Successors to string scuffle (physical assets/liabilities?)

[tom.stoer]

I do not know about what you are speaking

I am talking about deep inelastic scattering in QCD and the violation of Bjorken of the nucleon structure functions. The classical theory is scale-free, but experimental results show that the energy-dependence of the structure functions involves a new energy scale. Naively one would expect that the structure functions depend on x like F(x), where x measures the fraction of the hadron energy carried by the single quark involved in the quark-electron scattering. Careful analysis shows that F = F(x, Q²) where Q² = -q² is the 4-momentum transfer between electron and hadron. Instead of Q²-independence one observes log(Q² / Lambda²) corrections where Lambda is a new scale in QCD which dictates the typical mass scale and size of the hadrons.

http://www.nikhef.nl/pub/experiments/zeus/theses/wouter_verkerke/latex2html/node4.html

But I think it's not the right place here to discuss standard QCD ...

 

[marcus]

...
But I think it's not the right place here to discuss standard QCD ...

Personally I'm not bothered by departures from formal topic, in the case of this thread. Bob has decided to go along with the moderator's suggestion that he limit discussion of his own theories to the Independents forum, which I do think would be helpful. But otherwise I personally have no objection to bringing up QCD if it seems to you at all relevant.

For the moment I'm frustrated by the Corfu School talks not being posted. The school was last week and I have no idea when the media will go on line.

 

[marcus]

Someone steering a boat may pick a mark on the horizon or shore "to steer by". It might not be the actual destination, but it temporarily it serves as one.

To me it seems as if Hermann Nicolai has indicated what might serve as provisional goal for a line of 4D quantum gravity development.
Does this make sense to you?

As a rough paraphrase, Meissner and Nicolai (M & N) are saying if you can give a quantum 4D geometry for them to define fields on, which has a certain kind of symmetry in the flat classical limit, then thereon they can construct a version of the Standard Model QFT with all troubles pushed out beyond Planck scale. The model will predict that you take it all the way to Planck scale without finding new physics----then, as expected, lots of new Planck scale physics.

So incidentally it can be falsified in a completely straightforward manner! If new physics shows up at some intermediate scale, long before Planck, then this minimalist theory is wrong.

It looks like a reasonable provisional target. What would it take for a QG to satisfy M&N? One could try heading that way, and maybe some progress in the right general direction would be made, even if it ultimately turns out not to be the final destination.

Could any of the half-dozen QG approaches we've been looking at be adapted or modified in the desired way? How close or far are they from providing the missing piece in M&N's picture? Do any have a reasonable chance of filling that role?

Here is some background on the Meissner Nicolai papers, and Nicolai's talk at the Planck Scale conference.
https://www.physicsforums.com/showthread.php?t=339154
If for some reason you don't like the M&N approach, please let me know.
They are offering it as something that deserves to be worked out, which can be checked for consistency and possibly tested experimentally. It is subject to experimental refutation. So it is not being presented as The Answer, but as an interesting minimalist theory.

Here are five papers by Kris Meissner and Hermann Nicolai:
http://arxiv.org/find/grp_physics/1/AND+au:+Nicolai_H+au:+Meissner/0/1/0/all/0/1
"... in a UV finite theory of quantum gravity, if the latter admits a flat space limit which is classically conformally invariant. The mass spectrum and pattern of couplings observed in elementary particle physics could then have their origin in quantum gravity."

What prospects do any of our half-dozen approaches have of admitting a flat space limit that is conformally invariant?

 

[atyy]

Nicolai wrote a viewpoint on how insights from string theory contributed to evidence consistent perturbative finiteness of N=8 SUGRA. He draws on the role of modular invariance in string theory to suggest that interesting future directions would to understand the results more deeply from the point of view of an undiscovered symmetry. http://physics.aps.org/articles/v2/70

Another viewpoint which mentions N=8 SUGRA as a place to start looking for alternatives to string theory is Denef, Douglas and Kachru's http://arxiv.org/abs/hep-th/0701050 .

Not related to modelling of elementary particles, but still interesting I think, is the use of gauge/gravity duality without supersymmetry - maybe there is some connection here with the condensed matter viewpoints of Visser/Volovik/Wen/Horava:
Balasubramanian and McGreevy, Gravity duals for non-relativistic CFTs, http://arxiv.org/abs/0804.4053
Balasubramanian and McGreevy, An analytic Lifgarbagez black hole, http://arxiv.org/abs/0909.0263
McGreevy, http://arxiv.org/abs/0909.0518

 

[atyy]

Nicolai wrote a viewpoint on how insights from string theory contributed to evidence consistent perturbative finiteness of N=8 SUGRA. He draws on the role of modular invariance in string theory to suggest that interesting future directions would to understand the results more deeply from the point of view of an undiscovered symmetry. http://physics.aps.org/articles/v2/70

Another viewpoint which mentions N=8 SUGRA as a place to start looking for alternatives to string theory is Denef, Douglas and Kachru's http://arxiv.org/abs/hep-th/0701050 .

One of Denef et al's references for N=8 SUGRA is Green, Russo and Vanhove's http://arxiv.org/abs/hep-th/0611273 . It's very much in the spirit of Nicolai's commentary of looking for a theoretical explanation of Bern's computational results. They propose an explanation for up to 8 loops of the 4-graviton amplitude, and add "A priori, finiteness of N = 8 seems very unlikely and, if true, would cry out for a natural explanation. One possible framework for such an explanation might be a variant of twistor string theory [23], which naturally describes N = 4 Yang–Mills coupled to superconformal gravity [24, 25]. Perhaps one of the proposals for a N = 8 twistor string theory in [26] is on the right track."

Reference [26] is Abou-Zeid, Hull and Mason's http://arxiv.org/abs/hep-th/0606272 "A family of new twistor string theories is constructed and shown to be free from world-sheet anomalies. The spectra in space-time are calculated and shown to give Einstein supergravities with second order field equations instead of the higher derivative conformal supergravities that arose from earlier twistor strings. The theories include one with the spectrum of N = 8 supergravity, another with the spectrum of N = 4 supergravity coupled to N = 4 super-Yang-Mills, and a family with N ≥ 0 supersymmetries with the spectra of self-dual supergravity coupled to self-dual super-Yang-Mills. The non-supersymmetric string with N = 0 gives self-dual gravity coupled to self-dual Yang-Mills and a scalar. A three-graviton amplitude is calculated for the N = 8 and N = 4 theories and shown to give a result consistent with the cubic interaction of Einstein supergravity."

 

[crackjack]

Hi Crackjack, the poll question was: "Which of these potential string successors seem most promising?"

Since we don't know the future, we can't tell if the current shift of interest (e.g. into Horava QG) will be temporary or permanent. The list of competing ex-string research programs was meant to be open-ended:

Since we don't know the future, aren't you making an assumption (a big one, at that) when you even say 'string successors'?
I think the poll should have included an option like 'a new string' to successors to current string scuffle, if the motive is purely altruistic. If the motive is less than altruistic, then I don't have anything to add.

But I should also say that it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

 

[Haelfix]

String theory, supergravity, and field theory dominates the population of active research to an enormous extent, possibly 20 to 1. Its been that way for at least twenty years, it continues to be that way and anyone who is actually a researcher in academia knows as much.

If there has been a 'shift' in the last few years, its a shift from pure string theory into more applied areas like condensed matter/atomic physics as well as additional phenomenology and cosmology (eg dark matter) jobs that are available (b/c of the LHC).

Its not surprising either. The second it became clear that string theory had something to say about real world applications, departments jumped on the opportunity to get out of the esoteric and difficult subject matter of quantum gravity (which as a rule was overpopulated with little to no tangible rewards).

 

[Bob_for_short]

Since we don't know the future, aren't you making an assumption (a big one, at that) when you even say 'string successors'?

But science fans are already fed up with promises given by string people. “String scuffle” is in fact an agony so it is in the air to make a scientific guess what is a more promising and realistic.

In my opinion, we can arrive at better theories of “elementary” particles, including quantum gravity, if we apply a better physical and mathematical approach to building an “interacting theory”.

Let me remind that we can obtain reasonable solutions in case of knows sources: if we know the current, then we can calculate the radiation; if we know an external field, then we can calculate the particle state. The problem is in building a self-consistent theory. So far the self-consistent theory is built by analogy with Classical Electrodynamics, i.e. with self-action leading to non physical effects removed with non mathematical means (unnecessary corrections and their discarding).

I am sure that as soon as we are able to describe correctly QED, we will be able to advance in other theories (SM, QG, TOE, whatever).

What I mean by “to describe correctly QED”? I mean an initial state and perturbation are such that no conceptual and mathematical problems arise and the results are in agreement with experiments. No bare particles, no infinite vacuum polarization to screen something infinite, no Landau pole, nothing artificial and wierd. It is possible to achieve this goal if we look at the system (electron + quantized EMF) as at a compound system with its center of inertia and relative variables. In other words, if we understand the observed “particles” (photons, for example) as quasi-particles (elementary modes) in a compound system, then there is no problem with the energy-momentum conservation law and there are no mathematical problems like UV and IR divergences. It differs from the standard QED with new meaning of entities involved (quasi-particles) and the interaction term (no self-action). The rest is similar so we are nearly there. I find this direction to be very promising. I vote for it. Can we add this direction to the list?

 

[marcus]

...But I should also say that it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

Thanks, I think a lot of people are in situations similar to yours, though perhaps more in the USA than some other places. As a physics-watcher, I have the impression that 4dQG (4D quantum gravity) is an under-reported research category.

When I got interested in non-string approaches back in 2003 most people had only heard of Loop, if even that. There were some others (CDT, causal sets, asymptotic safety) but just a handful of researchers and a few institutions were involved.

Now researchers have swarmed in, new 4d approaches have appeared, and there is more of a tendency for these non-string groups to openly compete. That is what interests me in this present thread.

As I said earlier, interest in string research has slacked off some. Nonstring lines have taken up some of the slack. This could be merely temporary or it could be a permanent shift, we can't tell that. Either way, it is a present fact (among other things, reflected in recent faculty hiring.) So I'm interested in comparing the relevant physics features of these different nonstring approaches to see what their physical assets/liabilities are, and if possible to estimate their prospects.

 

[marcus]

... dominates the population of active research to an enormous extent, possibly 20 to 1...

Hi Haelfix, you have made a sociological point, so perhaps I should respond to make clear how my perspective differs from yours.

I think you are saying that people whose research interest is 4d quantum gravity are vastly out-numbered and therefore negligible.

That could be made precise in various ways---outnumbered by whom, in the context of what larger population---and we could make some true statements of the "vastly outnumbered" variety.

My perspective is from quite a different standpoint because I see large percentage increases starting from a small base.

You alluded to physics department hiring. What I see is a remarkable increase in the number of jobs in nonstring QG. The difference is that I see a dramatic increase in some numbers which started out very small.

Your attitude might be that if some sociological indices (funding levels, postdoc and faculty openings, conferences and workshops held, authors) are small, by comparison with some other numbers, they are negligible even if they, for instance, double or triple.

So you might dismiss what, from my perspective, is quite significant.

 

[marcus]

... dominates the population of active research to an enormous extent, possibly 20 to 1...

Since Haelfix has made a point of sociological and demographic issues, I should probably lay out some relevant facts, just so we won't be merely waving our hands and making broad pronouncements.

There is evidently a fresh pie of nonstring QG jobs and funding out there. This pie needs to be divided up, and may have something to do with these various nonstring approaches getting into a more competitive mode.

The ESF (Euro. Sci. Found.) created a QG agency in 2006. Big increase in the number of individual grants, conferences, workshops, schools. Check out the QG network website for a list of what they have supported, starting in 2007. Here's the link:
http://www.maths.nottingham.ac.uk/qg/Meetings.html
It's an impressive list--and it's not the only new ESF support agency, Renate Loll has been put in charge of another.

For whatever reason a lot more nonstring QG postdocs now, in more different places.
(Not just the well-known centers Marseille, Utrecht, Perimeter, Nottingham, AEI-Potsdam, Penn State, but more recently other places like Rome, Western Ontario, Davis, Morelia, Montevideo, Warsaw, Iceland, Sydney...possibly Vancouver...hard to keep track.)

In terms of recent permanent or semipermanent appointments (above postdoc) here's what I recall:
Freidel to Waterloo.
Corichi to Morelia
Livine to Lyon
Sahlmann to Karlsruhe
Noui to Tours
Saueressig to Mainz
Girelli to Sydney
Speziale and Perez to Marseille
Rovelli now advertising a professorship opening at Marseille
Alexander to Haverford (his research varies, includes some 4d QG)
Dah-wei Chiou to Beijing (I'm not sure of level, think it's above postdoc.)

Grants to young researchers enabling them to build and lead their own autonomous research group---intermediate or longer term. Humboldt Foundation awards these.
Oriti at the AEI (Albert Einstein Institute)
Dittrich at AEI

CDT computing used to be only at Utrecht, now also begun at UC Davis and Perimeter.
Spinfoam computing at Western Ontario.

Loop cosmology has grown so much since 2005 that it would require a separate discussion. Access to that field by new researchers is comparatively easy and the results are interesting, so activity has mushroomed. Anyone who wants can do a Spires search on keyword "quantum cosmology".

I would have to check details of the appointment to be sure in every case. Good sources are Rovelli's and Loll's websites where they list what their former PhD students, or former postdocs, are now doing.

I can see how all these developments could seem negligible from the perspective of someone who thinks differently from the way I do, because as Haelfix says the numbers are massively "dominated" by factors like 20 to 1. String may not be growing or progressing significantly, but in terms of sheer numbers of researchers it hugely outweighs 4-dimensional quantum gravity.

From a different perspective, however, there has been considerable improvement in QG prospects during the past 3 or 4 years, and the fact that only about 100 or so papers are published per year does not imply the science is insignificant.

 

[atyy]

One of the things I've wondered about Asymptotic Safety is: suppose it exists - that means that the metric field has a continuum limit, and there are a few parameters we need to measure to pin down the theory. Does that really mean it will be predictive? After all, it needs to make predictions about matter, and electroweak theory doesn't have a continuum limit, so wouldn't we still not be able to make predictions at an energy high enough that gravity comes into play? Or is there a known GUT that has a continuum limit and can be meshed with an asymptotically safe metric field? Similar questions with N=8 SUGRA.

Maybe matter is harder to get than gravity? I think string theory gets gravity but has problems getting known matter; Wen has some clues about how gravity, but not chiral interactions.

 

[tom.stoer]

...Maybe matter is harder to get than gravity? I think string theory gets gravity but has problems getting known matter; ...

Interesting point. I think we should distinguish between
a) theories trying to unify all interactions (strings, SUGRA, perhaps non-comm. geometry) and
b) theories trying to construct a well-defined QG on which one tries to add matter on top (LQG, asymptotic safe gravity, CDT).

As I said, I see a very small chance that LQG could do the job of unification with the help of braiding, but currently this seems to be wishful thinking ...

 

[atyy]

As I said, I see a very small chance that LQG could do the job with the help of braiding, but currently this seems to be wishful thinking ...

I'm not a big fan of mainstream LQG - but would love if the bilson-thompson braiding worked out! This is a talk about stat mech, but Kardar's introductory remarks include a very interesting history of the braiding idea: http://online.kitp.ucsb.edu/online/colloq/kardar2/

 

[atyy]

I'm not a big fan of mainstream LQG - but would love if the bilson-thompson braiding worked out! This is a talk about stat mech, but Kardar's introductory remarks include a very interesting history of the braiding idea: http://online.kitp.ucsb.edu/online/colloq/kardar2/

Hmm, Kardar actually jokes that "all that went to the wayside until 10-20 years ago, when it was resurrected as string theory"

 

[marcus]

Atyy I think a key issue is one you raised (actually in another thread, but applies here).

...Although a common classification of gravity theories is background and non-background independent, I believe a better classification is whether the gravitational field is fundamental or emergent. Asymptotic Safety and LQG treat the gravitational field as fundamental, while string theory and condensed matter approaches hypothesize that the gravitational field is emergent.

The question can be put more strongly, are causality and locality fundamental (not simply does a metric exist), or are they merely appearances arising from something else, an abstract algebra perhaps?

That's not for us to settle but we can, as you point out, attempt a practical classification of theories with the help of this distinction.

Is the theory constructed about a concrete mathematical representation of spacetime carrying causal and local structure, that we can plant matter fields on?

More simply, is there anything concrete in the theory that one can build matter fields on, and what is it?

I hope some other people will comment. At the moment I don't have definite ideas about this. Presumably matter fields live on a geometry, but maybe it needn't be a metric geometry. It might, I suppose, be a conformal geometry without definite scale.

This possibility was raised in 't Hooft's September 2009 talk at the Erice school. Here are the slides:
http://www.ccsem.infn.it/issp2009/professors/GtH_NoSingularities_09.ppt
Here is the corresponding paper on arxiv:
http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an "emergent" one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory. Use is made of the principle of "black hole complementarity", the notion that observers entering a black hole describe its dynamics in a way that appears to be fundamentally different from the description by an outside observer. These differences can be boiled down to conformal transformations. If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear, while causality and locality may survive as important principles for quantum gravity."

So it seems to me 't Hooft sees the same important issue that you do. Even more crucial than background independence (avoidance of a prior fixed geometry) is the question of whether or not a concrete spacetime geometry that one can hope to build matter fields on exists at all, in the theory.
I was interested that 't Hooft decided to use his time at Erice this year to make this point. He raises the issue forcefully.

The slides have a lot of intuitive graphic illustration that adds to the paper. I found it helps understand the arxiv paper if you look at the slides as well.

 

[Fra]

I'll just throw my personal opinon in on one key question.

Do we know for a fact there is a force unification? If nature does not unify forces at high energies, neither should our theories

There seems to be different views of what unification means. One idea seems to replace a bunch of distinct symmetry principles by one large symmetry principle, to somehow reduce the number of free parameters or prinicples.

From my point of view, which sees no difference to treat physical process different from general scientific inference processes, information can only be acquired be means of a physical interactions - this alone is a information-theoretic basis for unification - and it's the kind of unification I expect. From this point of view I find it conceptually confused or inconsistent to treat different kinds of information in different ways. Some are treated properly, while some are treated in a realist fashion. So in my view, the information theoretic unificaiton is a requirement from consistency of reasoning if we take the scientific perspective serious and don't think that human scientists should somehow be described by a logic totoally unrelated to that describing nonbiological systems.

I think the same "inference logic" that applies to say "particle physics" must apply also to "very complex selforganised systems". I can't find any rational defense for such fundamental decomposition other than as a plain simplificaiton.

/Fredrik

 

[marcus]

to re-establish space-time as an essential frame...rather than an "emergent" one...

Judging from their most recent talks, I think that Steven Weinberg, Gerard 't Hooft, and Hermann Nicolai have come to essentially the same conclusion about priorities--what urgently needs to be done to get theoretical physics back on track. I don't suppose there has necessarily been communication about this. They just seem to be, for the present, aligned towards a common objective.

't Hooft puts this most openly and radically.

... 't Hooft's September 2009 talk at the Erice school. Here are the slides:
http://www.ccsem.infn.it/issp2009/professors/GtH_NoSingularities_09.ppt
Here is the corresponding paper on arxiv:
http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an "emergent" one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory..."

The conclusions of the Erice talk, the next to last slide, say:
==quote==
We suspect that it is not correct to dismiss
space-time as "emergent", like it is
sometimes done in Superstring theories.
Space-time can be, and should be, the
essential backbone of a theory.
==endquote==

In the same conclusions slide he also makes the intentionally provocative statement that "Scale invariance is an exact symmetry, not an approximate one!"
(One should understand that in context, which either the slides or the paper can provide.)

The annual Erice school on subnuclear physics has a long tradition. It is where, in 1976, Steven Weinberg first proposed Asymptotic Safety. Dirac lectured at Erice. 't Hooft is one of the people who guides the Erice program. In 2008 he had Renate Loll lecture at the school. I believe he thinks carefully, when he chooses the subject of his Erice talks. There is a tradition of Erice talks sometimes turning out to have historical import. And it's a nice place.

I don't think we necessarily have to take the viewpoint that this or that approach is right. I am interested in what people with vision think should be worked on. All three of them seem primarily concerned with getting physics back on track.
Steven Weinberg indicated this year that he has decided to work on Asymptotic Safe quantum gravity and cosmology, with an eye to re-establishing the Standard Model of matter on the expected new version of space-time. He expressed the opinion that that might be how the world is, rather than some Superstring emergence scenario.
Well, it is something to try, and it is in the same general direction as 't Hooft, namely working to to re-establish space-time as an essential frame.

We probably all know Hermann Nicolai is one of Europe's most respected particle theorists. He has been on the scientific organizing committee of nearly every one of the annual String conferences for the past 10 years*. He directs a branch of the Max Planck Institute concerned with unification and quantum gravity. For a long time it seemed that his research was primarily in string theory, but I think he has a broader perspective. Since 2006 he has published a series of papers with Kris Meissner that also has this same thrust, namely to re-establish space-time as an essential frame rather than (again in 't Hooft's word) dismissing it as "emergent".

The recent talks by both 't Hooft and Nicolai have in common an intense interest in scale invariance, conformal symmetry.
Here are some Nicolai links:
https://www.physicsforums.com/showthread.php?t=339154
Here's a thread about Weinberg's July 2009 talk at CERN, with links to the video and related pdf, plus some discussion of AsymSafe QG.
https://www.physicsforums.com/showthread.php?t=324841

*Strings 1999 through Strings 2010, the sole exception being 2001.

 

[Bob_for_short]

...namely working to to re-establish space-time as an essential frame.

Good! What else should we re-establish and what for? Do we have at all a suit to follow, a working model of anything or shall we first try out all our personal "postulates"?

As I said, we have two working (quite nicely) limiting cases - when sources are known. The problem is to build a self-consistent theory. How to exchange with energy-momentum without obtaining non physical results?

As I showed in "Reformulation instead of Renormalizations", we can build an interacting theory of whatever because any field theory can be reduced to (practically non-relativistic in form) Hamilton dynamics. Hamilton dynamics of non-relativistic particles is free from conceptual and mathematical difficulties, such as bare particles and constant renormalizations. For two- and more-particle systems the natural variables are the center of inertia and relative ones, i.e., the quasi-particle variables. It is easy to show that in fact it is these variables that are observable in experiment, not "personal" particle degrees of freedom. Think, for example, of the total inertia (mass, rest energy), proper frequencies, etc. We have just to recognize it as experimental and theoretical facts - we observe quasi-particle degrees of freedom of compound systems. This should serve as a flawless model to follow if we think we know what degrees of freedom (photons-electrons, quarks-gluons, whatever) have to share the energy-momentum. Any self-action approach is non-physical in principle because it introduces a positive or negative feedback that violates the conservation laws (runaway solutions are not physical). Isn't it evident?!

 

[marcus]

to re-establish space-time as an essential frame...rather than an "emergent" one.

As with any research goal stated briefly in general terms we should probably take some time to understand the context of what 't Hooft was saying.
==quote 0909.3426==
...what happens to the horizon and the space-time singularities? An answer sometimes suggested by string theorists, as well as others, is that all of space-time is just “emergent”[2][3][4]; the theory should first be formulated without space-time altogether.

Or, perhaps, time alone is an emergent concept[5]. It was argued that, at least, locality would have to be abandoned[6][7].

In this paper, however, we dismiss all such options. In particular, we insist that any satisfactory theory should have built in a strong form of causality, as well as locality, in order to explain why cause precedes effect, and why events separated at some distance from one another appear to evolve independently. For this, space-time appears to be indispensable.

Something has to give, and in this paper we claim to have found a good candidate for that: the definition of scales in space-time. It should be done in a way that differs from conventional wisdom...
==endquote==

I'm not proposing here, nor do I believe in, automatic acceptance of authority. But I think that 't Hooft continues to earn the right to have us occasionally pay special attention to what he's saying.

He thinks this message is a good one to get out to his theory colleagues & students at this time, and to it he devotes his September 2009 Erice talk together with the followup posting on arxiv which I am quoting here:
http://arxiv.org/abs/0909.3426

My comment at this point is basically that I'd like to understand this better. Does anyone want to elucidate? Why does "something have to give?"
If we want to keep the causality/locality structure embodied in spacetime---and continue to base fields on some type of concrete mathematical realization of the lightcone structure, then why are we obliged to give up something else? Why do horizons and singularities like those associated with black holes imply that "something has to give?" Maybe this seems obvious to you---please explain anyway: it will reassure me (and possibly others) that we've got it right.

================
I'm considering the notion that we could evaluate our halfdozen rival approaches in terms of four ad hoc criteria:
1. Does the approach come to terms with renormalization and the running of couplings. (Which, as Weinberg observed, might give a natural explanation of inflation.)
2. Does it have spontaneous dimensional reduction, as discussed by Carlip. 4d down to 2d at small scale.
3. Does it have a concrete mathematical realization of spacetime--that gives meaning to causality/locality and you can define fields on.
4. Could it be adapted so as to fill the bill for Nicolai (and it sounds like 't Hooft would like this too.) Could it acquire conformal symmetry in the limit.

 

[tom.stoer]

Let's put it that way: we know that the combination of usual calculational tools with spacetime manifold leads to diverging = physically meaningless results. This applies both to classical physics (GR) and quantum field theories. So there are two options:

1) Let spacetime stay a four dim. manifold but change the way how to put matter on top in a fundamental way yet to be discovered.
2) Change the way how to represent spacetime

1) was the program followed by string theory over some decades; OK, spacetime was 10 dm., but that was the only difference.
2) is e.g. the idea of LQG and non-comm.geometry.

It's hard to say what an emerging spacetime means. Is spacetime emerging from spin networks? Or are spin networks only a different representation of spacetime? (they already have to right symmetry).

 

[Fra]

Here is a very basic comment from me, that sits at the bottom leve of the LQG type of constructs.

2) Change the way how to represent spacetime
...
It's hard to say what an emerging spacetime means.

My expectation is that the kind of programs that are likely to be most successful is those where spacetime and matter are required to emerge simultanesouly, rather than trying to patch a matter model ontop of a spacetime model without loosing the coherence of the constructions.

If you take the machian idea that spacetime is only a matter of relations between objects rather than anything absolute. One is naturally lead to question the ontological status of these relations. Rovelli has said that there are similarly (this is to me required by consistency of reasoning, once you start to walk in this direction) no absolute relations, but only RELATIVE relations; and the only way for systems to COMPARE their "establised/observed relations" is by interacting with each other (communicating).

So far I am totally with Rovelli.

I associate here "systems" with "matter system". So the observers communicating their subjectively establised relations to their environments, are simply the matter-matter interactions.

But then Rovelli conjectures that standard QM, describes this interaction. This is not satisfactory for me, and it's at this point, my lack of confidence in the rest of the construction is rooted.

IF we can rework it from here (which is what I like to do), I think the original relational kind of spirit of Rovelli can be continued even more coherently.

Somehow I think expectations from such reconstruction must reconstruct spacetime and matter at the same time, because matter is somehow the physical basis where the subjective relations to it's environment is encoded.

I don't know in detail all the details of the variations of LQG style reasoning, but I did start to read rovelli's book and his papers some years ago and was put off by these kinds of what I consider to be serious flaws in the reasoning.

/Fredrik

 

[tom.stoer]

... that the kind of programs that are likely to be most successful is those where spacetime and matter are required to emerge simultanesouly, rather than trying to patch a matter model ontop of a spacetime model without loosing the coherence of the constructions.

Yes, that's my feeling as well.
I liked strings in the sense that they were heading towards unifying matter and gravity both emerging from the fundamental vibrating string, but unfortunately they could not get rid of the spacetime background.
I think LQG is a step forward in the sense that they incorporate the lessons from GR (background independence, diff.-invariance) but as I said, emerging matter is still only wishful thinking (braiding is a good idea but as far as I can see not very actively studied).
Non-comm. geometry seems to do something like what we have in mind, but I am not an expert (just started to study some review papers).

... but I did start to read rovelli's book and his papers some years ago and was put off by these kinds of what I consider to be serious flaws in the reasoning.

I wouldn't agree. Writing his book Rovelli had a clear strategy, namely to describe in detail and in a rigorius manner the current knowledge about quantum gravity / quantized geometry. He left out unification - not accidentally but simply because he wanted to focus on gravity.
LQG is still not fully developed; it seems that the spin foam approach can answer questions regarding the classical limit and smooth 4-dim. spacetime, but there are still some puzzles regarding canonical quantization, Imirzi parameter, cosmological constant etc.
As soon as they have patched together all these pieces, they are definately able to start with the next step in their program, namely to understand if LQG allows not only to put matter on top, but to let matter emerge from geometry. In addition there is a different research area waiting to attract attention, namely if it's possible to quantize SUGRA according to the LQG methods and if one can understand if matter and gravity can be harmonized in that way.

I am pretty sure that Rovelly, Smolin, Ashtekar and others are well aware that LQG (as of today) is not the final answer, but maybe it's simply too early (even for them!) to attack these questions now and to skip the more basic ones. In addition they have to chec if they are still on the right track (LQC is just doing that: try to solve a simpler models and check for consistency and possibly experimental verification).

Do you think that Heisenberg, Pauli, Dirac, Schrödinger could have been successful w/o the early attempts (Bohr, Sommerfeld, ...)? Do you think that one could have skipped the 20 years between 1905 and 1925? I don't think so.

Tom

 

[Fra]

I wouldn't agree. Writing his book Rovelli had a clear strategy, namely to describe in detail and in a rigorius manner the current knowledge about quantum gravity / quantized geometry. He left out unification - not accidentally but simply because he wanted to focus on gravity.

Maybe my comments came out harsh. I like Rovelli, and parts of his writings and papers are great, no doubt about that. I also think he wrote a nice book, which contains great perspectives on the problem of QG in a reasonably neutral way in the intro parts.

But when I got that book, and before I actually knew anything about LQG, I already had my own questions, and my own sort of loose ideas. And I got an early vision of a possible interpretation of his spin networks, and I started to read up on his work to see if his ideas, could be the answers to my questions - I found out it wasn't, but maybe they could be, but it's the same as with string theory. If you rework a theory enough, would you still call it by it's old name?

Even from my very personal view, I can see possible reworkings of both string theory and LQG that might satisfy me, but then the reworkings would be significant and then one certainly starts to wonder what "characterises" say string theory? Would anything a string theorist come up with be called string theory? would string theory were the strings are emergent rather than fundamental be farily called string theory?

Now of course, it really doesn't matter what we call it though, but I can do nothing but to try to pose and answer my own questions, and the starting point provided by rovelli in the subset of his work I've seen is not convincing me.

My reason for reading rovelli was basically, could this research give my a jump start on my journey? As far as I understand it, it can't, because it suggests jumping in a direction that I can not defend. I have to defend my own actions, even if authorative people suggests otherwise.

I think I've pointed in previous threads on the key principles of construction of rovelli's reasoning as well as the string theory reasoning that I do not like. I think at least I have rational objections, but both programs contains great things as well of course. But the principles that are used to define the framework of LQG, or the framework of string theory are important and if I find reasons to doubt some of the principles and see how it could be done differently it wouldn't be rational of me to buy into them just because they are advocade by famous and intelligent researchers. But it would be equally irrational to not look at what they have done and try to learn! I have done that, I take what I like and leave what I don't like :)

/Fredrik

 

[tom.stoer]

Frederik,

I understand and appreciate your reasoning, but I didn't want to repeat the discussion from the other thread :-) That's the only reason why I didn't follow your ideas here.

Thomas

 

[marcus]

... So there are two options:

1) Let spacetime stay a four dim. manifold but change the way how to put matter on top in a fundamental way yet to be discovered.
2) Change the way how to represent spacetime

1) was the program followed by string theory over some decades; OK, spacetime was 10 dm., but that was the only difference.
2) is e.g. the idea of LQG and non-comm.geometry.

It's hard to say what an emerging spacetime means. Is spacetime emerging from spin networks? Or are spin networks only a different representation of spacetime? (they already have to right symmetry).

You hit the sensitive issue. The word "emergent" is used two different ways in this context.
One way is you don't give up trying to represent spacetime. Spacetime still exists in your model----there is still some mathematical object that you can point to and say it represents space time, only it may not be a smooth metric manifold.

In that first way you can say that spacetime is described by more fundamental degrees of freedom but it is still represesented, and the familiar smooth manifold is an epiphenomenon that arises or emerges from it.

In this way "emergence" is only a small step---rising from an underlying microscopic description to a more smoothed out macro. It is merely a "zooming out" emergence, like you zoom the camera.

In the other way, spacetime does not even exist in the model. The ground of existence is way off in some other department. This is a kind of unintuitive radical emergence, like in the movie "Matrix" or like in a holographic projection. Perhaps my analogies or my description of this way are bad because I don't really grasp this kind of radical emergence. I think 't Hooft is pulling away from this.
I think he is not arguing against the other. So this word, because of a double usage, could be obstructing clarity.

 

[tom.stoer]

You hit the sensitive issue. The word "emergent" is used two different ways in this context.

What do you mean exactly by "two different ways"?

 

[marcus]

What do you mean exactly by "two different ways"?

Thanks for the comment. I edited my post to explain what I meant.

As a reminder (mostly for my own benefit) there are some possible criteria to use in evaluating these 4D QG approaches.

...we could evaluate our halfdozen rival approaches in terms of four ad hoc criteria:
1. Does the approach come to terms with renormalization and the running of couplings. (Which, as Weinberg observed, might give a natural explanation of inflation.)
2. Does it have spontaneous dimensional reduction, as discussed by Carlip. 4d down to 2d at small scale.
3. Does it have a concrete mathematical realization of spacetime--that gives meaning to causality/locality and you can define fields on.
4. Could it be adapted so as to fill the bill for Nicolai (and it sounds like 't Hooft would like this too.) Could it acquire conformal symmetry in the limit.

In both the talks by Nicolai and by 't Hooft, conformal symmetry played a big role, so as a resource here is Sam's PF tutorial thread on Conformal Symmetry
https://www.physicsforums.com/showthread.php?t=172461

 

[marcus]

Readers might want to refer back to some of the interesting posts in this thread such as:

Nicolai wrote a viewpoint on how insights from string theory contributed to evidence consistent perturbative finiteness of N=8 SUGRA... http://physics.aps.org/articles/v2/70...

The article by Nicolai that Atyy links to is fascinating. Recommended. Role of string there is not as a physics theory about nature but as a set of mathematical techniques (the strings are eventually shrunk down to points, after they have greatly facilitated a calculation.)

... it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

Thanks The value of reporting is in howevermuch objectivity, and let the (less than) altruism chips fall where they may. If you mean favorism then altruism shouldn't enter as an issue.

... a shift from pure string theory into more applied areas like condensed matter/atomic physics ...
... string theory had something to say about real world applications, ... get out of the esoteric and difficult subject matter of quantum gravity (which as a rule was overpopulated with little to no tangible rewards).

Again the "real world applications" is as a set of mathematical techniques, not as a fundamental theory of nature. Like the application to QCD which Atyy mentioned, and the application to the study of superconductivity we heard about this year. Facilitating calculations at much larger scale, e.g. "condensed matter/atomic physics."

Here "pure" string seems to refer to the program of unification and [string] quantum gravity. Failed or stalled, so a way out is needed for both the researchers and the departments which have hired them. Redirection into the use of string mathematical techniques (to non-unification ends) is one way out.

... it is in the air to make a scientific guess what is a more promising and realistic...

Yes the problem of the QG succession, what will take up the slack in fundamental physics, remains an exciting problem.

 

[marcus]

Back around 20 October 2007, Christine wonderfully gave us some excerpts of a piece by Hermann Nicolai that was published in Nature. It illustrates something Haelfix just referred to (a couple of years later.)

Keep in mind that Nicolai is a leader in the string community, but avoids favoritism. He also does strong research outside of the string program and is a clear-eyed critic of string short-comings. Here (and in the recent piece Atyy linked) he warmly praises where he sees a useful string success. Views like this are potentially valuable information.

==exerpts from 18 October 2007 issue of Nature==
String theory: Back to basics

Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force.

Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley[1–8] indicate that the theory might be about to deliver something of its original promise: helping us to understand the physics of interactions mediated by the strong nuclear force. String theory was born in the 1960s, (...)

But initial attempts to describe the forces between the quarks, and why they form the bound states they do, failed miserably. So particle physicists started casting around for other ways of attacking the problem. In 1968, the Italian theoretician Gabriele Veneziano made a brilliant guess [9] and wrote down a concrete mathematical expression, the Veneziano amplitude, that explained some important features of high-energy scattering. But his formula could not be understood in terms of point-like particles; instead, it required the existence of extended objects — strings. (...)

The arrival in the early 1970s of quantum chromodynamics (QCD), the quantum-field theory of the strong interaction, dealt the final blow to these early attempts to understand nuclear physics in terms of string theory. But, unfortunately, QCD is incredibly complex. (...) In this ‘perturbative’ regime, we understand (at least in principle) how to work with QCD. But for the strong coupling that occurs over larger distances, one has to resort to computer-simulation techniques, known as lattice QCD. (...)

The new approach that revives the link to string theory first suggested itself in 1998, when Juan Martín Maldacena conjectured[12] a link between a close relative of QCD and a ‘superstring’ living in a ten-dimensional curved space-time. (...) The Maldacena conjecture raised a lot of interest, but seemed for a long time to be quantitatively unverifiable. (...)

Help came from an entirely unexpected direction. Following a prescient observation[13], the spectrum of the N = 4 theory has been found[1,2] to be equivalently described by a quantum-mechanical spin chain of a type discovered by Hans Bethe in 1931 when modelling certain metallic systems. (...) Indeed, even though the mathematical description of the duality on the string-theory side is completely different from that on the condensed-matter side, a very similar, exactly solvable structure has been identified here as well[3–5]. Puzzling out the details of the exact solution is currently an active field of research. (...)

Just recently, Beisert, Eden and Staudacher[8] have extracted the analogue of this observable on the field-theory side, and have been able to write down an equation valid at any strength of the coupling. Since then, work has established that their ‘BES equation’ does indeed seem, for the first time, to offer a means of reformulating theories such as QCD as string theories. Much still needs to be learned from this one exactly solvable case. There is justifiable hope that this solution will teach us how to go back to the physically relevant case of QCD and finally arrive at the long-sought dual description by a string theory. It may even take us closer to realizing the quantum-field theorist’s ultimate dream, unfulfilled for more than 50 years: completely understanding an interacting relativistic quantum-field theory in the four space-time dimensions that we are familiar with. Progress towards this goal can be judged independently of loftier attempts to use strings in the construction of a theory of everything.

Hermann Nicolai is at the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Mühlenberg 1, D-14476 Potsdam, Germany.

1. Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0303, 013 (2003).

2. Beisert, N., Kristjansen, C. & Staudacher, M. Nucl. Phys. B 664, 131–184 (2003).

3. Bena, I., Polchinski, J. & Roiban, R. Phys. Rev. D 69, 046002 (2004).

4. Kazakov, V. A., Marshakov, A., Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0405, 024 (2004).

5. Arutyunov, G., Frolov, S. & Staudacher, M. J. High Energy Phys. 0410, 016 (2004).

6. Gubser, S. S., Klebanov, I. R. & Polyakov, A. M. Nucl. Phys. B 636, 99–114 (2002).

7. Frolov, S. & Tseytlin, A. A. J. High Energy Phys. 0206, 007 (2002).

8. Beisert, N., Eden, B. & Staudacher, M. J. Stat. Mech. P01021 (2007).

9. Veneziano, G. Nuovo Cimento 57A, 190 (1968).

10. Ramond, P. Phys. Rev. D 3, 2415–2418 (1971).

11. Neveu, A. & Schwarz, J. H. Nucl. Phys. B 31, 86–112 (1971).

12. Maldacena, J. M. Adv. Theor. Math. Phys. 2, 231–252 (1998).

13. Lipatov, L. N. preprint available at www.arxiv.org/abs/hep-th/9311037 (1993).

14. Zaanen, J. Nature 448, 1000–1001 (2007).

NATURE|Vol 449|18 October 2007NEWS & VIEWS
==endquote==

Most of Nicolai's references are to papers already several years old. There is one 2007 paper that plays a pivotal role in what he has to say, the BES. I will put it here for convenience of anyone who wants to check it out as well:

http://arxiv.org/abs/hep-th/0610251
Transcendentality and Crossing
Niklas Beisert, Burkhard Eden, Matthias Staudacher
31 pages
(Submitted on 23 Oct 2006 (v1), last revised 14 Nov 2006 (this version, v2))

"We discuss possible phase factors for the S-matrix of planar N=4 gauge theory, leading to modifications at four-loop order as compared to an earlier proposal. While these result in a four-loop breakdown of perturbative BMN-scaling, Kotikov-Lipatov transcendentality in the universal scaling function for large-spin twist operators may be preserved. One particularly natural choice, unique up to one constant, modifies the overall contribution of all terms containing odd zeta functions in the earlier proposed scaling function based on a trivial phase. Excitingly, we present evidence that this choice is non-perturbatively related to a recently conjectured crossing-symmetric phase factor for perturbative string theory on AdS5xS5 once the constant is fixed to a particular value. Our proposal, if true, might therefore resolve the long-standing AdS/CFT discrepancies between gauge and string theory."

Incidentally, the byline says two authors at AEI (Beisert and Staudacher) where Nicolai is director and one (Eden) at 't Hooft's Utrecht institute. AEI and Utrecht are like Perimeter Institute in being strong in non-string QG as well as string research. They are places where string and non-string QG researchers work in neighboring offices, chat in the coffeeroom and can easily attend each other's seminars. One group is not frozen out by the other. Grad students have a choice. That is not how it typically is in the US.

 

[arivero]

N=8 SUGRA. But it is a compactification of D=10 or D=11 SUGRA, isn't it? And it is related to a limit of string theory. So yes it could be a good candidate, but not really an alternative to strings. I would say it is a hint about what way string theory should focus on

 

[marcus]

I would add SUGRA to the original poll, morally it is done. We now have 2 votes for N=8 SUGRA, earlier Tom Stoer said that, and now you, Arivero. We can count up now. Seven people have responded so far (six on the original poll plus Arivero with a "write in" vote) as follows:

4 for Loop (marcus, MTd2, tom.stoer, SW VandeCarr)
2 for AsymSafe (marcus, william donnelly)
2 for SUGRA (arivero, tom.stoer)
1 for CDT (marcus)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)

No votes for Horava.

====
Arivero, here is the earlier discussion of supergravity that came up in this QG succession thread:

Marcus,

why not considering supergravity?

- there are indications that it could be renormalizable
- there are versions possibly rich enough to contain the standard model
- SUGRA is a theory on its own and does not necessarily need strings

Tom, let's add SUGRA to the list of contenders. I keep seeing Kelly Stelle's name on speaker lists, might he serve as a persuasive advocate? Is there an introduction/overview to recommend?

 

[tom.stoer]

N=8 SUGRA. But it is a compactification of D=10 or D=11 SUGRA, isn't it? And it is related to a limit of string theory ...

I am not so sure about that.

It seems that duality in string theory relies basically on large-N limit considerations. So perhaps it's exactly the other way round: not ordinary gauge fields are (low-energy) limits of strings, instead string theory may be the large-N limit auf certain gauge theories. If this is true, SUGRA may very well be an own candidate w/o the requirements to derive it from strings.

One question: which SUGRAs are the limits of certain string theory and which are not?

 

[Bob_for_short]

Seven people have responded so far (six on the original poll plus Arivero with a "write in" vote) as follows:

4 for Loop (marcus, MTd2, tom.stoer, SW VandeCarr)
2 for AsymSafe (marcus, william donnelly)
2 for SUGRA (arivero, tom.stoer)
1 for CDT (marcus)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)
0 for Horava QG.
1 for VK quasi-particle divergence-less construction (Bob_for_short)

I added my not yet developed approach.

Vladimir Kalitvianski.

 

[atyy]

Since we don't know the future, aren't you making an assumption (a big one, at that) when you even say 'string successors'?
I think the poll should have included an option like 'a new string' to successors to current string scuffle, if the motive is purely altruistic. If the motive is less than altruistic, then I don't have anything to add.

But I should also say that it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

What do you think "new string" might be? I know Xiao-Gang Wen in his talks contrasts his approach with "old strings", leaving open the interpretation that there isn't a philosophical difference between the condensed matter approaches and "new string". My impression of what's cool is the link between strings and condensed matter through gauge/gravity duality, which maybe even extends to non-relativistic theories. The matrix theories are also interesting, but I don't know if those can be linked to condensed matter, except in the broadest sense of gravity being emergent.