Exploring the Six Rings of Smolin's Quantum Gravity Circus

[marcus]

Smolin looks to me like he has about halfdozen QG-related things under investigation right now at Perimeter Institute---maybe a halfdozen main quantum gravity issues, or lines of investigation. I know they do other kinds of research besides QG, but now I am just looking at the one sector.
The complexity is getting to be exciting and (without resorting to a checklist like this) a bit hard to follow.

1. Race to testability
2. DSR Soccerball
3. Dynamical triangulations
4. Immirzi parameter
5. Kodama state
6. Extended diffeomorphisms (if not already, will be working on this one)

I will discuss each ring of the circus, inviting comment as well. But first I want to list some recent papers, at least one dealing with each issue.
The paper can serve to illustrate the issue without my having to explain.

I am not including CNS (cosmic natural selection) which is Smolin's scientifically testable multiverse theory. We already have a thread on that and CNS is not logically a part of Loop Gravity. It is its own separate theory. One can think of it as a "sideshow" to the main circus.

 

[marcus]

papers illustrating each thing

1. Race to testability
Smolin An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048
(especially pages 27 and 28, The near term experimental situation)

Christian Testing Quantum Gravity via Cosmogenic Neutrino Oscillations
http://arxiv.org/gr-qc/0409077

2. DSR Soccerball
Hinterleitner Canonical DSR
http://arxiv.org/gr-qc/0409087

3. Dynamical triangulations
Markopoulou, Smolin Gauge fixing in Causal Dynamical Triangulations
http://arxiv.org/hep-th/0409057

4. Immirzi parameter
Dreyer, Markopoulou, Smolin Symmetry and entropy of black hole horizons
http://arxiv.org/hep-th/0409056

5. Kodama state
Malecki Inflationary Quantum Cosmology: General Framework and Exact Bianchi I Solution
http://arxiv.org/gr-qc/0407114

6. Extended diffeomorphisms
Fairbairn, Rovelli Separable Hilbert space in Loop Quantum Gravity
http://arxiv.org/gr-qc/0403047

 

[meteor]

Hi marcus. Interesting thread

What do you think of this?: Dynamical quadrangulations. Seems a modification of the dynamical triangulations business
http://arxiv.org/abs/hep-lat/0409028

 

[selfAdjoint]

Marcus what do your paper 5 (by Malecki) and 6 (by Fairbairn and Rovelli) have to do with Smolin?

 

[marcus]

Hi Meteor, thanks for catching this paper. I rarely look in arxiv's "hep-lat" department so I would likely have missed it. The junior author, Martin Weigel, is relocating to Uni Waterloo (next to Smolin's Perimeter I.). So he will likely be recruited the group at Perimeter doing Dynamical Triangulation-style quantum gravity. The senior author, Wolfhard Janke, has been doing this kind of thing (lattice field theory, Monte Carlo methods, simplicial quantum gravity) for over 10 years. I see that he has 71 papers going back to 1993. You asked about my reaction to the paper. I can't tell too much. But Ambjorn Jurkiewicz Loll did not put matter on the 4D worlds that they made in the computer by causal DT. Here these two people at Leipzig are wanting to put some kind (toy?) material on their computer model DT world----but it is lower dimension so everything is still just getting started (or so is my impression). the best I can do by way of response is just to quote a short exerpt:

The F model on dynamical quadrangulations
Martin Weigel and Wolfhard Janke
http://arxiv.org/hep-lat/0409028

---quote from the introduction---
Einstein gravity being perturbatively non-renormalizable as a field theory, constructive approaches towards a quantization of gravity have been an ever more active field of research in the past decades[1]. The dynamical triangulations model in its Euclidean and Lorentzian versions has proved a successful ansatz for the formulation of such a consistent theory of quantum gravity [2, 3]. Compared to the more fancy methods, such as string theory[4] and non-commutative geometry [5], it is rather more minimalistic in trying to directly model the quantum fluctuations of space-time by a probabilistic sum over an ensemble of discrete, simplicial manifolds [6].
---end quote---

 

[marcus]

Marcus what do your paper 5 (by Malecki) and 6 (by Fairbairn and Rovelli) have to do with Smolin?

Hi selfAdjoint, the connection with Malecki is very straightforward since he is at Perimeter Institute and has earlier co-authored with Smolin about this very thing----inflation, quantum cosmology.

For example:
Stephon Alexander, Justin Malecki, Lee Smolin
Loop Quantum Gravity and Inflation
Phys. Review D (2004)
http://arxiv.org/hep-th/0309045

(in this paper they reply tangentially to Witten's earlier paper saying
Kodama not normalizable---"it, or something like it, might or might not be depending on the setup" essentially---no public reply from Witten about this AFAIK so far)

However Smolin's interest in the extended diffeomorphisms is not so obvious! You ask what is the connection with the Fairbairn/Rovelli paper!

For this look at the bottom of page 9 of Smolin's An Invitation to Loop Quantum Gravity and see Smolin's reference [43] to the Fairbairn/Rovelli paper.

Smolin has bought on to extended diffeomorphisms. Maybe at Perimeter there is not yet anybody working on this (I have no way of knowing) but I think there is or will soon be because having the group be the extended diffeo instead of just the plain diffeo is a big change--- there will be a lot of mathematical work to do about this, I feel certain. Rovelli has reported that Lewandowski and Ashtekar have also moved in this direction. so it is a future scramble that may not yet be visible. here is what Smolin says in "Invitation":

"...Instead we will find we can construct exactly states corresponding to their spatial diffeomorphism invariant classes. It turns out that there are exactly enough of these, so that we end up with a separable basis of normalizable diffeomorphism invariant states..."

 

[marcus]

ring 1.the race to testability

the race is basically to get theoretical predictions established before the experiments start. GLAST and ICECUBE to mention a couple. I will add some more papers to the list

1. Race to testability
Smolin An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048
(especially pages 27 and 28, The near term experimental situation)

Christian Testing Quantum Gravity via Cosmogenic Neutrino Oscillations
http://arxiv.org/gr-qc/0409077

Piran Gamma-Ray Bursts as Probes for Quantum Gravity
http://arxiv.org/astro-ph/0407462

Jacobson et al Quantum Gravity Phenomenology and Lorentz Violation
http://arxiv.org/gr-qc/0404067

 

[selfAdjoint]

Marcus, is Physicsforums taking over S.P.R.? You, juju, and I all have posts over there today (both of you have two!). Not to mention Urs...

 

[marcus]

selfAdjoint, whatever attractions SPR may hold out I am glad you are at this thread. I want to try to say how these various research issues are connected.

testability is the key issue and Perimeter Institute is kind of a Petri dish for the culture of quantum gravity so one can see instances of the main concerns in who is there and what they are investigating

nearterm prospects for testing focus on dispersion relations (observed over cosmological distances: the sharp spike of a gammaray burst observed after traveling a billion years)

what this means to me is that one is testing LQG by way of DSR.
If one can falsify the version of DSR implied by some form of quantum gravity then one indirectly falsifies that approach to QG.

the introduction to Joy Christian's article is very clear on this.

this is why the soccerball problem is important because it is an obstacle to making sense of DSR and meanwhile DSR is at the crux of nearterm QG testing.

I also want to say why work in Dynamical Triangulations (computer simulations of dynamic geometry) is also critical----how it connects to LQG
and what Smolin and Markopoulou recent DT article was aimed at.
Maybe these are self-evident things but I want to say them explicitly and as clearly as possible.
In their recent DT paper Marko/Smolin were interested in arguing that DT does not have a preferred time or spacetime foliation singled out.

I think they see a way to connect DT with spinfoam and loop gravity so that DT can help certify the low energy limit of quantum gravity. Or else guide them to correct modifications of LQG.

 

[selfAdjoint]

I skimmed Christian's paper, and it looks like we should look for a PREDICTED SN more than 10,000 ly away. Predicted so we can set up to look for the neutrino oscillations. I wonder if the astronomers have any pre-SNs in view at the required distances? Also I have to caution that Christian says she uses a non-standard method of calculating the ocsilllation frequency (I didn't study the details of it). This could weaken the impact of any positive LQG results fron neutrino oscillations.

BTW, it looked to me like she was saying that work to date has ruled out QG predictions of a preferred frame, which I take to be your DSR connection. Would you care to comment on this?

 

[meteor]

http://arxiv.org/abs/gr-qc/0409031
Khriplovich proposes here a value of 0.27 for the Immirzi parameter. I remember that recently in gr-qc/0407052 Meissner proposed a value of 0.23

 

[marcus]

...
BTW, it looked to me like she was saying that work to date has ruled out QG predictions of a preferred frame, which I take to be your DSR connection. Would you care to comment on this?

would be glad to comment, though later.
I reckon there is no great need for comment, however,
since the logical connection with the growing interest in DSR is clear.
The whole point, one might say, of DSR is its freedom from a preferred frame.

I first read of the preferred frame approach being ruled out, or all but ruled out, last year in Smolin's basic paper How far are we from the quantum theory of gravity?, where he cited work by Jacobson, Liberati, Mattingly dealing with synchrotron emission from the Crab Nebula. I think Joy Christian is basically echoing what Smolin said there. There has been a surge of interest in DSR (and also TSR) since then.

what one wants observations to do is distinguish what version of DSR, to determine some of the coefficients in the dispersion relation (or, as Joy Christian also considers, to rule it out)

have to go but will be back later, hopefully with more comment

 

[wolram]

Marcus
Does QG predict a value for this refraction, if so can you give some idea
as to the scale?
Thanks.

 

[marcus]

Marcus
Does QG predict a value for this refraction, if so can you give some idea
as to the scale?
Thanks.

I have not seen any prediction of a value for any of the coefficients in the presumed dispersion relation.

Unfortunately that's all I can say in answer to your question! But just to exercise the words a bit I will try to say more about it. Refraction is slightly different, they talk about "dispersion" which is what happens when the speed waves travel depends on the wavelength.

When you hear "whistlers" over long-distance ham radio it is a dispersion effect. There was a bolt of lightning in Argentina which in a single instant sent out vibrations of many frequencies (a mix of all frequencies is white noise, but this was a brief burst). By the time the signal gets to you it sounds like a slide-whistle because the low frequencies traveled faster and come in ahead so you hear the lows first and it glides up the frequencies. I may have it backwards, could be remembering wrong. Anyway the components of noise are pure flute-like sounds and you can get them separated out in time by some traveling faster than others.

In LQG if you say that the Planck energy is an invariant scale (as well as the speed of light) then one possibility is a preferred coordinate frame. Now suppose you exclude that possibility.
then you get a choice of DSR options with a very slight energy-dependence of the speed photons travel.
But nobody can say yet how slight!
Only that the dependence is not zero.

You still have the constant c, but it is the low energy limit. Effectively it is the speed all the light we see and are used to goes. But for very high energy light, gammaray, the speed is just slightly faster so that over astronomical distances it might be possible to detect dispersion.

OR AT LEAST observations could RULE OUT dispersion. they could say "if there is a variation of the speed with energy then the coefficient must be less than so-and-so". So they could put observational constraints on it.

Traditionally in physics they expect coefficients to be "order one" when measured in the appropriate units. The idea is that nature favors coefficients like 1/2 or 2 pi, or one itself. So if you constrain some parameter to be way less than a millionth (when it was expected to be "order one") then people say "virtually ruled out" and give up on that one.
This is what has to happen.
they either have to constrain the dispersion coefficients to way less than order one (expressing everything in Planck units, considered natural for this job) in which they have vindicated pure Lorentz symmetry, that is pure SR.
or, in the process of trying to show the coefficient is virtually nil they might find that it ISNT. In that case pure Lorentz, or pure SR, gives way to the slightly modified SR called DSR----with some experimentally determined dispersion coefficient or coefficients plugged in.

I think you have been thru this on your own, wolram, but I thought I'd say it anyway in case some other reader finds it useful. I could also copy in the formulas for the "energy-momentum" relation you get in DSR which is where the dispersion relation comes from, but you saw that when we were discussing Smolin's March 2003 article "How far are we from the quantum theory of gravity?"

 

[wolram]

An extremely well written and thoughtfull reply thankyou Marcus.