Edward Witten researching non-string LQG-like quantum gravity

[ensabah6]

Edward Witten is the most influential string theorists in the world, is now doing research into gravity that is decidedly non-string, and very similar to LQG.

here's a link
http://gesalerico.ft.uam.es/strings07/040_scientific07_contents/041_speakers.htm

his research program is titled " Three-Dimensional Gravity Revisited"

and here's a discussion
http://www.math.columbia.edu/~woit/wordpress/?p=555

Peter Woit writes "If one wants to interpret this new work in light of the the LQG/string theory wars, it’s worth noting that the technique used here, reexpressing gravity in terms of gauge theory variables and hoping to quantize in these variables instead of using strings, is one of the central ideas in the LQG program for quantizing 3+1d gravity."

 

[marcus]

Last year the UC Berkeley math and physics departments hosted Witten and he gave three 90 minute talks, which I attended all of.

and at no time during the talks did he mention string theory (or superstring or brane or whatever)

it was about the geometric Langlands program which doubtless has implications for all kinds of mathematics including of course string mathematics but the talks did not explicitly bring up string

then at the end of the third talk, when it was time for questions, someone from the audience asked if he had any words about string and he said that he believed it would turn out to have something to do with nature.

 

[ensabah6]

Last year the UC Berkeley math and physics departments hosted Witten and he gave three 90 minute talks, which I attended all of.

and at no time during the talks did he mention string theory (or superstring or brane or whatever)

it was about the geometric Langlands program which doubtless has implications for all kinds of mathematics including of course string mathematics but the talks did not explicitly bring up string

then at the end of the third talk, when it was time for questions, someone from the audience asked if he had any words about string and he said that he believed it would turn out to have something to do with nature.

Hi,
So what's your interpretation of Witten's remark? Is he being more lqg- by the day?

 

[marcus]

Hi,
So what's your interpretation of Witten's remark?

As a member of the audience I knew people from both departments who were present and I felt the physicists' frustration with Witten's singleminded focus on his current interest (geometric Langlands). These were the only talks he gave in the course of his brief visit and he avoided saying anything about string or even general ideas like unification.

Beyond reporting what I thought was striking about the emphasis I having nothing to add about Witten.

I think it is more interesting to watch the shift of interest of a whole class of people (the prominent successful string mathematicians of the 1990s) and try to get an idea of where they are going. The example of just one person, no matter how famous, doesn't do it for me.

so I want to take something you said and generalize it to make a broader question:

... research into gravity that is decidedly non-string, and very similar to LQG.
...

that raises a very interesting general topic, that is not even restricted to the one example of Edward Witten

what other string theorists do you know of who have crossed over into non-string QG or moved in that direction-----despite earlier career success in string?

I can think of several. Jan Ambjorn has many string papers but he was giving the dynamical triangulations talks at the European Union QG school this spring at Zakopane---he made some remarks about string at the beginning that persuade me he is not so amphibious now but is more concentrating on non-string QG.

Steve Giddings and Don Marolf have been successful and prominent in string research, but their most recent joint paper had LQG citations and looked as if it would fit usefully into the non-string QG program. I think of them now as both more ambidextrous---able to contribute ideas and results either way.

Giddings most recent solo paper cited 4 papers by Rovelli, one by Smolin, two by Dittrich, several by Gambini and Pullin---it showed unusually thoughtful reading of non-string QG research (this was his contribution to this year's Gravity Foundation essays)

Of course Leonardo Modesto got out of string at Genoa and went to work with Rovelli in Marseille. Likewise Sergei Alexandrov got his PhD in string in Paris and now is doing interesting work at Montpellier related to LQG. there must be quite a few people who made the transition early, before they had any chance to become known as string mathematicians. What I am wondering about is more the prominent string people who seem to be acquiring a non-string QG interest.

 

[arivero]

Witten went into strings because of the KK extra dimensions, as he had gone into supergravity before, due to the same reasons. He always kept pushing the theory towards d=11, to meet the standard model embedding, and insisting that string theory at the end should be very diferent from the starting theory.

 

[ensabah6]

As a member of the audience I knew people from both departments who were present and I felt the physicists' frustration with Witten's singleminded focus on his current interest (geometric Langlands). These were the only talks he gave in the course of his brief visit and he avoided saying anything about string or even general ideas like unification.

Beyond reporting what I thought was striking about the emphasis I having nothing to add about Witten.

I think it is more interesting to watch the shift of interest of a whole class of people (the prominent successful string mathematicians of the 1990s) and try to get an idea of where they are going. The example of just one person, no matter how famous, doesn't do it for me.

so I want to take something you said and generalize it to make a broader question:that raises a very interesting general topic, that is not even restricted to the one example of Edward Witten

what other string theorists do you know of who have crossed over into non-string QG or moved in that direction-----despite earlier career success in string?

I can think of several. Jan Ambjorn has many string papers but he was giving the dynamical triangulations talks at the European Union QG school this spring at Zakopane---he made some remarks about string at the beginning that persuade me he is not so amphibious now but is more concentrating on non-string QG.

Steve Giddings and Don Marolf have been successful and prominent in string research, but their most recent joint paper had LQG citations and looked as if it would fit usefully into the non-string QG program. I think of them now as both more ambidextrous---able to contribute ideas and results either way.

Giddings most recent solo paper cited 4 papers by Rovelli, one by Smolin, two by Dittrich, several by Gambini and Pullin---it showed unusually thoughtful reading of non-string QG research (this was his contribution to this year's Gravity Foundation essays)

Of course Leonardo Modesto got out of string at Genoa and went to work with Rovelli in Marseille. Likewise Sergei Alexandrov got his PhD in string in Paris and now is doing interesting work at Montpellier related to LQG. there must be quite a few people who made the transition early, before they had any chance to become known as string mathematicians. What I am wondering about is more the prominent string people who seem to be acquiring a non-string QG interest.

Thanks. I do think should LHC fail to find evidence of SUSY, what is now a trickle may become a stream.

Lee Smolin I think started out in the supergravity/string theory camp.

 

[ensabah6]

Witten went into strings because of the KK extra dimensions, as he had gone into supergravity before, due to the same reasons. He always kept pushing the theory towards d=11, to meet the standard model embedding, and insisting that string theory at the end should be very diferent from the starting theory.

Do you think Witten still feels this way, with his current research interests? "then at the end of the third talk, when it was time for questions, someone from the audience asked if he had any words about string and he said that he believed it would turn out to have something to do with nature."

doesn't sound like a vote of confidence. Michio Kaku and Briane Greene and other stringers called String theory the Theory of Everything, and the language which God wrote the universe.

 

[arivero]

asked if he had any words about string and he said that he believed it would turn out to have something to do with nature."

doesn't sound like a vote of confidence.

Well, it is more of the same: to tell that the "final string theory" will be very different from the current one, is a optimistic way of saying the same thing.

String theory has grown sort of randomly, with no respect with the empirical evidence. So, the fact of having a very adecuate number of extra dimensions (11 better than 10, in any case) and room enough to contain previous extensions of the standard model (GUT models, supergravity) can be interpretated either as telling us that string theoy has something to do with nature or that as string theory has explored in a unified way a range of mathematical objects which have something to do with nature.

 

[marcus]

Arivero, I think the question on several people's minds can be put more simply than Ensabah stated it. And you may have an illuminating answer.

We see that this is the second year in a row* that Witten goes to the big annual international String conference and gives a paper that is not string.

Naturally people wonder, is this just an isolated case with no special significance?
Or should we draw some conclusion from it?

For my part, I tend to ascribe a lot of stuff to mere random variation---Witten is just one guy, I see him to a large extent as a mathematician and the interests of mathematicians are changeable. If he finds himself not getting results in one field he can jump over and see what he can do in another field.

I would be impressed if there were a trend in the overall NUMBERS involving a lot of experienced top people. But not by one guy, even if he is some people's tribal totem

*Last year at Beijing Strings '06, it was “Gauge Theory and Geometric Langlands Program”. Woit commented: "He has given talks at almost all the Strings conferences since the first in 1995, but this will be the first one at which he won’t be talking about string theory."

 

[arivero]

Witten is just one guy, I see him to a large extent as a mathematician and the interests of mathematicians are changeable. If he finds himself not getting results in one field he can jump over and see what he can do in another field.

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+WITTEN%2C+E&FORMAT=www&SEQUENCE=ds

mathematician?

I would be impressed if there were a trend in the overall NUMBERS involving a lot of experienced top people. But not by one guy, even if he is some people's tribal totem

Well it depends of the initial motivation to be in strings. If it is just training, why to move now that you have got the training?

Also, it is about particle vs cosmology. Even this forum reflects the modern feeling that strings are primarely about cosmology. So people in strings are not ashamed of the failure to predict the particle spectrum.

 

[arivero]

Check

http://www-hep.phys.cmu.edu/beauty2003/beauty2003/talks_web/Edward_Witten.pdf

Except Gross and perhaps Polchiski, I can not imagine the other main speakers of String07 giving a talk on flavour such as this one.

 

[Dcase]

Hi Arivero,

I do not know how accurate this Wiki biography is, but Witten has a remarkable academic history:

- bachelor's degree in history (with a minor in linguistics) from Brandeis University

- attended the University of Wisconsin for one semester as an economics graduate student before dropping out

- enrolling in applied mathematics at Princeton University before shifting departments and receiving a Ph.D. in physics in 1976 under David Gross

- first physicist to win the Fields Medal for mathematical physics

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


LD Faddeev. chair Fields Medal Committee comments.
http://www.mathunion.org/Prizes/Fields/1990/Witten/page1.html


M-theory may be an extension of Morse Theory?
http://en.wikipedia.org/wiki/Morse_theory

 

[marcus]

mathematician?
...

Yes. since the categories are not exclusive, if you want to challenge you need to bring evidence that he is NOT a mathematician.

 

[marcus]

...the modern feeling that strings are primarily about cosmology. So people in strings are not ashamed of the failure to predict the particle spectrum.

Alejandro, I should think that string contribution to cosmology would be an embarrassment to many in the community. If that is a primary ground of self-respect, things must be in a bad way. I don't see how that could be so. Perhaps you would explain.

 

[ensabah6]

Peter Woit responds to Aaron Bergman on the stringness/LQGness of Witten's latest research here http://www.math.columbia.edu/~woit/wordpress/?p=570#comments


PW
"...During the talk he brought up the question of LQG, noting that in 3d you could covariantly express gravity in terms of a gauge theory, that the way this was done in 4d (LQG) was non-covariant..."

"...There was nothing at all about string theory in his talk..."

 

[Boris Leykin]

is there a LQG landscape?!

...someone from the audience asked if he had any words about string and he said that he believed it would turn out to have something to do with nature.

Sorry for my dilettantish curiousity.
But isn't everything like I was asking https://www.physicsforums.com/showthread.php?t=171673". Loop quantum gravity is more fundamental than string theory. Look at the picture. What is this quantum foam? Isn't it made of loops?

No?

 

[marcus]

I think the topic of the thread is interesting in its own right---we don't have to get off into our favorite speculations about the nature of reality. Here is what the thread-starter said

Edward Witten is the most influential string theorists in the world, is now doing research into gravity that is decidedly non-string, and very similar to LQG.

here's a link
http://gesalerico.ft.uam.es/strings07/040_scientific07_contents/041_speakers.htm

his research program is titled " Three-Dimensional Gravity Revisited"

and here's a discussion
http://www.math.columbia.edu/~woit/wordpress/?p=555

So let's try to follow up on that theme. An influential string theorist is doing non-string gravity research. Is that part of a more general trend?
Yes. As most of us know---perhaps all of us know---the research publication rate is declining, and the production of heavily-cited papers has dropped to almost nothing compared with 5-10 years ago.

It could just be a fallow period and will pick up later---just as Witten could post an important M-theory paper later this year. We don't know the future. But it is still worth looking at quantative indices like this because they are part of the picture too.

 

[marcus]

In case anyone is not already aware of this.
If you count peer-review publication of articles whose summary abstracts have string-related keywords (superstring, brane, M-theory, heterotic, AdS/CFT) using the Harvard database you get this:

for the entire year
2002: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1

2006: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1

2007: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1For the first six months of the year

2002: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1

2006: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1

2007: http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1

Obviously not everything is in for the first six months of 2007, much less for the entire year---so those are numbers you can try guessing and see how close you come. The numbers for 2002 and 2006 illustrate the trend in string community activity.

More significant, perhaps, is the drop-off in the number of recent papers which other string theorists consider important enough, or sufficiently significant for their own research, to cite as references. Each year the number of recent (past five years) papers getting 100 or more citations has declined. From 10 - 20 a few years back, to 2 or 3 last year. Judging by citations there has been an order-of-magnitude drop in the value of recent string research as perceived by the string researchers themselves.

This slowdown in quantity and quality has been discussed by leading people in the string community. Witten minimized its importance, saying that these things go in cycles and in any line of research there are periods of a few years when it is very hard to get significant results. That seems pretty sensible---don't get excited and wring your hands, just go over into some other line of research and find an interesting problem to work on.

Others, like David Gross, have had a more profound reaction (in my humble opinion). He has thought deeply about the impasse and has been sounding frustrated and worried on several occasions over the past year.
Gross remarks after the Solvay Conference and at the end of the recent Jerusalem-Haifa conference were interesting and provocative, I thought.

Both ways of coping have my respect, needless to say.

 

[ensabah6]

I think the topic of the thread is interesting in its own right---we don't have to get off into our favorite speculations about the nature of reality. Here is what the thread-starter said



So let's try to follow up on that theme. An influential string theorist is doing non-string gravity research. Is that part of a more general trend?
Yes. As most of us know---perhaps all of us know---the research publication rate is declining, and the production of heavily-cited papers has dropped to almost nothing compared with 5-10 years ago.

It could just be a fallow period and will pick up later---just as Witten could post an important M-theory paper later this year. We don't know the future. But it is still worth looking at quantative indices like this because they are part of the picture too.

In NEW Peter Woit's view is that the popularity of string theory owes largely to Witten, and that many HEP followed Witten there. If this is true, then should Witten focus more on non-string QG, presumably QG theorists will follow suit, esp in light of a hypothetical null SUSY result from LHC.

 

[arivero]

In NEW Peter Woit's view is that the popularity of string theory owes largely to Witten, and that many HEP followed Witten there.

Yes, this point of view is old; it comes from Green & Schwartz, who time ago enjoyed to explain how they got to involve Witten in the project as an strategical move. And after that, the 1995 revolution (dualities) increased his role as prominent figure.

Others, like David Gross, have had a more profound reaction (in my humble opinion). He has thought deeply about the impasse and has been sounding frustrated and worried on several occasions over the past year.
Gross remarks after the Solvay Conference and at the end of the recent Jerusalem-Haifa conference were interesting and provocative, I thought.

Frustrated? Pity I have never heard Gross so I can not compare. I will try to check Friday and Saturday, as he has scheduled one technical lecture and then a public one.

 

[marcus]

... I will try to check Friday and Saturday, as he has scheduled one technical lecture and then a public one.

That should be interesting! Please let us know the highlights, and anything new he may have found to be hopeful about.

 

[arivero]

Well the new paper of Witten is out and it does not seem, to me, a paper about gravity, but a paper about ads/cft.

 

[marcus]

Well the new paper of Witten is out and it does not seem, to me, a paper about gravity, but a paper about ads/cft.

It is interesting that there seems to be so much confusion about this. I suppose for me the question should be is it string theory? or is it something else, whatever. Here is the paper Alejandro referred to.

http://arxiv.org/abs/0706.3359
Three-Dimensional Gravity Revisited
Edward Witten
82 pages
(Submitted on 22 Jun 2007)

"We consider the problem of identifying the CFT's that may be dual to pure gravity in three dimensions with negative cosmological constant. The c-theorem indicates that three-dimensional pure gravity is consistent only at certain values of the coupling constant, and the relation to Chern-Simons gauge theory hints that these may be the values at which the dual CFT can be holomorphically factorized. If so, and one takes at face value the minimum mass of a BTZ black hole, then the energy spectrum of three-dimensional gravity with negative cosmological constant can be determined exactly. At the most negative possible value of the cosmological constant, the dual CFT is very likely the monster theory of Frenkel, Lepowsky, and Meurman. The monster theory may be the first in a discrete series of CFT's that are dual to three-dimensional gravity. The partition function of the second theory in the sequence can be determined on a hyperelliptic Riemann surface of any genus. We also make a similar analysis of supergravity."

Because he deals with PURE gravity (no matter, only empty space) throughout the paper, I do not see any contact with current Loop gravity research in the 3D case.
A typical Loop paper about 3D, these days, would be one like those by Laurent Freidel and collaborators. Matter is included and the cosmological constant, where it plays a role, is positive.

Excluding matter and assuming a negative cosmo constant makes things very different, almost a different subject of study altogether.

Apparently Witten presented this paper in a talk at a conference in New York in May of this year, which Peter Woit attended
http://www.math.columbia.edu/~woit/wordpress/?p=555
Peter Woit's long post covers a lot that is in the paper and gives an accurate commentary as far as I can see. The only "LQG-like" thing that Woit points out is that Witten is using Ashtekar variables (instead of string) which is similar to what LQG people do.
(But there the similarity ends AFAICS, and Woit does not push the comparison any further.)

One should be careful not to over-interpret what Woit said.

 

[chrisina]

I went through the paper quickly, I do not see anything in there that looks like ST.
On the contrary, if you look at the "meat" of the paper, on partition functions (starting p.26), he wants to derive an EXACT spectrum of physical states of 2+1 G with a negative Lambda.
Seems quite opposite to the 10^500 sets of solutions of string theory and its 10/11 dimensions...

 

[marcus]

Hopefully, some of us will make the COMPARISON between what Witten has to say about pure 3D gravity with negative
cosmological constant and the 15 May online seminar talk of Laurent Freidel
about 3D matter and gravity with positive cosmo.

Matter coupling to 3d quantum gravity and effective field theory

The link to PDF of Laurent's slides is at

http://relativity.phys.lsu.edu/ilqgs/

The direct link to the PDF is
http://relativity.phys.lsu.edu/ilqgs/freidel050807.pdf

The direct link to the MP3 audio is
http://relativity.phys.lsu.edu/ilqgs/freidel051507.mp3

A key point which Laurent makes at the outset is that because matter is present the system has infinitely many degrees of freedom and that he is intentionally USING ONLY TOOLS WHICH APPLY TO THE 4D CASE AS WELL.
That is, the 3D case is special in ways that do not carry over to 4D but he is restricting to analytical methods which apply to 4D as well, and he says that while this appears at the beginning to make the analysis harder, in the end because it is the "right" way one finds that it is actually more effective.

He explicitly excludes using AdS/CFT in treating the case of 3D gravity with matter. So the work in progress addressing the 4D case must not be using AdS/CFT either.

In Laurent's work, described here in the 15 May talk, he GETS FEYNMAN DIAGRAMS OF MATTER OUT OF SPINFOAMS (which are the "paths" of spacetime geometry evolving in a path-integral picture).

So this recent seminar talk by Freidel should afford an illuminating comparison with what Witten is currently doing with pure 3D gravity.

 

[arivero]

I still think that all the goal of Witten exercise (for him it is) is to bring some light on the uses of AdS/CFT, perhaps to eventually get a hint of some method for AdS/QCD

 

[marcus]

here is a sample exerpt from the Madrid talk, slides 14 - 16:
==quote==
First of all, I am only going to consider the case of negative cosmological constant.
Currently there is some suspicion that quantum gravity with Lambda > 0 doesn't exist non-perturbatively (in any dimension) with positive cosmological constant. The reason for this is that it does not appear to be possible, with Lambda > 0, to define precise observables. This is natural if it is the case that a world with positive cosmological constant (like the one we may be living in) is always unstable.

If that is so, then a world with Lambda > 0 doesn't really make sense as an exact theory in its own right but (like an unstable particle) must be studied as part of a larger system.

Whether that is the right interpretation or not, since I don't know how to define any precise observables, I don't know what it would mean to try to solve 2+1-dimensional gravity with Lambda > 0, since it isn't clear what we'd want to compute.
==endquote==

Any comment?

 

[Boris Leykin]

So let's try to follow up on that theme. An influential string theorist is doing non-string gravity research. Is that part of a more general trend?

I thought that maybe Witten is going to merge String theory and LQG, that maybe he will cook up something like spin foam representation of String theory, this is what I wanted to ask for.

 

[marcus]

I thought that maybe Witten is going to merge String theory and LQG, that maybe he will cook up something like spin foam representation of String theory, this is what I wanted to ask for.

I don't expect that from Witten at all! But other people are working on stuff like that. There was posted just yesterday by Khavkine and Conrady.

the nice thing when you use spinfoam methods is you can get a version of string-like object that does not require higher dimensions----the action happens in 4D or maybe 5D.
A string theory purist would not like this---maybe he would hate it---but some people find it interesting.

http://arxiv.org/abs/0706.3423
An exact string representation of 3d SU(2) lattice Yang--Mills theory
Florian Conrady (Penn State U.), Igor Khavkine (Western Ontario U.)
12 pages, 7 figures
(Submitted on 23 Jun 2007)

"We show that 3d SU(2) lattice Yang--Mills theory can be cast in the form of an exact string representation. The derivation starts from the exact dual (or spin foam) representation of the lattice gauge theory. We prove that every dual configuration (or spin foam) can be equivalently described as a self--avoiding worldsheet of strings on a framing of the lattice. Using this correspondence, we translate the partition function into a sum over closed worldsheets that are weighted with explicit amplitudes. The expectation value of two Polyakov loops with spin j becomes a sum over worldsheets that are bounded by 2j strings along a framing of the loops."

Also there are several papers by John Baez which get interesting results from string-like objects in 4D. Seeing Baez work one is tempted to ask: why bother with higher dimensions??!
http://arxiv.org/find/gr-qc/1/au:+Baez_J/0/1/0/all/0/1

1. http://arxiv.org/gr-qc/0605087 [ps, pdf, other]
Title: Quantization of strings and branes coupled to BF theory
Authors: John C. Baez, Alejandro Perez
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Mathematical Physics (math-ph)
2. http://arxiv.org/gr-qc/0603085 [ps, pdf, other]
Title: Exotic Statistics for Strings in 4d BF Theory
Authors: John C. Baez, Derek K. Wise, Alissa S. Crans
Comments: 41 pages, many figures. New version has minor corrections and clarifications, and some added references
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Geometric Topology (math.GT)