction. This is where this meeting is markedly different. The aim of this week-long workshop is to bring together experts in each of the major representative approaches toward quantizing gravity, to critically and openly discuss the successes and failures of each approach and on that basis to consider pointers towards a satisfactory resolution.

==endquote==
http://www.mth.uct.ac.za/~jeff/Quantum_Gravity/About.html

The meeting starts in a little less than 7 weeks from now, on August 10. It is sponsored by FQXi and by the Cambridge University Press. I imagine the CUP might get a book out of it.

The title of the meeting, or maybe conference is a better term for it, is
Foundations of Space & Time: Reflections on Quantum Gravity.

 

[marcus]

It is probably too much to ask that a lot of people are going to be moved to post in this thread because the title is so crazy.

We have a stereotype of stringfolks as all being acting offended by LQG, or sneering, or simply dismissing the idea that it could ever succeed in contributing to our understanding of the quantum dynamics of geometry.

So we expect only negative putdowns from them. It sounds like a contradiction in terms to ask, as this thread does, what string-minded folks might find interesting, even enviable, about developments in the Loop community.

But I think this is a stereotype expectation we have. There are probably things in nonstring quantum geometry research that should interest and do interest a fair number over in the stringy sector.

And we can try to identify what some of those things are.

Because the title of the thread goes against preconceptions I don't have great hopes, but still at least one person considered posting something here. Here's what someone said in another thread (referring to this one.)

... I think [LQG]provides insights regarding new technical aspects how to quantize classical systems, how to address background independence, how to do non-perturbative calculations, ... Therefore they should not ignore it - but this is something I should write in Marcus' thread :-)
...

 

[MTd2]

Hmm, I mistook George Ellis for John Ellis, LOL! I deleted that message. Well, this meeting is a homage for his 70th birthday, and since he is South African, this is a good reason to go there:

http://en.wikipedia.org/wiki/George_Ellis

He is the one that wrote "The Large Scale Structure of Space-Time" with Stephen Hawkins.

 

[marcus]

Yes! George Ellis is a great guy. It is not just that he was co-author of Hawking's best known real book (not counting the massmarket pap.) He's been a leading light in foundational cosmology and foundations of physics in general. And to me this select workshop bringing different qg approaches together, with a possible Cambridge press book coming out---this seems like a possible light at the end of the tunnel.

==quote from Ellis==
This is where this meeting is markedly different. The aim of this week-long workshop is to bring together experts in each of the major representative approaches toward quantizing gravity, to critically and openly discuss the successes and failures of each approach and on that basis to consider pointers towards a satisfactory resolution.
==endquote==
http://www.mth.uct.ac.za/~jeff/Quantum_Gravity/About.html

Foundations of Space & Time: Reflections on Quantum Gravity.

 

[marcus]

Just incidentally, seems hardly worth mention, but another thing string-minded people might find admirable about the LQG community is the high rate of peer-review professional journal publication.

I would not have imagined that was an issue, but I was amazed a week or two ago when a string-minded visitor here came around asking if Loop folks published in peer-reviewed places, or did they just, like, put it up on arxiv. It seemed important to him.

I was boggled. Did not know how to respond. If faced with that sort of question one reaction might be a simple Spires search with keywords "quantum cosmology", and another with "spin, foam", ranked by cite count to get the more important. There has been so much progress since 2006 that I normally use that as a cut-off.

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29
(this gets some non-LQG quantum cosmo but if you look at the Loop stuff it is mostly PRD with a few PRL)

The next gives a more representative list:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+SPIN%2C+FOAM+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29
You see PRD, PRL, CQG, several Nucl.Phys.B, and one Europhys.Letters.

I didn't fully understand where the stringy poster was coming from but his line of reasoning seemed to be that peer-review forces you to be open to citing outside your immediate community and thus makes your community more open, and this improves the quality of reasearch. Openness has good consequences for the physics, I think was the poster's idea. Probably right, even though it's a semisociological consideration, with only tenuous connection with research quality. But someone came here asking so we should try to be prepared to answer.

Anyway there is an extensive peerreviewed LQG literature---many hundreds of papers. And the community is traditionally open--e.g. string-folks attend Loops conferences and give papers. Invited speakers have included Robbert Dijkgraaf (Loops 2005) and Alex Muloney. But also invited speakers from other approaches, like Reuter, Loll, Ambjorn. Opposite from exclusionary. Non-loop papers have traditionally been welcomed, if addressing the conference themes and goals.

 

[ccdantas]

I can't tell for other fields, but in astrophysics, peer-reviewed publications are extremely important. In that field the arxiv mostly serves for two purposes: to have your preprints freely accessible and available in advance of (journal) publication. To be seriously considered in the astrophysics community, you must have peer-reviewed publications.

This may be changing, I don't know.

 

[marcus]

... you must have peer-reviewed publications.

This may be changing, I don't know.

I hope it is not changing in other fields. I see no sign of that in QG. There must be thousands of peer-reviewed publications by LQG researchers, it would seem ridiculous to even count.
I strongly agree with you about the importance!

If peer-review institutions connected with dead-tree publication become obsolete then new peer-review institutions must be invented to take over their role, because the idea of working for the recognition and reputation accorded by one's colleagues is at the heart of the science tradition and community. It is far more important than mere money, and more valuable to the scientist. In a strange way, we have a residual self-selecting aristocracy living inside an otherwise largely commercial marketbase society.

I am very glad to see your comments Christine, my past irritation was not at all with you! (but happened by context and misfortune to seem so.)
==================

But let's move on and get back to the main topic! Here is a clue to help us see what stringies find interesting about LQG. (Since many clearly are interested.) We all know Robert Helling, indeed from back as early as 2003. Here is what Helling said today 29 June:

http://atdotde.blogspot.com/2009/06/more-conference-blogging.html
==quote Robert==
Instead of String '09 I decided to attend this year the Born Symposium on the Planck scale. There are a number of stringy speakers as well as quite a few people from the loop camp. Watch this space for some reports. The talks (including video) will be online as well (as opposed to Strings '09).
==endquote==

Back in April I reported about the Planck Scale conference (Born XXV) partial list of talks here:
https://www.physicsforums.com/showthread.php?p=2154099#post2154099,
and the very hopeful sign that it had a mix. One of several indicators of a trend, noted recently.

What Robert Helling is saying is just what we have been hoping to hear from young string theorizers. Abandoning the exclusionary "only game in town" mentality and getting a healthy interest in what other approaches might be able to contribute. There can still be lots of competitiveness and elbowing. Testosterone never goes away! What one hopes to go away is the exclusionary chosen-people delusion with blindness to what the others are accomplishing.
(because "it cannot possibly succeed" )

So we will see how this year's Born Symposium, with its mix, works out. For me, even more significant is the mix later in August in Cape Town, at the smaller more selective workshop that Ellis has put together.

 

[marcus]

We are currently seeing a trend for string-people to get more interested in lines of research that share certain features with LQG. They may not cross over into LQG proper but this migration into areas with common features and concerns goes along with greater openness---and a fading of the "only-game-in-town" delusion.

This is a very hopeful sign, I think. Very glad to see Robert Helling going to the Born XXV Planck Scale symposium. And Brian Greene and Hermann Nicolai giving talks at George Ellis' Cape Town QG workshop (a real synthesis move with focus on fundamental concepts.)

Of course some of the best LQG people are ones who were in string earlier and got out. Like Sergei Alexandrov and Leonardo Modesto. Jan Ambjorn was trying to find a way to formulate M-theory in the 1990s and now is a major force with Loll in the Triangulations QG business. That's just a few.

But I am not talking about line-crossers (intellectual jay-walkers who don't wait for the light to turn green ) What I mean is dozens of people piling into things like:
1. Horava QG
2. Bounce cosmology
3. anything with only four dimensions.
4. anything without T.O.E. pretensions, whether it is QCD, or just plain QG ("pure" in the sense of using generic matter instead of including the whole cast of Standard particles.)

Whether or not they declare an interest in LQG, these people are demonstrating an interest in research that shares common features. What we need to do, I think, is try to identify those common features and see the common research vectors.

 

[turbo]

I can't tell for other fields, but in astrophysics, peer-reviewed publications are extremely important. In that field the arxiv mostly serves for two purposes: to have your preprints freely accessible and available in advance of (journal) publication. To be seriously considered in the astrophysics community, you must have peer-reviewed publications.

This may be changing, I don't know.

It appears to be Springer's policy to recommend that the authors pre-publish on arxiv as soon as their papers have been peer reviewed and accepted (at least for the journal Astrophysics and Space Sciences). I was quite surprised to see this development coming from a subscription-only journal. Springer followed up with an on-line version to their subscribers, and finally, the print version. If paper journals fade away, Springer appears to be ready to transition to digital formats.

As quickly as some research in some fields progresses, subscriber-only digital journals should have a premium value over print journals.

 

[atyy]

Whether or not they declare an interest in LQG, these people are demonstrating an interest in research that shares common features. What we need to do, I think, is try to identify those common features and see the common research vectors.

Maybe it's that they are all related to condensed matter?

Horava - http://arxiv.org/abs/0901.3775: "the novelty of our approach is that it takes advantage of theoretical concepts developed in recent decades in condensed matter physics, in particular in the theory of quantum critical phenomena."

Asymptotic Safety/CDT - renormalization group, which considered condensed matter as long ago as Ambjorn's '90 Les Houches lectures http://www.nbi.dk/~ambjorn/reviews.html: "The unification of the theory of critical phenomena and Euclidean quantum field theory ... There is a special area of field theory which is intimately linked to statistical mechanics, but ..." [I think CDT was only DT in '90.]

Even strings - the most intriguing thing is AdS/CFT - which also has applications to condensed matter. http://qpt.physics.harvard.edu/talks/upenn.pdf