# Union of LQG and string theory

I've read in a couple books (e.g. in Lee Smolin's three roads to quantum gravity) about the possible union of LQG and string theory. In other words, that string theory and LQG can possibly be reformulated into a single TOE. i've been unable to find anything more on this idea through google and was wondering if anyone knew of a few papers/books outlining progress on this topic.

## Answers and Replies

Yeah I think it's something that we've all thought about. It may be possible, but I don't see how it would work. It would be good for the LQG people because most attepts to get realistic particle phenomenology out of Quantum Gravity (besides string theory) are pretty remedial.

In some sense, the two theories start from different points---string theory starts from quantum field theory, and treats the string as fundamental. LQG starts at GR and treats geometry as fundamental (inasmuch as I understand it). Space-time in string theory is an emergent'' phenomenon, because GR is an effective field theory. So the two approaches seem to be contradictory.

This is not to say, of course, that I know at all what I am talking about :)

I've read in a couple books (e.g. in Lee Smolin's three roads to quantum gravity) about the possible union of LQG and string theory. In other words, that string theory and LQG can possibly be reformulated into a single TOE.

What did you learn from the book?

I also bought smolin's 3 roads along with other in my last book order. But a quick peak it seemed unstructured which just page after page with bulk text. Anyway I haven't had time to read it yet.

Perhaps if you read you can briefly explain what I can expect to extract from the book?

I guess my initial perception from the title was that he actually lines out 3 different tentative scientific strategies to QG at some level of detail. Is that what he does?

/Fredrik

i've been unable to find anything more
I would be quite surprised if Smolin did not give reference to his own work ! In fact, you can find on arXiv Smolin's paper, and he is (AFAIK) the main contributor to the attempts to link LQG and STs.

Perhaps if you read you can briefly explain what I can expect to extract from the book?

I guess my initial perception from the title was that he actually lines out 3 different tentative scientific strategies to QG at some level of detail. Is that what he does?

Yep he looks at black hole thermodynamics; LQG and string theory separately and in some detail. It's a good book; you should definitely read it when you have time.

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marcus
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Something interesting along these lines: The speakers for the Strings '08 conference have been announced. One of the speakers is Carlo Rovelli, one of the two leaders in LQG.

interesting. I'll toss in some detail and maybe a slightly different viewpoint. At Loops '05 conference in Germany one of the featured plenary speakers was Robbert Dijkgraaf, a prominent world-class string thinker. And there were three other string people who gave contributed talks at that conference. It was really open to the competition!

There was no big international Loops conference in 2006, but then Loops '07 had Moshe Rozali give an invited plenary talk. By mischance the main strings conference got scheduled exactly the same time as Loops '07, so it was something of a sacrifice for Moshe to give his talk (he had to miss Strings '07). Had it not been for the scheduling conflict I am sure more string folks besides Moshe would have show up. The Loop community has always been very open to hearing about other approaches. They WANT to know.

They also regularly invite people like Reuter and Loll come and talk about Asymptotic Safety and CDT, other rival QG approaches.

So there has been quite a bit of communication, but it hasn't always been symmetric. This case of Rovelli being invited to give a plenary talk (I assume it is plenary, maybe it is in divided session) is something new.

String conferences have been closed to LQG and, in my experience, people can go thru graduate school in string without ever getting an accurate picture of what the nonstring QG folks are doing. They tend to have views based on what other stringfolks have told them, and which are apt to be out-of-date.

So it is in light of this I would say that it's a very hopeful sign that Rovelli is invited to talk at Strings '08. It will begin to fill an educational and communications need.
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About LEADERS the way I see it is different from how you do, Coin. It is true that in the HISTORY of LQG the three most prominent have been Ashtekar, Rovelli, and Smolin.

but if you focus on current research, Rovelli and Ashtekar are leaders of the Loop mainstream. Rovelli sort of defines the Loop/Spinfoam mainstream and Ashtekar the Loop Cosmology mainstream. Their grad students and postdocs are the ones that are somehow typical.
Smolin on the other hand has ventured farther afield. that isn't bad, it is just restless and innovative and risk-taking. He has attracted and fostered a small group of people working on braid-matter in the past 3 years. That is not at all like what the other people (at Penn State and Marseille) are working on. He has also tried some other things.

However the media has seized on Smolin as a kind of icon of LQG, and alternatives to string in general. so if someone doesn't know what LQG is and doesnt know what the other non-string QG people are researching and doesnt know what Smolin works on then they may tend to equate it all. And make Smolin symbolize it.
And then they make Lee Smolin personify the competing alternative to string, whatever that might be.

I am not saying your viewpoint is wrong. I know you are very well informed (in many cases better-informed than I, so I listen). But my attitiude is different. I'm not sure what leader means in a research context, maybe it is different things and several different kinds of people (an arbiter of the peer-review process on one hand, or an avantguard scout, or the manager of the most productive team, or the person that reporters call on as a kind of spokesman...and so on)

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marcus
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...about the possible union of LQG and string theory. In other words, that string theory and LQG can possibly be reformulated into a single TOE...

that's a good question. I hope you keep asking it, and that you UPDATE your information about current research.

Nonstring Quantum Gravity is a very rapidly changing field and is no longer dominated by what used to be called LQG.
For example the spinfoam approach Rovelli is working with is not about loops at all, it really is not proper to call it LQG although out of laziness we do.
The approach called CDT (causal dynamical triangulations) is not much like LQG and it is getting a lot of attention.

then on the string side there is something called NONCRITICAL string which can involve abandoning the extra dimensions so you can get back to a more familiar 3D or 4D world.
Noncritical string has been around for a long time and it might be stimulated by a MERGER.

This is a long way round of saying that there was just a paper that appeared proposing a
merger of CDT with a noncritical string model of an unusual sort.

The paper was by Ambjorn, Loll, Watabiki, Westra, and Zohren. the curious thing was that in the string model there was only one string, and it was the whole world. The worldsheet of this one string was the 4D spacetime itself. I hope someone who has read the paper will correct me if I am wrong. It sounds highly bizarre, and not like usual string stuff either.

So I guess this illustrates that if there is some cross-fertilization between the competing research lines it is apt to happen in ways you don't expect. It wont necessarily happen in ways envisioned by Lee Smolin back in 2001 when he wrote that book you mentioned! On the other hand Smolin has a far-reaching vision and has been prophetic in the past (he began dealing with the String Landscape problem back in the early 1990s and it didn't hit the mainstream string community until ten years later.) He has been right sometimes, but there's no guarantee he will be in the future.

There were also some other papers by John Baez et al that were stringy in a sense, but without the extra dimensions. the strings didn't vibrate, it was their topology that was interesting---braids were involved. Someone on Rovelli's team worked on that with Baez. That was last year. It looked like signs of a merger between the spinfoam approach that Baez helped invent and a very nonstandard sort of string that folks might not even recognize as such.

Anyway this kind of thing HAPPENS and if you are curious enough to try the technical papers I can get you links to the Baez et al and also that other one. In fact there are THREE recent ones by Ambjorn, Loll, Watabiki, Westra, Zohren
1. http://arxiv.org/abs/0804.0252 [ps, pdf, other]
Title: A Matrix Model for 2D Quantum Gravity defined by Causal Dynamical Triangulations
Authors: J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
Comments: 13 pages, 1 figure
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
2. http://arxiv.org/abs/0802.0896 [ps, pdf, other]
Title: Topology change in causal quantum gravity
Authors: J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
Comments: 4 pages, proceedings of the workshop JGRG 17 (Nagoya, Japan, December 2007)
Subjects: High Energy Physics - Theory (hep-th)
3. http://arxiv.org/abs/0802.0719 [ps, pdf, other]
Title: A String Field Theory based on Causal Dynamical Triangulations
Authors: J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
Comments: 29 pages, 4 figures
Subjects: High Energy Physics - Theory (hep-th)

I've highlighted two which seem to have merger implications.
Bear in mind that nothing is settled. Nonstring QG is a rapidly shifting research field.

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Hi Marcus, I wasn't thinking very specifically when I made that comment about "leaders" and had just plain forgotten about Ashtekar. Yikes! Thanks for the perspective.

Thanks for the links/info Marcus

Yep he looks at black hole thermodynamics; LQG and string theory separately and in some detail. It's a good book; you should definitely read it when you have time.

I at least started to read it last night. I usually do bed-time reading, so I'll keep this book as bed-time reading for a while and see.

/Fredrik

then on the string side there is something called NONCRITICAL string which can involve abandoning the extra dimensions so you can get back to a more familiar 3D or 4D world.
Noncritical string has been around for a long time and it might be stimulated by a MERGER.

I don't think that too many people take non-critical stringtheory seriously, but I could be mistaken. I am far from an expert in that field, so I don't feel comfortable commenting on the things that are wrong with it. I hope that one of the more stringy people can answer that question, though.

marcus
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I don't think that too many people take non-critical stringtheory seriously, but I could be mistaken. I am far from an expert in that field, so I don't feel comfortable commenting on the things that are wrong with it. I hope that one of the more stringy people can answer that question, though.

Thanks for the helpful comment, Ben. I mean that seriously and I also hope we get response from some of the others. What you are making is a sociological observation (or guess) about the consensus in the string community, I believe, and such observations can be very useful guides.

I will add a comment of my own about John Ellis (and Nick Mavromatos). Ellis is probably the most prominent phenomenologist/theorist at CERN who is actively engaged with string research and he is one of the most highly cited researchers. For what it's worth I think he is pushing non-critical. It is quite possible he will take a bunch of other talented people along with him.
I need to check this.

Now you and I both know that things like citation numbers and community consensus (what you say about "not too many people seriously") don't constitute scientific proof. These things are interesting signposts and dimensions of the overall situation, not litmus tests of truth.

But I would say that your guess about the consensus is probably RIGHT in the sense that the vast majority of the world's string people completely disregard non-critical and consider it proven invalid. My (nonexpert) guess is that your guess is right! And ultimately that is meaningless.
Also I would say that there is probably a handful of brilliant and inventive like John Ellis who have decided that noncritical is OK and interesting after all. That too has no very definite meaning. Just a straw in the wind.

But we pay attention to sociological clues anyway. In case anyone is interested, the possible noncritical comeback issue came to my attention last summer around August 2007 with the publication of a paper by Ellis Mavromatos and the socalled "MAGIC" collaboration of about 120 some people.

I may be wrong about several details here, you might classify him as a phenomenologist only (not theory), or have a different take on this altogether. The guy has 343 papers on arxiv so it is hard to pigeonhole him in one category or other. I will try to check some of my details here. But at the moment, off the cuff and merely intuitively, I would throw Ellis in the balance against several hundred more consensus people who don't see any potential. Please correct me if you see some flaw in what I'm saying

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marcus
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Yeah I checked John R Ellis on Spires and he has 809 publications
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=ea+Ellis,+John+R [Broken]
and one published just last year was about noncritical. Here is the 2006 preprint
http://arxiv.org/abs/hep-th/0611228
I've heard him talk on video and mention noncritical and I think this is probably just the tip of the iceberg
but the guy is so prolific it is hard to tell.

the keyword "noncritical" brings up these seven papers by John Ellis
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+EA+ELLIS%2C+JOHN+R+AND+K+NONCRITICAL&FORMAT=www&SEQUENCE=ds%28d%29 [Broken]

the keyword by itself brings up 250
with the most highly cited authors being Gubser, Polyakov, Seiberg, Susskind, Kazakov etc
not even mentioning Ellis. So I was wrong. Ellis is not particularly a leader in this!
other people are more in the lead.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+NONCRITICAL&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

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Haelfix
Noncritical strings is an active research area, but I don't know much about it.

The hope I gather is really that there might be some mysterious dualities hidden in the mathematics. So they might get lucky and find something physical afterall.

Otoh, you have much greater freedom or said another way, less constraints. So you go from a unique theory, to an infinite amount of theories. Its thus seen as less promising to find a fundamental theory therein since you have much less analytic control, and presumably a greater chance of finding something that you want to see (read man made).

Its also hard to justify on physical grounds and all the old arguments against it are just as strong today as they were twenty years ago.

marcus
Gold Member
Dearly Missed
Noncritical strings is an active research area, but I don't know much about it.

The hope I gather is really that there might be some mysterious dualities hidden in the mathematics. So they might get lucky and find something physical afterall.
...

I'm glad to hear that it is active, Haelfix. Thanks for your comment!

BTW I see that Peter Woit was not very happy with the John Ellis and MAGIC paper (which was what put noncritical on the map for me, last year).
==quote Woit==
Hendrik, a commenter here, pointed out that there’s more of the latest string theory hype concerning results from the MAGIC telescope, originally discussed here. Now New Scientist has weighed in with an article entitled Finally, a MAGIC test for string theory? According to the article, Mavromatos and collaborators say that their (non-critical) string theory model “predicts the 4-minute delay exactly”. Polchinski is quoted to the effect that this would falsify (critical) string theory. LQG is completely cut out of the deal,... The way things are now, any supposed evidence of quantum gravitational effects is automatically evidence for string theory, in one version or another.
==endquote==
That was in this:
http://www.math.columbia.edu/~woit/wordpress/?p=598

I suspect that the MAGIC collaboration did not actually measure a repeatable 4-minute delay attributable to dispersion and that the paper will be forgotten, but what impressed me about Mavromatos and Ellis is that they were right in there explaining it, as if it had really happened, with their version of string theory

Wikipedia calls Ellis a theoretical physicist
http://en.wikipedia.org/wiki/John_Ellis_(physicist)
and says he is best known for some string theory work he did with Nanopoulos
also that he invented the phrase "theory of everything"
none of this is intended as a recommendation

Anyway, noncritical string is alive and well! Very glad to hear this. We don't have to have exactly this or that number of dimensions, apparently.

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Thanks for the helpful comment, Ben. I mean that seriously and I also hope we get response from some of the others. What you are making is a sociological observation (or guess) about the consensus in the string community, I believe, and such observations can be very useful guides.

They're only guides inasmuch as you actually plan to go somewhere. You can sit around talking about guides'' all you want and never actually do any physics.