Strings 2011 started today in Uppsala, with attendance quite a bit lower than in the past (259 registered participants, versus 500 or so at some of the past such conferences). One reason for this may be the high conference cost (discussed here), another may be that excellent video of the talks is available, so why bother traveling to Uppsala?

The opening talk was by David Gross, who tried to address the question “Where do we stand?” for string theory. He claimed the field is “extremely healthy”, “vibrant and exciting”, “making enormous progress in a variety of areas'”, with “stupendous progress” in N=4 planar SYM. At the same time, he acknowledged that it was “very sobering” that string theory was 43 years old.

In the past, Strings XXXX conferences often featured a call for progress towards making predictions that could be tested at the LHC. With LHC data now coming in, Gross acknowledged that this had been a failure: there are no string theory LHC predictions. He put a positive spin on this by noting that the lack of any BSM signal at the LHC so far is not a worry for string theory, since string theory can’t be tested at the LHC. As for the lack of any supersymmetry signal so far, he says that “I personally am not yet worried”, while acknowledging that some people are becoming pessimistic. While no SUSY is not a worry for string theory, he feels that “it would be awfully nice for string theory if SUSY appeared”. Supposedly he has made bets on SUSY at the LHC, but he gave no indication of when he would start to worry (or pay off the bets) if SUSY continues to not be there.

The main area of progress he sees is the usual gauge-gravity duality that has dominated the field for years, together with progress on N=4 SYM amplitudes. He sees Verlinde’s “Entropic Gravity” as an “exciting development I find enormously interesting”. Evidently later this week Verlinde will discuss his latest ideas about this which supposedly include an explanation of dark energy and dark matter.

Gross went over quickly the questions about string theory he first raised in a similar talk 26 years ago, which mostly remain unanswered, including the basic one of “What is String Theory?”. The additional questions raised by attempts to understand the emergence of spacetime in a deSitter background were one factor that inspired him to end with the quote that:

The most important product of knowledge is ignorance.

To which he added “After 43 years of string theory , it would be nice to have some answers.”

Surprisingly, not a word from Gross about anthropics or the multiverse. I assume he’s still an opponent, but perhaps feels that there’s no point in beating a dying horse. Susskind isn’t there and oddly, the only multiverse-related talks are from the two speakers brought in to do public lectures (Brian Greene and Andrei Linde, Hawking’s health has kept him from a planned appearance). So the multiverse is a huge part of the public profile of the conference, but pretty well suppressed at the scientific sections. Also pretty well suppressed is “string phenomenology”, or any attempt to use string theory to do unification. Out of 35 or so talks I see only a couple related to this, which is still the main advertised goal of string theory.

I’m looking forward to the talks of Witten, Gaiotto and Gukov, which I hope will provide a gentle introduction to their intriguing recent long papers on the arXiv. To the extent I find time to watch talks this week and have any comments about them, I’ll try and add updates to this posting.

**Update**: After looking at most of the talks online, the most remarkable thing about Strings 2011 is how little there is about string theory. One of the speakers, Chris Hull, started off his talk with the comment:

At lunch today one of the organizers was observing that my talk was unusual in being one of the few talks actually about string theory. It would be interesting to speculate on what that might mean about the state of the field, but it would be invidious to do so here.

One of the main themes of the conference so far has been study of mathematically interesting supersymmetric QFTs in 3,4,5 and 6 dimensions, often obtained from a specific class of 6d theories, which themselves remain poorly understood (what is known about them was reviewed by Greg Moore). Witten gave an overview of his work relating Khovanov homology and QFT, which involves a chain of various 6d, 5d, 4d, 3d and 2d QFTs. Nati Seiberg reviewed the technology used for constructing these theories on various special backgrounds, noting that this was all about “rigid” SUSY theories, with supergravity and string theory making no appearance.

**Update**: The videos of the talks are now all up. I took a look at the Verlinde talk, and the ideas he is putting forward still strike me as pretty much empty of any significant content. In Jeff Harvey’s summary of the conference, he notes that many people have remarked that there hasn’t been much string theory at the conference. About the landscape, his comment is that “personally I think it’s unlikely to be possible to do science this way.” He describes the situation of string theory unification as like the Monty Python parrot “No, he’s not dead, he’s resting.” while expressing some hope that a miracle will occur at the LHC or in the study of string vacua, reviving the parrot.

That the summary speaker at the main conference for a field would compare the state of the main public motivation for the field as similar to that of the parrot in the Monty Python sketch is pretty remarkable. In the sketch, the whole joke is the parrot’s seller’s unwillingness, no matter what, to admit that what he was selling was a dead parrot. It’s a good analogy, but surprising that Harvey would use it.

The talk that Verlinde is scheduled to give later this week at Uppsala has the same title as one he gave last week (22 June) at Perimeter. Anyone interested can watch the Perimeter video of the talk, which got some lively reactions from the audience.

http://pirsa.org/11060065/

The Hidden Phase Space of our Universe

Erik Verlinde

By combining insights from black holes and string theory we argue for the existence of a hidden phase space associated with an underlying fast dynamical system, which is largely invisible from a macroscopic point of view. The dynamical system is influenced by slow macroscopic observables, such as positions of objects. This leads to a collection of reaction forces, whose leading order Born Oppenheimer force is determined by the general principle that the phase space volume of the underlying system is preserved. We propose that this adiabatic force is responsible for inertia and gravity. This fact allows us to calculate the hidden phase space volume from the known laws of inertia and gravity. We find that in a cosmological setting the appearance of dark energy is naturally explained by the finite temperature of the underlying system. The adiabatic approximation that leads to the usual laws of inertia and gravity breaks down in the neighborhood of horizons. In this regime the reaction force degenerates into an entropic force, and the laws of inertia and gravity receive corrections due to thermal effects. A simple estimate of these effects leads to the conclusion that they coincide with observed phenomena attributed to dark matter.

Date: 22/06/2011 – 9:00 am

“While no SUSY is not a worry for string theory, he feels that “it would be awfully nice for string theory if SUSY appeared”.”

I confess I keep being puzzled about this situation. As I understand the situation with string theory without SUSY is pretty ugly, isn’t it? I mean, what is left is the bosonic ST, so why people like Gross keep on insisting no SUSY is not necessarily bad for ST? It seems to me that more than half of their business is gone without SUSY.

If the Higgs but no Susy is found by the LHC , I would say it would be a fatal blow to String Theory’s reputation. It’s kind of exciting that after 43 years, “the day of judgment” for all my friends that are String Theorists is less than 2 years away.

Bernhard,

strings without SUSY are pretty ugly. but the point is that at string scales SUSY can be restored in a lot of models.

what makes SUSY “appealing” for the rest of us is the potential to explain or soften the hierarchy and naturalness problems and to provide a DM candidate. for this you need a relatively low SUSY breaking scale in the region and somehow connected to EW symmetry breaking. but for string theory this is irrelevant. the SUSY breaking scale might be anywhere and it might not explain a single thing at TeV scales or below and yet it is good enough for strings.

We have some slides from Witten’s talk in Berkeley at the Michael Freedman 60th birthday conference available here (as well as from a few other talks):

http://web.math.berkeley.edu/~matthias/conference/abstracts.html

Peter, what do you think of this?

http://arxiv.org/abs/arXiv:1009.5414

(Gravity is not an entropic force, Archil Kobakhidze: We argue that experiments with ultra-cold neutrons in the gravitational field of Earth disprove recent speculations on the entropic origin of gravitation. )

Ian,

Thanks. I watched Witten’s talk today, it’s a very general overview of what he’s doing. The slides you link to give a more detail about one piece of this.

King Ray,

My quick attempt to understand Verlinde’s derivation of Newton’s law left me suspicious that there was nothing to it but dimensional analysis, in which case, you can’t prove it wrong. I don’t know if the paper you link to refutes Verlinde in any sense. If Verlinde now claims to explain dark energy and dark matter, that’s more substantive. I presume that it will soon get a lot of attention, personally I’ll wait for experts to sort out what is going on with these ideas.

A few years ago, Gell-Mann said in an interview that the biggest failure of string theorists was the failure to clearly specify its principles. After several decades of research into the field still nobody came up to the challenge. I recently did a literature search (both papers and books) and there is indeed nothing on the principles of string theory. If we recall that most string theorists are part-time mathematicians, and mathematics is built on axiomatic systems of some sort, the lack of writing down axioms and principles is an important sign.

Together with the remark that almost nobody is giving talks about string theory anymore, this is a strong hint that people are finding, maybe only unconsciously so far, that the emperor is naked after all.

@marc

In the same vein you can ask “What are the principles of Yang-Mills theory?”. There’s many results about perturbative Yang-Mills in four dimensions, but we lack a complete understanding beyond that. For string theory the situation is about as good or bad depending on your point of view. Definition-wise that is. For pure mathematicians string theory is a black box which generates conjectures.

Proudmemberofthecult,

That’s just ridiculous. We have a rigorous definition of non-perturbative quantum YM in 4d, even if it is an open problem to prove that it has all the properties that it appears to have (for which there is lots of evidence, numerical and otherwise). At every major conference he speaks at, Gross makes the point repeatedly that the big question is “what is string theory?”. No one has ever stood up at a major conference and said that the problem with quantum YM is that we don’t know what it is.

General conjectures about the properties hoped for in string theory have in the past led to interesting mathematical conjectures. The interesting thing about Strings 2011 is that most of the speakers have stopped trying to do this, instead sticking to QFT itself.

Forgive me, but how is this possible? It has always been my understanding that String Theory _depends_ upon SUSY. That without SUSY String Theory fails completely as a theory of gravity or unification. If the LHC does not see SUSY or rules out a large parameter space how is this not staggeringly bad for the proponents of String Theory?

I’m just a layman so I’d really appreciate any answers as this has me baffled.

Since string theory is dual to YM theory, there is no reason to discuss the ill-defined string theory anymore, we can stick to things we understand, and this is what most people are doing.

manyoso,

String theory never has been able to say anything about what causes supersymmetry breaking, in particular, it doesn’t even say anything about what the scale of such breaking should be. So, it could be an energies high above where the LHC could reach. It’s a generic phenomenon: string theory predicts absolutely nothing, and in particular it predicts nothing at all about what the LHC will or won’t see.

anony,

Great. So, Strings 2012 is being renamed “QFT 2012”?

So, if they really rename it and do just QFT from now on, Peter could peacfully terminate this blog because it`s aim would finally be achieved

Cheers

The kind of QFT discussed this year is really bizarre (i.e. maximally far from things relevant to nature, except maybe for 1 or 2 talks). So the most suitable name is probably ”bizarre QFT 2012.”

If we magically knew that there was no supersymmetry at any energies, I think it is a pretty strong argument against string theory. Unfortunately LHC will check the existence (or not) of SUSY only upto a very low energies (“TeV”) compared to Planck scale. This is the sense in which it is “encouraging” for string theorists if LHC finds SUSY, but admittedly nothing more.

As to the reason why some supersymmetric QFTs are being studied at Strings 2012 is because many of them are interesting because of their nuemrous connections to 2D CFTs, string theories, etc. Many string theorists including myself, love to berate our field (who wouldn’t like to make a testable prediction?), but that should always be taken with a grain of salt. Sorry to speak truth to the power here, but anyone who claims in this day and age that N=2 (in 4D and analogous things in other dimensions) supersymmetric quantum field theories have nothing to do with string/M-theory is agenda-driven. Seiberg-Witten solution which looks magical from 4D is AUTOMATIC from a brane-engineering point of view. Gaiotto’s construction of his N=2 theories which started the recent interest came DIRECTLY from M2-branes wrapping Riemann surfaces. The AGT conjecture whch is another focal point of the current interest immediately relates these theories to 2D CFTs. Not to mention the are numerous connections these things have with topological strings, matrix models, etc.

This message only serves a public service point, Peter of course has been well known on this blog to describe anything interesting to come out of string theory, as NOT string theory.

Interesting somebody,

thanks for these explanations 😉

Hm. The claim I saw is that N=2 SUSY has nothing to do with nature (“bizarre QFT”). Does not N=2 have a number of peculiar properties (e.g. non-renormalization theorems) that are manifestly false for the SM.

peaceful termination,

The disappearance of string theory would certainly mean that this blog would get a lot less attention, not necessarily that it would disappear…

Somebody,

One can make as much hype as one wants claiming that all good ideas in the hot topics of today (amplitudes, N=2 SUSY, various applications of gauge-gravity dualities, etc.) historically come from string theory, but the undeniable fact of the matter is that if you watch the talks at Strings 2011, virtually no one is talking about string theory itself or using string theory anymore to do anything. A student who wants to work on any of the hot topics has no reason to bother to learn string theory anymore.

What’s remarkable is that this seems to be true even in those areas where string theory has had some success, far away from the heavily promoted ones. Besides virtually no talks about string theory and unification, there’s also almost nothing about string theory as a theory of quantum gravity. The hot topic of recent years, the idea that string theory would explain heavy ion physics, seems to have completely disappeared.

Thomas,

Personally I find some of the work being discussed about these SUSY gauge theories quite fascinating. There’s some beautiful mathematics, and techniques being used (e.g. “SUSY localization”) that may some day have important applications to physical theories. These theories are not physical, but they are tantalizingly close to physical theories. That said, much of what is being done is “topological quantum field theory”, which inherently uses techniques special to topological, not physical, observables.

Of course I disagree with Peter’s claims despite his vehemence. But we already knew that.

The real reason for this comment is that I wrote “M2-branes” wrapping Riemann surfaces, when I meant “M5-branes”.

I don’t see any vehemence in Peter’s post. I think too often we read too much into conversations on the internet. Imagining emphasis and body language that is not inherently conveyed by the printed words on the page. That emphasis and body language we imagine is all too often just a manifestation of our own minds and not actual communication.

Close your eyes and see if you can see the posts above coming in a jovial manner from two respected colleagues frankly disagreeing, but not in a disagreeable way.

“… virtually no one is talking about string theory itself or using string theory anymore to do anything. ”

That’s like saying that people doing AdS/CFT are “just doing field theory” (since the string theory side of the duality “isn’t defined”).

To pick one example, consider Witten’s talk on Khovanov Homology. He starts with the D3-NS5 system in Type IIB, S-dualizes to the D3-D5 system, T-dualizes on a transverse circle, to get the D4-D6 system in Type IIA and then, finally, lifts the whole construction to M-theory (where he, finally, obtains a description in terms of the 6D (2,0) theory).

I suppose one could characterize this work as “just field theory” (a “followup on his work on Chern-Simons theory”). But his paper (and most of the talks, however “field theory”-oriented, at Strings 2011) would be completely incomprehensible if you don’t know a good deal about string theory.

In fact, what you need to know is “modern” string theory. In his concluding remarks, Jeff Harvey noted that most of the talks would be utterly incomprehensible to even a well-trained string theorist who, Rip Van Winkle-like, had slept through the past 15 years.

Wolfgang,

The terminology Witten uses in his talk is based on string theory, but what he actually does doesn’t use string theory. Those names for dualities and boundary conditions correspond to constructions that are quite independent of string theory. If you look at the more detailed description of some of the same material that Witten gives when talking to mathematicians (see the slides Ian Agol links to), you’ll see no reference to string theory at all, with string theory/M-theory terminology replaced by explicit boundary conditions. It’s true that the way Witten and others found those boundary conditions was from studying string theory, but that quite possibly is just an historical accident, you could imagine coming upon them from other starting points.

Better yet, look at the main paper of Witten’s that his talk is an overview of. All the calculations are in field theory, not string theory. Here’s how Witten himself describes the situation (see pg. 12 of the “Five-branes and knots” paper):

“One goal is to give a gauge theory definition of Khovanov homology (as opposed to a definition that requires a full knowledge of string/M-theory). String theory and branes will be used as clues, but the results can be expressed as a gauge theory construction.”

My understanding is that he thinks the fundamental object here is the 6-dimensional (0.2) field theory and he would argue that, since that isn’t well-defined, much of what we know about it comes from string/M-theory. It seems to me that we’ll have to see what the future holds as this theory is better understood. It’s quite possible that it will have some good definition and way of studying it that has nothing to do with string/M-theory.

What Witten is finding here (and I think it’s true in a lot of the other “physical mathematics” work described at the conference) is that the huge amount of knowledge developed over the last 20 years relating string theory and gauge theory is quite useful in providing hints as to what to look for, but at the end of the day, the interesting results don’t require string theory calculations to get to, or string theory ideas to understand.

Jeff Harvey noted that many people at the conference were asking “where did the strings go?”, and Chris Hull made much the same point. I don’t think the people they were talking to who were asking this question were people who have been asleep for the past 15 years. Quite the opposite: I think they’re people who are very aware of what the state of the field is, and are seeing a quick swing away from string/M-theory to return to more conventional field theory ideas and methods.

The relative absence of “real” string theory in the recent conference has been noticed elsewhere.

Lubos Motl is ascribing it to string theorists trying not to arouse the envy of their non-stringy colleagues. Always entertaining, this Lubos.

“The relative absence of “real” string theory in the recent conference has been noticed elsewhere. Lubos Motl is ascribing it to string theorists trying not to arouse the envy of their non-stringy colleagues.”

That could be funny. Do you have a link? I think the relative absence of string (and the relative presence of what Weinberg in a talk at CERN called “good old QFT”) needs to be seen in the context of declining string jobs compared with first-time faculty hires in particle theory/cosmology as a whole. Peter reported on that earlier.

“Quite the opposite: I think they’re people who are very aware of what the state of the field is, and are seeing a quick swing away from string/M-theory to return to more conventional field theory ideas and methods.”

You reported on Weinberg’s CERN talk almost exactly 2 years ago:

http://www.math.columbia.edu/~woit/wordpress/?p=2199

He plotted the “stock price” of conventional QFT over the past 80 or so years and extended the curve with a dotted line where he expected it to go. Basically he predicted this “quick swing” or QFT resurgence—and described a general pattern or mechanism using several historical examples. The talk video is online and could be worth watching a second time.

“If you look at the more detailed description of some of the same material that Witten gives when talking to mathematicians (see the slides Ian Agol links to), you’ll see no reference to string theory at all …”

Nor to quantum field theory. In the end, the problem comes down to solving certain PDEs. And you can explain those PDEs to mathematicians, without any reference to string/field theory.

It would, however, be impossible to deduce those equation without using string theory (and field theory) as a guide. And, if you read Witten’s paper (and the previous papers with Gaiotto), the same can clearly be said of the relations between the various field theories (in 3, 4, 5 and 6 dimensions) that he uses.

It is all very well to fantasize about an alternative universe where a purely field-theoretic explanation of those dualities between field theories was discovered. Perhaps, somewhere in the multiverse, that universe exists. But it’s not our universe …

The video of Frank Wilczek’s talk is no longer online.

I especially liked that talk and would like to watch it again. Does anybody know of an alternative link? I don’t see that any of the other talks have been taken offline.

What to expect?

It used to be number 24.

Now you get some kind of message in Swedish.

Frank Wilczek’s talk is up. The sound is not well synchronized with the image though:

http://media.medfarm.uu.se/flvplayer/strings2011/video24

What did he mean with the Portals though? I suspect a Wheatley joke was planned.

Wilczek’s “I certainly don’t recognize it” response to Witten’s very insightful comment was, to say the least, puzzling. How can he NOT be aware that low scale SUSY + a QCD axion implies a Polonyi-type problem? Wow!

For your benefit:

here is the parot´s joke:

http://www.youtube.com/watch?v=e6Lq771TVm4

and here is the suggest theme song of strings2011

http://www.youtube.com/watch?v=AQcmBUc0OQc

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