Answer your string questions for a nickle!

[Martin]

Are we talking about particle physics or not?

 

you were talking quantum gravity Ben. You flatly said that non-string QG approaches require a lot of "tickling" (and you did not qualify that or restrict to particle physics).

 

that is a false statement as it stands, someone could get the idea that non-string QG takes a lot of made-up stuff put in by hand (as we are used to hearing about in the string context)

 

QG approaches normally do not do particle physics----they are only just getting started really. It is not clear that one has to do the two together since Reuter has shown that plain old gravity is renormalizable.

 

QG approaches typically just do gravity with generic matter and they are MINIMALIST-----the absolute minimum made-up stuff like extra dimensions branes AdS asymptotia and all that.

 

You made a flat unqualified statement about QG. So we are talking about QG and NOT particles of course. And your statement was extremely misleading. Please correct it!

Thanks

 

... advisors don't appreciate when students come running to them with questions.

 

that depends. Asking a question in the coffee room about Alain Connes derivation of the Standard Model from non-commutative geometry might even impress someone that you were exceptionally bright

I didn't say ask your advisor. Ask one of the OSU theorists like Sami if he has ever heard of Connes. Or your good friend Josh Friess, anybody.

Connes is a major guy and it is incredible that he would commit his theory to predictions of phenomena already ruled out by, as you said, "astrophysics".

I think you must be the only person in the world that thinks this.

 

Oh here!

So either I am confusing what Connes is talking about (most likely) or he is off by ten orders of magnitude.

That sounds more reasonable. In your original post you said "clearly he is wrong". Flat, no qualification. It would would mislead people here by giving them the mistaken impression that you know what you are talking about.

But now you are allowing for the possibility that you might be wrong. That is fine with me.

 

The day Alain Connes is off by ten orders of magnitude, having be the whole NCG standard model on it, and NCG is falsified, will be a great day:-)

 

If you want me to start explicating stuff to you, BTW, please look over some Reuter papers and get a taste of his online recorded talk. It is relatively simple and I think you have the background for it.

 

 

This is exactly the point---dimensions and gauge groups are added by hand, just as in traditional approaches. This is tickling.

 

Great! This is a wonderful SCIENTIFIC issue to discuss, Ben! You have an idea of tickly----putting odd made-up stuff in a theory to get it to work right--that is very different from mine.

 

I look around and see that the world is 4D and no experiment has ever proven otherwise. So I take that as given! Now let's say I'm a quantum gravitist building a QG model: I naturally go for the 4D version! Most things like Causal Dynamical Triangulations (CDT) or Spinfoam work easier in lower dimension like 2D or 3D and the trick is to get them to work in 4D without "cheating"

 

But you seem to think that I am cheating when I go for the 4D version. Your attitude seems to be that when I pick the 4D version to study that is tickling. You are PROUD, apparently, that some stringy theory won't work in 4D or in any dimension besides TEN.

 

So you put in extra rolled up Ds and wrap fluxes around to stabilize it and do what all--and that you say is not putting in anything by hand because...

 

We seem to have different ideas of what is a clean minimalist theory quantizing Gen Rel without a lot of made-up stuff. And different ideas of a "tickled" theory.

 

And I could be wrong, maybe you are thinking about some other dimension version of string, not a 10D version. Perhaps you should say what it is you are busy getting Standard Model numbers out of.

 

I'm curious about something. Isn't the stringy model you are setting up in order to extract Standard Model numbers, isn't that MORE COMPLICATED than the Standard Model itself?

 

I guess my question this time is, why should one bother constructing a low-energy stringy model just to replicate Standard Model results?

You already said, I think, that it was not supposed to be fundamental or to work at higher energy. So this seems NOT to be about nature but about doing something that has already been done some years ago with SM. Presumably I'm missing something. What's the motivation?

[vincent]

Hi BenTheMan,

 

Strings are really wrapped M2-branes. Also, what did you mean by GR pops out of an S-matrix calculation?

[ajb]

Hmmm. Hopefully I won't make any mistakes, but if ajb is floating around here, maybe he can correct them for me

 

Thanks for the vote of confidence, but you have got this one.

 

If people are generally interested in string theory then they should consult Zwiebach or the two volumes of Polchinski.

 

Another good lace to start is Szabo's BUSSTEPP letures, or his book based on these lectures.

 

Kaku I have found also useful, if a bit rushed. It is a good reference book overall.

 

The "grand daddy" of all string references is Green, Schwarz and Witten Vol1 and Vol2. However, it should be noted that this was written before the "second string revolution" and before the importance of D-branes was realised. As such branes are not discussed.

 

Good luck

[vincent]

There are also the following four more recent texts:

 

String Theory and M-Theory: A Modern Introduction

 

String Theory in a Nutshell

 

Supersymmetry and String Theory: Beyond the Standard Model

 

D-branes

[ajb]

There are also the following four more recent texts:

 

String Theory and M-Theory: A Modern Introduction

 

String Theory in a Nutshell

 

Supersymmetry and String Theory: Beyond the Standard Model

 

D-branes

 

 

I am aware of these books, but as I have never used them I cannot directly say how "useful" they are, but they are all written by well know authors.

[BenTheMan]

Hi vincent----

 

I can't speak about M2 branes.

 

As far as getting GR from string theory, first you have to believe in gravitons Then I think you just have to look at the IR limit of the four graviton scattering amplitude. The non-linear behavior of GR pops out of this, I think, or can be understood in terms of it. It can't reproduce EXACTLY GR, but that's not what we'd want anyway---GR has singularities which we're trying to get rid of by quantizing the theory. There will probably be alpha' corrections that GR doesn't know about, but this (again) is exactly what I'd expect.

 

I should say I've never done this calculation myself, but there's probably a section on it in one of the textbooks you linked to.

[vincent]

As far as getting GR from string theory, first you have to believe in gravitons

 

I believe! I believe!

 

I think you just have to look at the IR limit of the four graviton scattering amplitude. The non-linear behavior of GR pops out of this, I think, or can be understood in terms of it.

 

Okay, I think you’re referring to the way that one can obtain from string scattering amplitudes the low-energy effective field theory. In this case, since it is a graviton scattering amplitude, one would obtain the usual Hilbert action. But this is not what is usually meant by the remark that string theory implies General Relativity.

 

As you know, different particles in string theory are represented by strings with different sets of internal modes of oscillations excited. Then assuming that strings represent a unified model of all interactions, every field with which strings can interact must be contained in the spectrum of states associated with the quantization of a string’s free oscillations. The point is that it’s the requirement that the dynamics of all background fields , even at the classical level, must be determind so as to be consistent with the results of string quantization that requires all background fields satisfy the Einstein equations.

 

The demonstration of how this works begins with so-called nonlinear sigma models which describe how the world-sheet theory is embedded in spacetime. It’s terms represent the interactions of strings with all of the background fields. General Relativity is a result of requiring the two-dimensional conformal invariance of the world-sheet theory present in all string models and which is crucial for obtaining the quantum spectrum in which the background fields themselves are included not be destroyed by any of these interactions.

 

The condition that there be no conformal anomaly is just the condition that the beta functions vanish. What is suprising is that the expressions for the beta functions turn out to be nothing but the Einstein equations for the background fields.

[BenTheMan]

vincent---

 

Thanks for the clarification. As I say---I haven't done these calculations, so I will differ to your expertise

[Wormwood]

Nono. It only has a length, no depth or height.

How is that comprehendible though? How can it have length, yet absolutely no width or height? I know there are more knowledgeable people around and I am holding you back with a fundamental concept, but I can't make myself understand how this is possible. If your size were relative to the string, it would HAVE to have height or width for it to have length. Length denotes a physical existence, but there must be at least a second trait to which this length can be attributed, or else there is no object to have length.

 

Also, one dimensional would mean that strings exist outside of time right?

 

 

I had something else to ask about too. This is fun, isn't it wormwood?

It is. However, you guys are a good bit smarter than me, so the fun for me is just understanding what you guys are talking about

[vincent]

Hi Wormwood,

 

How can it [strings] have length, yet absolutely no width or height?

 

What’s required here is a fundamental shift in your understanding of how meaning is attributed to ideas in physics. In a nutshell, in physics you have to follow the math. The question we ask as physicists is not how we feel about the idea of a string, but whether such a theory can describe our universe. The consensus among researchers continues to be that it probably can and quite probably does, though it may take a while before we fully understand how this happens.

 

As an example, string theory emerged from an attempt to describe the strong force using so-called dual resonance models. Gabrielle Veneziano discovered in a table of mathematical functions an expression which seemed to have the properties required for it to serve as the relevant scattering amplitudes. However nobody really understood what physical objects these amplitudes actually described. Eventually Leonard Susskind, by following the math, figured out that they describe objects having finite spatial extension in precisely one direction - i.e., they described strings. Leonard was very happy about this, and you can be too (unless you can find a better theory whose properties reflect your own personal world view).

[Wormwood]

^^^ That actually makes sense if I understood it as well as I think I did. So it is one dimensional as a mathematical expression, though the actual physical manifestation of this isn't really decipherable? Strings are just fundamental objects that behave as an extension in one direction for all intents and purposes, whether we can ever experience them as such. Does that sound close?

[vincent]

^^^ That actually makes sense if I understood it as well as I think I did. So it is one dimensional as a mathematical expression, though the actual physical manifestation of this isn't really decipherable? Strings are just fundamental objects that behave as an extension in one direction for all intents and purposes, whether we can ever experience them as such. Does that sound close?

 

Yes, this is just as good as the way I put it.

[Wormwood]

^^^Thanks

 

You too Ben.

[BenTheMan]

Martin---I haven't forgotten about you. I will respond to you eventually.

[Martin]

Martin---I haven't forgotten about you. I will respond to you eventually.

I am not impatient. I am enjoying the thread.

Actually my most pressing question was what about the quantum state of the geometry of spacetime (string framework doesnt seem to have a quantum state of geometry, i.e. a quantum state of the GR metric)

 

and if I remember you DID respond saying you didn't know about that, which is fine---not everybody should know about everything for goodness sake!

So that puts me at ease for the time being. But if you find out anything by talking to others, let me know.

[Guest026]

what is a string question?

[BenTheMan]

ummm a question about strings?

Specifically, do you have any questions about string theory?

[BenTheMan]

Martin---first of all apologies for taking so long to respond. I wanted to think about a coherent response, and I have been busy trying to finish a project I am working on, before taking a few days off to drink beer, catch fish, and get sunburnt back home in Texas.

 

I think that the problem is that we may be on different wavelengths. If I read correctly, when you say phenomenology, you are thinking primarily of cosmology. I have been using the word in a more limited context, referring specifically to the particle physics side. String theory DOES have problems getting inflation right, which leads to things like KKLT. You can take all of my comments about ``phenomelology'' as pertaining solely to the particle physics side of things---this is the only side that I really know about, anyway.

 

you were talking quantum gravity Ben. You flatly said that non-string QG approaches require a lot of "tickling" (and you did not qualify that or restrict to particle physics).

 

that is a false statement as it stands' date=' someone could get the idea that non-string QG takes a lot of made-up stuff put in by hand (as we are used to hearing about in the string context)[/quote']

 

Do chiral fermions come for free? Well sure they do, because they are representations of the Lorentz algebra. But, wait---you can only have chirality in even dimensions, which were put into your theory by hand. So, in a sense, chirality is not something that you get out of the theory---you had to specify the number of dimensions.

 

The comment about tickling still stands, (it cannot be ``true'' or ``false''---it is subjective) and is much softer than other comments I could have made. As far as anything I've ever read about other approaches to quantum gravity, the programs all attempt to do the same thing---find a way to make quantum mechanics consistent with gravity, starting from what we know about GR. I had assumed that there was some push to explain the things we see. To get low energy phenomenology, you wave your hands and say ``general fields and couplings''. But that's what we've been doing for a long time, and something that sring theory at least gives a hope of constraining.

 

For example, when building an SO(10) GUT (see Raby, Dermisek, or papers by Shafi, or Dorsner), you basically add SO(10) reps untill you fit the low energy data. The only requirement is freedom from anomalies in the LEEFT. But in strings, there are stringent modular invariance conditions that you must satisfy. So, for example, things like technicolor may not work---the modern technicolor theories require fermions in large representations, and getting reps above the adjoint in most approaches to string phenomenology that I am familiar with is pretty difficult. (This is just an example, and I haven't checked myself, so I don't know for sure.) Plus the main motivation for technicolor is to get rid of scalars in the LEEFT, and there are TONS of scalars in stringy EFT's. So whereas you may be able to make some SO(10) GUT with a huge higgs sector consistent with LQG, it may be impossible to get from string theory.

 

You made a flat unqualified statement about QG. So we are talking about QG and NOT particles of course. And your statement was extremely misleading. Please correct it!

 

I will not. But perhaps I am expecting too much from the program. To get good particle physics you still have to do the same things that people do in traditional GUTS---you put fields and couplings into the theory, instead of getting them FROM the theory. Surely you can see the difference?

 

I didn't say ask your advisor. Ask one of the OSU theorists like Sami if he has ever heard of Connes. Or your good friend Josh Friess, anybody.

Connes is a major guy and it is incredible that he would commit his theory to predictions of phenomena already ruled out by, as you said, "astrophysics".

I think you must be the only person in the world that thinks this.

 

The comment that I made is that it is quite odd that someone would say that top quark and tau neutrino yukawa couplings had to be of the same size, when this naively means they should have the same mass. You have made no real attempt to explain this to me---your argument seems to be that we should rely on the fact that Connes is Connes--- ``Well, Connes is famous and you're just a grad student.'' All of the things that I've found online say that yukawa couplings to a driac neutrino are constrianed to be less than 10^-10, by experiment. Now it is your job to explain to me why this bound (aparently) doesn't apply here. If you don't know, then you don't know.

 

And Josh doesn't do physics anymore, he works in finance and watches Packers games.

 

If you want me to start explicating stuff to you, BTW, please look over some Reuter papers and get a taste of his online recorded talk. It is relatively simple and I think you have the background for it.

 

When things slow down (I have three projects right now!), I will try to learn some of Reuter's stuff. My GR is pretty weak, because I've never really needed it for anything, so maybe I can learn something there, too.

 

But, what guarantee is there that Connes' work has any relation whatsoever to Reuter's work? When I tried to ask general questions concerning ``LQG'' in another thread, you became quite elusive with your answers, like ``Well, Smolin is wokring on something else'', or ``There are many different approaches to the problem''. So which is it? As far as I know, you don't have dualities hanging around, so there is no hope to connect Connes work to Reuter's work by taking a large coupling limit, or something.

 

I look around and see that the world is 4D and no experiment has ever proven otherwise. So I take that as given! Now let's say I'm a quantum gravitist building a QG model: I naturally go for the 4D version! Most things like Causal Dynamical Triangulations (CDT) or Spinfoam work easier in lower dimension like 2D or 3D and the trick is to get them to work in 4D without "cheating"

 

This is, of course, the canonical response. Martin---I thought you were more creative I've heard BIOLOGY professors come up with better You will get a stock response from me as well. The thing is, suppose we DO find 6 dimension physics at LHC, at a TeV. Or suppose the gravity experiments in Univ. of Washington get positive results. This is pretty unlikely, but still possible. Will you say ``Well, non-string QG doesn't predict extra dimensions, so it is wrong'', or will you go around to all of your papers and erase ``SO(3,1)'' and write in ``SO(5,1)''? My guess is the latter, of course. Then you will still use the same argument that the world is six dimensional, and string theory predicts all of these extra dimensions that we don't see.

 

The irony of it all is that, even if we did stuble on six extra dimensions, you theory STILL wouldn't be ruled out because you would just erase SO(4,1) and write in SO(9,1), and complain about background independance.

 

And besides, can you see electrons? I can't. But I know what the experimental consequences are. We observe electron transitions in hydrogen spectra, for example, so we know there are electrons jumping around in there somewhere. Likewise, we have things like generations (and, hopefully) N=1 SUSY in our QFT. String theory says these things are all topological quantities. So we can see the consequences of extra dimensions even though we don't see the extra dimensions directly.

 

One could argue that untill we find a vacuum selection mechanism, we are really tickling the theory by choosing the appropriate Calabi-Yau. This is still a significant improvement, because we have a finite choice of Calabi-Yaus, and (in principle at least) you can choose ANY number of dimensions and still make your theories work.

 

So you put in extra rolled up Ds and wrap fluxes around to stabilize it and do what all--and that you say is not putting in anything by hand because...

 

We choose a compactification consistent with the low energy data, which is pretty restrictive, actually. I still think that predicting a number of dimensions is MUCH better than putting the space-time symmetry in by hand. The moduli stabilization is a huge problem, no doubt---mainly because so things depend on getting moduli to take specific values.

 

I'm curious about something. Isn't the stringy model you are setting up in order to extract Standard Model numbers, isn't that MORE COMPLICATED than the Standard Model itself?

 

More complicated in what sense? Are there more fields? Yes, of course---any LEEFT you get from string theory will be ``more complicated'' because E8xE8 is so big. But the low energy spectrum is exactly the same---good hypercharge directions, exotics decouple, realistic higgs sector, and (hopefully) good predictions about unification. We put in some constraints on the internal symmetries (boundary conditions and wilson lines), and we choose an orbifold. Everything else is specified from there. For example, in the specific compactification we are working in (T6/Z6-II), there are only 61 consistent ways to give boundary conditions to the internal degrees of freedom.

 

I guess my question this time is, why should one bother constructing a low-energy stringy model just to replicate Standard Model results?

You already said, I think, that it was not supposed to be fundamental or to work at higher energy. So this seems NOT to be about nature but about doing something that has already been done some years ago with SM. Presumably I'm missing something. What's the motivation?

 

Sometimes, late at night, when I have been coding for seven or eight hours, I ask the same question:) As you are no doubt aware, there is no one compactification of ANY string theory that gives good phenomenology. The closest, I think, is in a paper by Burt Ovrut and Volker Braun, followed by Kobayashi, Raby, and Zhang, and other models by Wingerter, Nilles and the people in Bonn, and Buchmueller, Leudling and Schmidt. Also, check papers by Cleaver (my master's thesis advisor at Baylor), Nanopoulous and Faraggi. Less succesful approaches are by Verlinde, and Cvetic, Blumenhagen et al. Without a doubt, though, the heterotic string is the way to go for good phenomenology.

 

The idea is kind of like sighting in your rifle---you shoot, and check the target. You're a bit off, so you adjust and shoot again, each time getting a little closer to the bullseye. So, for example, Kobayashi, Raby, and Zhang don't get good unification. Others don't get good Yukawa couplings. The Type IIA people (Cvetic, Blumenhagen, Shiu, ... ) end up with chiral exotics.

 

Anyway, once we have something that looks like the MSSM, with good Yukawa couplings and such, then we can start making predictions about experiments, which is what people like Smolin and Woit are bitching about. Presumably, there is not just one good model, but an ensemble of many models which give good physics. We can check the dualities and see what the model looks like in the other string theories. We can see how gauge coupling unification works. We can see how to get good cosmology. ...

 

So really it's like a Jackson problem. Part a is derive the MSSM from string theory, which takes you 30 years to finish. THEN you have to start part b.

[Martin]

Hi Ben, just saw your post. Too many issues to address all at once but I'll respond to some. I should tell you that my reaction to all your posts has been guided by a declaration you made early on that in the QUANTUM GRAVITY department, string has NO COMPETITION. For me that flat statment makes no sense. Quantum gravity means quantizing General Relativity---the geometry of spacetime. It does not mean reproducing the generations of particles that come out of the Standard Model.

 

It seems obvious that string has serious competition from Alain Connes in reproducing the Standard Model, that is, in the PARTICLES department. But that is even a side issue. When we talk QUANTUM GRAVITY, then string has very active growing rivals---maybe it is not even a front runner in quantizing General Relativity. GR is about the state of spacetime geometry, the metric. Any QG should have a quantum state of spacetime geometry, or be working towards getting one. I don't see this in string--maybe you can show us.

 

So I have been guided by the sense that you, and the people around you that you are talking with, probably have no idea what the real competition is. So my job was to steer you in the direction of a wider awareness.

 

The first thing that I have been trying to get you to do is simply glance at the TABLE OF CONTENTS so to speak---for example the list of talks at LOOPS 05, or LOOPS 07 (and the 2007 QG SCHOOL would be good too)

 

Earlier, it seemed to me you had a fixed idea that the competition was something from the 1990s called "LQG" and you had been listening to talks by Nima Arkani-Hamed about why that particular "LQG" couldn't possibly work. I witnessed this kind of narrowed perspective before on the part of stringfolks. It is so important to take an objective look at what the real competition is and simply glancing at the list, reading a few abstracts, would put you way ahead. At least you'd know generally what the competition is up to.

 

If you choose to use "LQG" as a loose catch-all terminology for the main non-string QG approaches, and many people do use the term that way, then you need to wake up to the fact that LQG is totally different from anything you thought you knew about LQG, or that Nima A-H thinks he knows, judging by your report. It is a rapidly changing field and the terminology at this point is inadequate. It isn't clear what "LQG" means. I can tell you about definite non-string approaches that feature prominently at the Loops conference, but they may seem strange and new if all you know is what you hear within the string circle.

 

I am not being elusive when I tell you that Smolin is working on something entirely different. I see his thing as very edgy, not central, but interesting all the same---it didn't exist before 2005. It is a way of generating the standard model which is completely different from Alain Connes approach, and elegant in the sense that there are very few if any choices and the underlying materials are very simple. It's hard. He has 4 or 5 people working on it. When I say that Smolin's research is not central to QG and not characteristic, I'm just reporting a fact. String people tend to use Smolin as a symbol of what they imagine is "LQG" and what I'm saying is don't do that, take a fresh look at what is actually presented at the big international conferences.

 

Because I know you must be busy, I didn't want to suggest learning about every approach-----an alphabet soup! (Reuter QEG, Ambjorn-Loll CDT, Spinfoam...). there are a lot of important Spinfoam names---Rovelli, Freidel, Livine, Krasnov. And Alain Connes NCG of course which has gained special importance for non-string QG because of work by Grimstrup which PUTS IT TOGETHER with a new version of LQG. I am only mentioning lead authors and leaving out their co-authors to keep it brief. Sorry about the alphabet soup!

 

Rather than talk about all these approaches, I tried to pick out one name for you to check out and that was MARTIN REUTER.

He has been something of a rising star at Loops 05, Loops 07, and the 2007 QG School. Attracted a lot of interest among the grad students and young researchers. I sense a move of researchers in that direction. There were 80 or so grad students at the QG School, it was for people looking to start research in QG, and Reuter was the surprise hit.

 

Right now my best advice to someone who wants a realistic idea of non-string QG----where the field is going at this moment in time---would be to check out some Reuter papers and also to look at the way Grimstrup is doing a fusion of a new version of LQG with Alain Connes noncommutative geometry.

Grimstrup is at Copenhagen, that's where NEXT year's QG School will be. I anticipate a big research drive to integrate LQG with Connes NCG.

 

I'm reading the most accessible Alain Connes paper (a 1996 "kindergarten" presentation) right now because I want to be able to understand Grimstrup's work better.

 

I'll try to respond to some of your comments.

 

When things slow down (I have three projects right now!), I will try to learn some of Reuter's stuff. My GR is pretty weak, because I've never really needed it for anything, so maybe I can learn something there, too.

 

GREAT! That would be wonderful. No good can come being unaware of the competition.

Quantizing GR is the whole idea of Quantum Gravity! GR says there is a state of the geometry of the universe---called the metric. Quantum GR says there is a quantum state of the geometry of the universe. It is that simple.

So don't say you never NEED it You might not need GR to study string but you need at least a surface familiarity with GR to talk about Quantum Gravity. Not to worry. I'm sure your grasp of GR is plenty strong for conversational purposes. I wouldn't claim any great expertise in it myself.

The good thing about Reuter is he shows GR is renormalizable. Finite at high energies.

 

But, what guarantee is there that Connes' work has any relation whatsoever to Reuter's work? When I tried to ask general questions concerning "LQG'' in another thread, you became quite elusive with your answers, like "Well, Smolin is wokring on something else'', or "There are many different approaches to the problem''. So which is it? As far as I know, you don't have dualities hanging around,..

 

There are no guarantees. I don't think string is guaranteed to match nature either, not in any simple unique way and perhaps not at all. No more is Connes' It is hard to doubt that Reuter has it right because when you have shown renormalizability you are back in safe Quantum Field Theory territory, but you can doubt Reuter too if you want

 

Now you asked about CONNECTING different approaches. Are there RELATIONS among approaches?

I already mentioned Grimstrup connecting Connes' work with a new version of LQG. This is quite interesting! The new LQG does not have a foliation--it is 4D rather than 3+1D. The new Grimstrup version LQG has other nice features, I am discovering as I read his papers, compared with say the 1990s traditional LQG.

 

You asked about connection specifically between CONNES AND REUTER.

I'm picturing an equilateral triangle with Connes, Grimstrup LQG, and Reuter at the vertices. If Connes and Reuter both describe nature correctly, then their work should connect because nature is one. that is the best guarantee I can give.

It is easier to see if you go around the other two sides of the triangle. Reuter was already talking about how he envisages linking LQG with his approach. He was talking about that at Loops 07 in Rovelli's discussion section.

And Grimstrup is connecting LQG with Connes.

This field of converging approaches, exploring equivalences, will, I think, be a good research area to get into.

 

The best thing on that was when Reuter gave specifics about a possible equivalence between his QEG and some version of LQG. It was speculative but the man's hunches are suggestive. that's in the Loops 07 audio.

 

String theory says these things are all topological quantities. So we can see the consequences of extra dimensions even though we don't see the extra dimensions directly.

 

There's a thought! Smolin's recent work (since 2005) has been concerned with getting the particles of the Standard Model out of topology. But the topology is that of knots---graph embeddings in 3D.

The goal of that research is to realize matter as topological complications in the geometry of space. Tangles in the quantum state of geometry.

It is certainly not central to current QG research but as a long shot it might turn out to be important.

 

One of the directions you see in QG is a tendency to try to get comparable things to what you get in conventional string approaches but without the extra dimensions.

 

I don't understand your objection to 4D. So far the string framework has not shown a testable unique theory and I see no firm evidence that it has anything to do with nature. So why should I suppose there exist other dimensions besides the usual four?

 

It seems to me that stringfolks are still struggling to repeat the success of the Standard Model.

By all signs it is a dreadfully difficult and complicated job just to get basically to where the SM was circa 1980. I don't see one unique way to do this coming out of their work---I see a Landscape. And I don't see them predicting any NEW numbers. All this effort just to match appearances with the OLD model.

But it looks like the competition, in the form of Alain Connes, has finessed the old SM rather elegantly and PLUS HE HAS PREDICTED NEW NUMBERS so that his approach can be tested by LHC or other doable experiments.

This doesnt look so good for string, so I tend to view the alleged EXTRA DIMENSIONS with skepticism.

Why should I believe in the existence of extra dimensions because they were found necessary in the 1990s when now it seems that other people can do as well or better without them?

 

Sometimes, late at night, when I have been coding for seven or eight hours, I ask the same question:) As you are no doubt aware, there is no one compactification of ANY string theory that gives good phenomenology. The closest, I think, is in a paper by Burt Ovrut and Volker Braun,...

 

...Anyway, once we have something that looks like the MSSM, with good Yukawa couplings and such, then we can start making predictions about experiments,..

 

Well your description of the laborious process to some extent speaks for itself. Alain Connes has raced ahead with some rather beautiful algebra and geometry concepts and gotten "something that looks like the Standard Model" and has already started making predictions.

 

You are at different places on the same track. but you have taken on the burden of assuming extra dimensions, in a way that he has not. So your work is made more cumbersome by the existence of 10⁵⁰⁰ different compactifications which might not be stable unless you something to prevent the extra dimensions from unrolling etc etc. And none of the compactifications seems to give quite the right numbers.

And once you have the right numbers THEN you can venture forward and make predictions of new numbers so that the model will be testable.

 

I can well believe that this can lead to asking questions late at night.

 

I see signs in the string community of confusion, discouragement, and gloom, and the general dwindling of the numbers seems to bear that out.

I try to keep an eye on things by watching video of the presentations at converences like Toronto Strings 05 and Madrid Strings 07. And of course when Witten was out here giving a series of three talks I went to all of them.

Each was an hour-and-a-half talk---I went to all three. He never mentioned string theory. At the end of the last talk somebody raised his hand and said "What about string theory?" And Witten made a one sentence comment.

 

At Strings 07, the main strings conference of the year Witten gave a talk which seemed to have nothing to do with string and at the end someone else raised their hand and asked "What about string?" Very brief reply again. It's online free for download if anybody wants. Kind of interesting.

 

Grimstrup is ex-string. One of Reuter's co-authors, Saueressig, is a former string theorist and still does some. Ambjorn (I mentioned earlier in connection with one of the leading QG approaches) used to do more string. It looks to me as if the people who are ABLE to move out into other fields, many of them, are doing just that. Crossing over, either on a full or parttime basis.

 

I watched David Gross closing talk at Strings 07, especially the last 5 minutes where he stops congratulating everybody (as the speaker at the end of a conference is supposed to do) and gives his own personal thoughts about the state of the field. It was blunt and revealing.

 

 

BTW I noticed you seemed to separate things into a PARTICLES pigeonhole and a COSMOLOGY pigeonhole. Reuter's work does not fall into either. Most of his papers are straight QG. Some papers do derive cosmology results--I'm very excited by the most recent one like that. But there is a core field of quantizing GR and I'd say most of his work is in that department. Likewise Rovelli, Ambjorn and all the others I mentioned. The only QG leader who is mainly focused on cosmology is Bojowald, whom I havent talked about much in this thread. So the particles/cosmology split is something of an oversimplification.

 

If anyone else is reading this thread and wants some links to papers or to talks at Strings 07, Loops 07, please let me know.

[iNow]

If anyone else is reading this thread and wants some links to papers or to talks at Strings 07, Loops 07, please let me know.

You mentioned audio versions. Links would be cool. Thanks.

 

 

String question: What does string theory have to say about discussing politics and religion? I'm seeing some parellels.

[Martin]

You mentioned audio versions. Links would be cool. Thanks.

 

 

 

As a courtesy, I mention Strings 07 first

http://gesalerico.ft.uam.es/strings07/060_marco.htm

Each talk has slides and video links. Alphabetical by speaker

If you scroll down you can find Witten "3D Gravity Revisited"

and Gross end-of-conference talk. I will fetch the Loops 07 audio links in a moment (they do not have video, only slides and audio)

 

String question: What does string theory have to say about discussing politics and religion? I'm seeing some parellels.

Enjoyed this sly and amusing comment:D

 

I'll be back with some Loops 07 links

 

Here is the main program page

http://www.matmor.unam.mx/eventos/loops07/program.html

and from there you get the INVITED TALKS (aka "plenary" talks, by mostly well-known people)

http://www.matmor.unam.mx/eventos/loops07/plen_abs.html

 

and you get the CONTRIBUTED TALKS (usually by post-docs younger less well known but also by senior people as well)

http://www.matmor.unam.mx/eventos/loops07/cont_abs.html

 

and what i thought was the most amazing of the invited plenary talks was Reuter

which you scroll down to here

http://www.matmor.unam.mx/eventos/loops07/plen_abs.html#reuter

and it is best if you download the PDF of his slides first, then you have the slides ready to scroll thru as he speaks on the audio.

Here are the slides:

http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf

Here is the audio:

http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3

 

But you can get any of the talks on the list, it is alphabetical and most of them have both the slides and audio.

 

There is also the same things for Loops 05 which was at Albert Einstein Institute at a place near Berlin (but all in English thankfully) and also the same things for the 2007 QG School, which was more aimed at graduate students. Let me know if you want more links. I tend to think latest is best. And I'm rather selective. There are only a few of the Loops 07 talks that I've listened to and I've listened intensively and several times thru to those.

Reuter, Rovelli, Ambjorn, and some other obvious ones like that. Actually come to think of it I've listened to quite a few: Ashtekar, Bojowald, Smolin, Rideout, and still more but I wont list all.

Some were talking about work I already knew so it didn't stand out so much for me.

[BenTheMan]

I should tell you that my reaction to all your posts has been guided by a declaration you made early on that in the QUANTUM GRAVITY department, string has NO COMPETITION. For me that flat statment makes no sense. Quantum gravity means quantizing General Relativity---the geometry of spacetime. It does not mean reproducing the generations of particles that come out of the Standard Model.

 

Well, then---I would say that string theory STILL has no competition, because one theory quantizes gravity AND explains some strange properties of the standard model, and another just quantizes gravity.

 

QG should have a quantum state of spacetime geometry, or be working towards getting one.

 

Why? Strings and loops aside, this is not at all obvious to me. GR is a classical theory, which means it is an effective theory. We have seen several places in physics where quantum corrections GREATLY alter the classical behavior of a physical system. So to say that ``Any quantum theory SHOULD quantize geometry'' is not an obvious statement. Any quantum theory of gravity should look like geometry in the low energy limit, of course, but quantizing geometry is not something that everyone thinks is a good idea.

 

One of the directions you see in QG is a tendency to try to get comparable things to what you get in conventional string approaches but without the extra dimensions.

 

Martin this statement is empty. You say that you get it without the extra dimensions, but YOU HAVE SPECIFIED THE NUMBER OF DIMENSIONS. GR is consistent in any number of space and time dimensions. I can get the standard model from SO(10), by adding arbitrary matter and higgs representations, but I've done it all by hand, so it is not a surprise that you get the standard model and no extra dimensions.

 

I don't understand your objection to 4D. So far the string framework has not shown a testable unique theory and I see no firm evidence that it has anything to do with nature. So why should I suppose there exist other dimensions besides the usual four?

 

Likewise I don't understand your objections to ten dimensions Why should you suppose that there exist four dimensions? The only statement that you can make is that we have checked directly to roughly 10 microns, and haven't found any. Any statements more than that are speculative. String theory makes no excuses about this---ten dimensions is a prediction of the theory.

 

If you claim your theory explains space-time, don't you think you should explain its most fundamental property?

 

But it looks like the competition, in the form of Alain Connes, has finessed the old SM rather elegantly and PLUS HE HAS PREDICTED NEW NUMBERS so that his approach can be tested by LHC or other doable experiments.

 

And if his predictions are wrong? Won't he go back and tinker with his models until he fits the experimental data? This is a trick question, of course, because it's what every theorist does. The problem in string theory is that parameter spaces are just too big right now.

 

I see signs in the string community of confusion, discouragement, and gloom, and the general dwindling of the numbers seems to bear that out.

 

If you're counting the indeces that you make up, then of course this is what you see... It is what you WANT to see.

 

We must know different string theorists, is all I have to say.

[ajb]

I don't know many string theorists, but those that I do seem very excited by twister sting theory, AdS/CFT and their connection with QCD and integrability.

 

True so far no "realistic models", but just the idea that strings and quantum field theory could be the same thing is fascinating and needs exploring.

[fredrik]

The problem in string theory is that parameter spaces are just too big right now.

 

From a strategy point of view, that sounds to me like a substantial problem after all? Doesn't it even suggest a fundamental strategy problem? How could it not?

 

It doesn't sound right to me to induce a parameterspace but induce no selection strategy? What is the qualifying or supporting evidence/indices for this parameterspace in the first place, when there are no selection principle? Shouldn't they sort of go hand in hand? Ie. shouldnt the original reason why you introduced this parameterspace in the first place, come with a measure of discrimination. If not, I would personally question the soundness of this induction in the first place.

 

That doesn't however mean it's wrong, I just sense it's very speculative from a scientific perspective, and yet doesn't address my fundamental questions.

That's all motivation I personally need to look for something better.

 

/Fredrik

[BenTheMan]

From a strategy point of view, that sounds to me like a substantial problem after all? Doesn't it even suggest a fundamental strategy problem? How could it not?

 

Well, ask yourself... how big is the parameter space in quantum field theory? How many different quantum field theories can we build? Well, there are AT LEAST as many quantum field theories as there are string vacua, because every string vacua (to my knowledge, at least) is a consistent quantum field theory.

 

In fact, I would wager a guess that there are MUCH fewer string theories that quantum field theories. (In fact, I think there is a paper about it, by Cumrun Vafa and Lubos Motl, called something about the ``Swampland''.) And no one doubts quantum field theory's ability to describe nature. So it may turn out that string theory is just a framework for quantizing gravity, and it's predictions are all vrey general.

 

This is what I have gathered about non-stringy QG---they don't explain things like the standard model, and are completely fine with that.

 

It doesn't sound right to me to induce a parameterspace but induce no selection strategy?

 

But this is exactly the approach that one SHOULD take. You have to know what you're picking before you know how to pick it.

 

What is the qualifying or supporting evidence/indices for this parameterspace in the first place, when there are no selection principle?

 

You look at the ways to write the action down in ten dimensions, and think of the ways you can get consistent solutions in four dimensions. This means compactifying on a Calabi-Yau three-fold (six dimensional space), of which there are many. Generally there are many ways to put conditions on internal degrees of freedom, and some moduli that you have to deal with.

 

This is how I understand the vacuum problem---there is only a few ways to write down a ten dimensional theory, but many ways to get four dimensional physics.

 

Shouldn't they sort of go hand in hand? Ie. shouldnt the original reason why you introduced this parameterspace in the first place, come with a measure of discrimination. If not, I would personally question the soundness of this induction in the first place.

 

Again, this is simply not the way things are done. There is now way for a low energy effective field theory to know very many things about dynamics in the UV---I can give you two or three examples, if you like. The higgs boson is probably the most expected discovery at the LHC, and there are about a million ways to break the electroweak theory. Everyone expects the higgs is a scalar, but we cannot rule out the possibility, for example, that one of the competing theories is right. Supersymmetry and SUSY breaking is another example, as are grand unified theories.

 

You have to understand the parameter space of your theory very well before you can understand how Nature behaves.

 

That doesn't however mean it's wrong, I just sense it's very speculative from a scientific perspective, and yet doesn't address my fundamental questions.

 

So it doesn't address YOUR fundamental questions! I'm not of the opinion that physics should answer to anybody:)

 

The point is that string theory (speaking in general) explains a huge number of things that we have just taken for granted. For example, gauge symmetries. String theory explains why we have gauge symmetries in our universe. Other things, like number of generations and number of supersymmetries are topological.

 

That's all motivation I personally need to look for something better.

 

This is what the quantum gravity people like Martin are doing. (I assume he's a scientist.) But they all have their problems. For example, string theory is the only way to quantize gravity that constrains the number of dimensions. In other QG approaches, this number is put in by hand. Scroll up to see the heated discussion

 

====================

I don't know many string theorists, but those that I do seem very excited by twister sting theory, AdS/CFT and their connection with QCD and integrability.

 

This is why martin's index about ``String Research Quality'' is pretty useless. Everybody is citing Maldacena's AdS/CFT paper, which was written ten years ago, and so falls outside of Martin's arbitrary five year window.

 

As an asside, I met some students from Michigan State and Minnesota this summer at Princeton (phenomenologists who work with Tony Gherghetta, and these people), and they all did higgsless models a la AdS/CFT. If higgs scalars aren't found at LHC, and we see something like technicolor, we could learn a lot about how AdS/CFT works in nature.