Successors to string scuffle (physical assets/liabilities?)

atyy

Some answers as to how Asymptotic Safety might still make some prediction, even though the correct theory of matter isn't known are given by Niedermeier and Reuter http://relativity.livingreviews.org/...es/lrr-2006-5/: "Compared to the effective field theory framework the main advantage lies not primarily in the gained energy range in which reliable computations can be made, but rather that one has a chance to properly identify ‘large’ quantum gravity effects at low energies."

I guess the other possibility is that as a fixed point is approached, there will be approximately "universal" behaviour for some quantities. I think this is why CDT folks look at the scaling behaviour of the computational results and say those suggest a fixed point - presumably of Asymptotically Safe gravity.
Litim http://arxiv.org/abs/hep-th/0503096: "Wilsonian flows play an important role in the study of universal scaling phenomena in gauge theories and gravity"
Codello et al http://arxiv.org/abs/0805.2909: "For example, the critical exponents should be universal quantities and therefore cutoff–independent."

atyy

Or maybe Weinberg is postulating that the whole SM, not just gravity, is asymptotically safe?

marcus

Atyy I think I can give some relevant perspective on this by quoting Nicolai's slide #5:

==quote Planck scale conf. talk==
The demise of relativistic quantum field theory

• With SM-like bosonic and fermionic matter, UV and IR Landau poles are generically unavoidable.

• Thus breakdown of any extension of the standard model (supersymmetric or not) that stays within the framework of relativistic quantum field theory is probably unavoidable [as it appears to be for λφ₄⁴].

• Therefore the main challenge is to delay breakdown until M_Pl where a proper theory of quantum gravity is expected to replace quantum field theory.

• How the MSSM achieves this: scalar self-couplings tied to gauge coupling λ ∝ g2 by supersymmetry, and thus controlled by gauge coupling evolution.
⇒ m_H ≤ √2m_Z
in (non-exotic variants of) MSSM.
==endquote==

What he said is there is no use trying to fix the SM. Eventually it will blow up (with its built-in Landau dynamite) but nature may have arranged so that this does not happen until Planck, when a new completely different theory would be expected to take over in any case.

So we can explore this possibility by ourselves devising an absolute Occam minimal barebones modification of SM which pushes the Landaus out past Planck scale!
And other people have already tried this delaying tactic! But Nicolai says his new way is considerably simpler.

The MSSM was one attempt, but it leads to predicting a lowest lying Higgs mass less than √2m_Z = 130 GeV. If a Higgs is not found under 130 GeV then MSSM is dead, he observed. The MSSM is already rather elaborate, with "tons of Higgs". But he says you pay a heavy price in terms of economy if you go to more exotic versions---i.e. NMSSM. "Even more Higgs!"

Nicolai's proposal (which still has some things to work out) has so to speak "one and a half Higgs"----a basic one with predicted mass of 207 GeV, plus it has a kind of shadow or "fat twin brother" roughly estimated around 477 GeV. So two really, but that's it. He says one of them (I forget which) should be very easy to produce and detect---a clear signature.

The proposal is amazingly clean and economical. There are also some other predictions or clear signatures to look for.

So I doubt that Weinberg is including the SM when he says asymptotic safety.
Weinberg, in his July talk at CERN was talking about the UV fixed point of gravity and a revival of interest in the "good old" standard model. The good old SM is not going to have a UV fixed point because it has Landau poles and will blow up. The only thing to do is see if you can delay the blow up until you are out to Planck scale where new physics in any case. I think that is the basic situation within which Weinberg would be thinking and working, just like Nicolai is. It is a mental environment that is not special to Nicolai or any one person.

atyy

I understand Nicolai's point of view - am trying to figure out what Weinberg means by "It is just possible that the appropriate degrees of freedom at all energies are the metric and matter fields, including those of the Standard Model." http://arxiv.org/abs/0908.1964 since I'm sure Weinberg knows all the Landau poles very well, and is one of the people who's been saying the standard model and gravity are probably just effective theories. So I guess he must be saying maybe one can et round the Landau poles if the SM is asymptotically safe. I would guess that it's asymptotic safety of the SM and SM extensions that is keeping Nicolai from saying that we definitely need to go beyond 4D relativistic field theory - he just says probably.

atyy

Yes, I think in the past Weinberg only meant AS for gravity. But now that he's seriously considering it, he needs a theory of matter that works when quantum gravity kicks in at the Planck scale. It's conceivable one can have a matter effective field theory with Landau poles above the Planck scale for that purpose, though Arkani-Hamed et al's "Gravity as the weakest force" suggests maybe not. However, I think Weinberg is seriously considering to get around the Landau poles of the SM or SM extensions by having them asymptotically safe. A recent paper http://arxiv.org/abs/0901.2459 (which I found out from http://motls.blogspot.com/2009/01/he...on-monday.html) tries to see if the Landau poles in the Higgs sector of some models can be gotten round by asymptotic safety in those sectors - they proved that the UV fixed point doesn't exist in many of these models - but it nonetheless shows that Landau poles only suggest, not prove that the theory has no continuum limit - one must also prove that asymptotic safety doesn't exist.

marcus

I encountered (thanks to Nicolai) another player, whose paper may interest you. It is along the same lines as "It is a mental environment that is not special to Nicolai or any one person." We both seem to be filling in the picture of that mental environment and noting other related work.
http://arxiv.org/abs/0708.3550
Is there a new physics between electroweak and Planck scales?
Mikhail Shaposhnikov
10 pages, talks given at the Workshop on Astroparticle Physics, Budapest 2007 and at the 11th Paris Cosmology Colloquium 2007.
(Submitted on 27 Aug 2007)
"We argue that there may be no intermediate particle physics energy scale between the Planck mass M_Pl ~ 10¹⁹ GeV and the electroweak scale M_W ~ 100 GeV. At the same time, the number of problems of the Standard Model (neutrino masses and oscillations, dark matter, baryon asymmetry of the Universe, strong CP-problem, gauge coupling unification, inflation) could find their solution at M_Pl or M_W . The crucial experimental predictions of this point of view are outlined."

"In this paper we describe a (hopefully) consistent scenario for physics beyond the StandardModel (SM )that does not require introduction of any new energy scale besides already known, namely the electroweak and the Planck scales, but can handle different problems of the SM mentioned in the abstract."

"This point of view, supplemented by a requirement of simplicity, has anumber of experimental predictions which can be tested, at least partially, with the use of existing accelerators and the LHC and with current and future X-ray/γ-ray telescopes."

Background on Shaposhnikov:
http://itp.epfl.ch/page58722.html
He has 232 papers on Spires, eight of which have been cited 250+ times. One cited over 1500 times. Several cited over 500 times.
He seems to have a command both of particle physics (including beyond standard) and cosmology/astrophysics. Interesting guy. Born 1956, so a bit over 50, still extremely productive. Respected but also pushing the edges.
That paper about nu-MSM from 2007 was not an isolated one. He has written several followups, including in collaboration with other authors. He was appointed director of the ITP at the Swiss Federal Ecole Poly (Lausanne) and also directs the Laboratory for Particle Physics and Cosmology. Good combination. Early universe so important to understanding high energy physics.
http://www-spires.slac.stanford.edu/...tecount%28d%29

Here's are those publications listed most recent first, to get an idea of his recent work:
http://www-spires.slac.stanford.edu/...ENCE=ds%28d%29

BigFairy

voted.

marcus

Thanks adding your vote BiFa, and thanks everybody! So we are 9 in all, so far. 8 people have voted on the original poll plus Arivero registered a point after we decided to include 4D SUGRA. In order of most points the results are:
Loop 5
CDT 2
AsymSafe QG 2
4D Supergravity 2
Regge QG 1
Wen 1
Horava 0

One of the things discussed in this thread is what assets/liabilities different 4D QG approaches have that could attract researchers and give interesting prospects/promise to the approach. Like a natural inflation mechanism (e.g. AsymSafe).
Like spontaneous dimensional reduction at small scale (several, including Loop, CDT, AsymSafe).
An interesting issue is which if any approaches might be adapted to suit the Meissner-Nicolai proposal for continuing a minimalist Standard Model all the way from weak scale up to Planck.

In another thread, up in High Energy forum, Haelfix listed some questions which any proposal like that should address.
I'm not sure what proposal he was talking about but the checklist is a useful one that one could apply to any proposal which posits no new physical scales between weak (100 GeV) and Planck (10¹⁹ GeV). We should copy it down and go through one by one, to see which of these points Nicolai addressed in his talk. IIRC he addressed a good many of them, but we'll see.

hierarchy and naturalness---both addressed in detail--Nicolai shows "naturalness in the sense of 't Hooft" (the way 't Hooft originally defined the concept when introducing it)

neutrino physics not left wide open of course--M&N model employs see-saw and gives a prediction for neutrino mass

it also predicts 207 GeV for lowest mass Higgs, also something around 400 GeV for the only other Higgs (the "fat twin" Higgs)

dark matter---I don't recall Nicolai addressing this, will check

free parameters---half a dozen running couplings are determined by equation but since eqns hard to solve numerical methods were used and the running was plotted. Nicolai explained paraeters at some length to give intuition about why the various couplings ran as they did, and why they did not blow up. Plots running over the whole energy range from weak to Planck.

atyy

But what is new look Loop?

Is it secretly Asymptotic Safety with a UV fixed point which guarantees the theory is fundamentally continuous, as Bahr and Dittrich or Krasnov in non-metric mood envision?
http://arxiv.org/abs/0905.1670
http://arxiv.org/abs/0907.4064

Or could it be in cahoots with condensed matter by being fundamentally discrete with a continuum emerging due to an IR-like fixed point, as Freidel, Gurau and Oriti hope for?
http://arxiv.org/abs/0905.3772

marcus

Atyy, you may have a more crystalized vision of the alternatives than I. After reading your post again I think you probably do.
I am a bit bothered by the fact that the Corfu site seems slow to put the lectures on line. On their main page they have an item "online lectures" which if you click says "Lectures will be put on line as soon as they are available". But as far as I can tell none are yet. And that is true not just for the QG school but also for two other schools held there by the same organization earlier.

All I have to go on is Rovelli's abstract summary:
==quote==
Title: Covariant loop quantum gravity and its low-energy limit

Abstract and content (tentative)

I present a new look on Loop Quantum Gravity, aimed at giving a better grasp on its dynamics and its low-energy limit. Following the highly succesfull model of QCD, general relativity is quantized by discretizing it on a finite lattice, quantizing, and then studying the continuous limit of expectation values. The quantization can be completed, and two remarkable theorems follow. The first gives the equivalence with the kinematics of canonical Loop Quantum Gravity. This amounts to an independent re-derivation of all well known Loop Quantum gravity kinematical results. The second the equivalence of the theory with the Feynman expansion of an auxiliary field theory. Observable quantities in the discretized theory can be identified with general relativity n-point functions in appropriate regimes. The continuous limit turns out to be subtly different than that of QCD, for reasons that can be traced to the general covariance of the theory. I discuss this limit, the scaling properties of the theory, and I pose the problem of a renormalization group analysis of its large distance behavior.
==endquote==

What continuous limit? How is it taken? How does one take a continuous limit in the spinfoam context? Or is he not working with spinfoam?

We can see that he is not using canonical LQG because he says that this approach which he is using reproduces results and agrees with canonical LQG. So he is using some other approach. But is it actually just a lattice, where the size of the lattice can be taken to zero?

I don't have much to go on and I don't feel able to take jumps of imagination. So my reaction is to refrain from asking the question you just asked and go think about something else until we get some PDF from this series of lectures.
==============================

The papers you offer, as a way of "triangulating" to guess where Rovelli is, are in themselves quite interesting. The Dittrich, the Krasnov, and also the Freidel Gurau Oriti, which I had not tried to read earlier. I found the diagrams bewildering. FGO talk about generalized Feynman diagrams made of cell complexes, they do some kind of renormalization. It might actually be very good what they do, but it's just a beginning and I can't imagine that it could be already be transformed into school lectures. They were still working on a 3D toy model of it in May 2009.

Krasnov's work is again exploratory. It seems to have a renormalization angle----you termed it something like sub rosa Reuter. Maybe Loop research will eventually move in that direction. Maybe, but Krasnov's innovation is still preliminary: he has first to publish a spin foam vertex derived from his new Lagrangian.

I'm glad they are working on such things but I'm not ready to take these innovative projects seriously as yet. So in the meanwhile I'm studying Meissner and Nicolai's proposal.

You have a knack for finding really interesting papers though.

marcus

Here's a link to the Haelfix post I was quoting back a couple, right before Atyy's post:
http://physicsforums.com/showthread....75#post2379775
It has a useful list of issues which we can check to see if they are addressed
by Nicolai in his Planck Scale talk.
Video
http://www.ift.uni.wroc.pl/~rdurka/p...=1.3%20Nicolai
Slides
http://www.ift.uni.wroc.pl/~plancksc.../3-Nicolai.pdf

atyy

Ambjorn et al http://arxiv.org/abs/0906.3947

"It should be mentioned that the asymptotic safety picture is not the only suggestion for a continuum quantum theory of gravity using only “conventional” ideas of quantum field theory. .......... The other model goes by the name of “scale-invariant gravity” [5, 4]. ...

4. M. Shaposhnikov and D. Zenhausern, Quantum scale invariance, cosmological constant and hierarchy problem, Phys. Lett. B 671 (2009) 162-166 [arXiv:0809.3406 [hep-th]]; Scale invariance, unimodular gravity and dark energy, Phys. Lett. B 671 (2009) 187-192 [arXiv:0809.3395 [hep-th]].

5. M.E. Shaposhnikov and I.I. Tkachev, Quantum scale invariance on the lattice, 5 pages [arXiv:0811.1967 [hep-th]]."

marcus

Shaposhnikov! You found another reference to his scale-less gravity coming from another direction.
This time the reference is in a CDT paper by Renate Loll's group
"Quantum gravity as sum over spacetimes"
For convenience, I repeat the link you gave http://arxiv.org/abs/0906.3947
To save people trouble, if they want to follow, I will quote a longer excerpt from pages 5 and 6 of the paper
==quote Loll et al==
Thus we have created a picture where the underlying lattice spacing goes to zero while the physical mass (or the correlation length measured in physical length units, not in lattice spacings) is kept fixed. This is the standard Wilsonian scenario for obtaining the continuum (Euclidean) quantum field theory associated with the critical point g₀^c of a second order phase transition ...

We would like to apply a similar approach to quantum gravity, and thus obtain a new way to investigate if quantum gravity can be defined non-perturbatively as a quantum field theory. The predictions from such a theory could then be compared with the renormalization group predictions related to the asymptotic safety picture described above. It should be mentioned that the asymptotic safety picture is not the only suggestion for a continuum quantum theory of gravity using only “conventional” ideas of quantum field theory. Very recently two other scenarios have been suggested. One is called Lifshitz gravity [3] and is a theory where the non-renormalizability of the Einstein-Hilbert theory is cured by adding higher-order spatial derivatives in a way somewhat similar to what Lifshitz did many years ago in statistical models.

In fact, the setup of the theory has some resemblance with the lattice-theory setup of “Causal Dynamical Triangulations (CDT)” , to be described below, since a time foliation is assumed and the infrared limit is that of GR. However, contrary to Lifshitz gravity, we do not attempt to put in higher spatial derivatives in the lattice theory. However, when a continuum limit in the lattice theory is taken in a specific way which is not entirely symmetric in space and time one cannot rule out that higher spatial derivatives can play a role.

The other model goes by the name of “scale-invariant gravity” [5, 4]. It modifies gravity into a renormalizable theory by introducing a scalar degree of freedom in addition to the transverse gravitational degrees of freedom. Also this model has interesting features not incompatible with the results of computer simulations using the CDT lattice model.
==endquote==

marcus

One purpose of this thread is to choose a strong advocate for each approach (wherever a current presentation is available) and put the spotlight on a short up-to-date list of presentations that can serve to introduce the various competing approaches. Here is an up-date of an earlier list:

CDT
Loll at the Planck Scale conference is tops--best available single lecture on the subject.
http://www.ift.uni.wroc.pl/~rdurka/p...tul=1.4%20Loll
Video: "Causal Dynamical Triangulations and the Quest for Quantum Gravity"

AsymSafe
The last 12 minutes of Weinberg's CERN talk is the best video.
http://cdsweb.cern.ch/record/1188567/
Video: "The Quantum Theory of Fields: Effective or Fundamental?"
To save time jump to minute 58.
Percacci has an excellent paper, that just appeared. I wanted online video, but this is so good I'll list it with the others:
http://arxiv.org/abs/0910.5167
"Gravity from a Particle Physicist's perspective"

Horava QG
A video lecture by Horava himself. Fixed camera though. We may get something better after the November conference.
http://online.itp.ucsb.edu/online/ad.../rm/flash.html
Video: "Quantum Gravity with Anisotropic Scaling"

new look Loop
For the time being there is no 2009 online video that can serve. Here is the intro to Loop-Foam QG that Rovelli gave at Strings 2008:
http://cdsweb.cern.ch/record/1121957?ln=en
http://indico.cern.ch/getFile.py/acc...s&confId=21917
For a recent overview we have an online audio+slides talk by Lewandowski that is very good:
http://relativity.phys.lsu.edu/ilqgs...wski102009.pdf
http://relativity.phys.lsu.edu/ilqgs...wski102009.wav
more information if needed at http://relativity.phys.lsu.edu/ilqgs/
and the corresponding recent arxiv paper http://arxiv.org/abs/0909.0939
"Spin-Foams for All Loop Quantum Gravity

4D SUGRA No online video introduction available, but here are Powerpoint slides from Lance Dixon's two talks at Erice 2009:
31 August: http://www.ccsem.infn.it/issp2009/pr...rs/Dixon-I.ppt
1 September: http://www.ccsem.infn.it/issp2009/pr...s/Dixon-II.ppt

Hamber Regge QG
Hamber does a great job on PIRSA
http://pirsa.org/09050006/
Video: "Quantum Gravitation and the Renormalization Group"

Condensed matter approach
Atyy has suggested a 2008 PIRSA video of Xiao-Gang Wen.
http://pirsa.org/08110003/
Video: "The Emergence of Photons, Electrons, and Gravitons from Quantum Qbit Systems"
I think Fotini Markopoulou makes a strong presentation along what seem to be similar lines.
http://pirsa.org/09030018/
Video: "Quantum Graphity: a Model of the Emergence of Locality in Quantum Gravity


Some of these approaches may teach us something valuable, probably will in fact, and of course some may not. The idea here was to limit the poll to active well-known 4D QG research programs which can take up some of the slack resulting from current loss of focus and interest in the string unification program.
===================

So far nine people have responded!: eight on the original poll plus Arivero with a "write in" vote for SUGRA.

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

Thanks to all who have responded so far!

marcus

Has the picture changed?

Have any new approaches that you think are important come to light in the past 3 months?

We now have papers and conference talks by Percacci, Weinberg, Thiemann, Krasnov, Rivasseau that hadn't appeared when this thread was started. Does the paper by Stephon Alexander that appeared on arxiv yesterday change the picture?

Does anyone think the "Strand" model should be added to our list of contenders?

===================
So far 10 of us have responded: 9 have registered choices on the poll as posted, plus arivero who contributed a "write in" vote for Sugra.

Asymptotic Safe QG has gained favor, as compared with a month or two ago. The count is now:

Loop 5
AsymSafe 3
Sugra 2
CDT 2
Regge 1
Wen 1

heinz

I am discussing with Schiller on his "strand model" on another thread. There are many open issues, in my opinion; but he obviously sees the strand model as a candidate for a theory of everything. Despite the many open issues, I would agree that it is a candidate. This does not mean that I think that it is correct; in my opinion, it is too early to say that, or to say the opposite.

marcus

In retrospect, a lot of what we were discussing in this thread is actually part of the
no-frills unification trend in fundamental physics!

One of the things we already discussed a lot in this thread is Nicolai's unification gambit, which is kind of the archetypical no-frills proposal. He expounded it in that Wroclaw Planck Scale talk that in the other thread I said would be the "manifesto" of the movement if it is a movement.

And also remember that nearly the top thing in the poll, in this thread, turned out to be ASYMPTOTIC SAFE GRAVITY! And that fact about gravity---its apparent UV fixed point---has provided kind of a backbone for the no-frills approach to get off the ground.
Asymsafe gravity means a number of different things---takes part in several different approaches. I realize now, more that when I set up the poll, that it is not just one thing.

BTW Garrett, who is in touch with the pro theory scene, responded to the poll and chose Asymptotic Safety and the category with most interest/potential. Things like that can serve as straws in the wind for us.