# Leading lines of development in LQG, main issues, Loops 15

1. Apr 16, 2015

### marcus

The Loops conference held every two years, or so, gives us a good window on what the active lines of development are, what the organizers consider the main issues to be addressed and who they see as leaders in the various LQG research areas.

LQG is a research community rather than a single "official" formulation of a single agreed-on theory (that would be kind of ridiculous ) and there are in the community a number of active productive lines of development. It's the business of conference organizers to identify these and get them in focus in their list of PLENARY SPEAKERS and in their schedule of PANEL DISCUSSIONS.

Plenary speakers are the ones who usually speak in the morning sessions and present their talks to the whole conference (which might be 300 people in the main hall). Then in the afternoon the organizers let people divide up into 3, possibly more, "parallel sessions" run in smaller rooms. So there is time for many more parallel talks.

So you can see how this year's Loops 2015 might provide a kind of snapshot of who the people are and what the promising developments are in the LQG community.

Fortunately the list of plenary speakers has been posted, and also the list of 4 panel discussions (one on each of four evenings M T Th F.)

It's worth studying, if you are interested in learning about LQG.

Last edited: Apr 17, 2015
2. Apr 16, 2015

### marcus

July 6-10, Erlangen Germany
http://www.gravity.physik.fau.de/events/loops15/home.shtml

The invited plenary speakers are:
• Emanuele Alesci (Warsaw)
• Benjamin Bahr (Hamburg)
• Aristide Baratin (University of Waterloo)
• Norbert Bodendorfer (University of Warsaw)
• Astrid Eichhorn (Imperial College)
• Jonathan Engle (FAU Boca Raton)
• Hal Haggard (Bard College)
• Muxin Han (FAU Erlangen)
• Anna Ijjas (Princeton University)
• Wojciech Kaminski (Warsaw)
• Stefano Liberati (Trieste)
• Mercedes Martin-Benito (Radboud University Nijmegen)
• Karim Noui (LMPT Tours)
• Javier Olmedo (LSU)
• Parampreet Singh (LSU)
• Matteo Smerlak (Perimeter Institute)
• Wolfgang Wieland (IGC Penn State)
• Edward Wilson-Ewing (Albert Einstein Institute)
The invited panel speakers are:
• Abhay Ashtekar (PSU)
• Aurelien Barrau (Grenoble)
• John Barrett (Nottingham)
• Eugenio Bianchi (Penn State)
• Steve Carlip (UC Davis)
• Bianca Dittrich (PI)
• Laurent Freidel (PI)
• Stefan Hofmann (Munich)
• Jerzy Lewandowski (Warsaw)
• Tomasz Pawlowski (Warsaw/Santiago de Chile)
• Roberto Percacci (Trieste)
• Alejandro Perez (Marseille)
• Jorge Pullin (LSU)
• Carlo Rovelli (Aix-Marseille University)
• Lee Smolin (PI)
Here's the list of evening panel discussion TOPICS:

Monday: The issue of the continuum limit in QG
Tuesday: Status and prospects in (Loop) Quantum Cosmology
Thursday: Quantum Black Holes
Friday: Relation between canonical and covariant LQG (Dynamics)

Here is a timetable that has additional information embedded in it.
http://www.gravity.physik.fau.de/events/loops15/program.shtml
For instance, in future when plenary speakerswill have turned in their talk titles and abstracts you will be able to hover over an icon in any of the morning sessions and have a "pop-up" appear giving speaker's names and short descriptions of the planned talks.
But for now the "pop-ups" only give the names of the speakers and say "TBA"

Last edited: Apr 16, 2015
3. Apr 16, 2015

### marcus

Looking at the line-up some things immediately come to mind. In the "invited plenary speakers" list it is remarkable that these three are all there:
Haggard, Han, Kaminski
They were all three co-authors (with Riello) of a paper that put forward a new way to incorporate the cosmological curvature constant Lambda in simplicial LQG (i.e. spinfoam path integral dynamics). The idea is that instead of FLAT simplices one can use ones that all have this small curvature Lambda built into them.
This is at once a simple idea and one with far-reaching consequences.

It has the effect of making the phase-space of GEOMETRY compact and giving a kind of quantum jump discreteness to time and transitions from one state of geometry to the next. It reinforces the idea that the cosmological curvature constant has a deep connection to time. In cosmology I am finding it actually defines a natural scale of time and distance which greatly simplifies the equations.

So I have strong associations with this work of Haggard, Han, Kaminski, Riello (the socalled "HHKR" paper) and am alerted by three of the authors being invited to give morning talks to the whole conference. What the talks will be about, I do not know. I feel that at least ONE of them must be about how to include the cosmological constant in LQG.
HHKR paper:
http://arxiv.org/abs/1412.7546
Work by CL in several ways parallel to HHKR
http://arxiv.org/abs/1501.00855
which cites it as reference [22] and points out the parallels at a number of points (twice on page 3, twice on page 17, twice more in the conclusions)
An important extension, building on HHKR, see Vidotto's Tux3 slides and the paper they are based on:
https://www.gravity.physik.fau.de/events/tux3/vidotto.pdf
http://arxiv.org/abs/1502.00278

Last edited: Apr 16, 2015
4. Apr 16, 2015

### marcus

One way to tell the Loops 15 topics from the list of speakers is to check out the slides of the talks given at the February 2015 Tux3 conference:
https://www.gravity.physik.fau.de/events/tux3/tux3.shtml
You can see immediately that Wolfgang Wieland and Edward Wilson-Ewing, who are both invited plenary speakers at the July Loops 15, gave talks at Tux3, and the links are to their slides. So if you click on them you stand a good chance to get a foretaste of their talks at Erlangen in July.
Wieland's work is (to the extent it succeeds) transforming LQG, and Wilson-Ewing's incorporates the Loop cosmology bounce in the standard cosmic model obtaining observable predictions
Both are important developments in LQG.
You see several other Loops 15 speakers in the Tux3 lineup: Muxin Han, Martin-Benito, Bodendorfer...
their slides are there to check out as well.

Last edited: Apr 16, 2015
5. Apr 17, 2015

### julian

But no talks by Thiemann et al? Some recent progress made by Thiemann et al encourages me that a completion of the programme of canonical quantization of LQG may not be out of reach. (If anybody could do this it would be Thiemann et al).

One of the things that caught my eye was in "Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity" was: "some preliminary results on the semi-classical limit of graph-changing operators might be obtained through the use of the proposed Weyl quantisation". This was a major block to establishing that the canonical theory had the correct semi-classical limit.

Last edited: Apr 17, 2015
6. Apr 17, 2015

### marcus

Julian, good comment! Thiemann has posted major work just in the last year, over a half-dozen long papers, with Suzanne Lanery and with Alex Stottmeister. His collaborators are sure to be talking in the Parallel Sessions, and to be getting a lot of attention.
A possible consideration here is that Thomas Thiemann is the host of the conference, and there is an old custom (as I understand it I may be wrong) that the host does not showcase his own work.

Last edited: Apr 17, 2015
7. Apr 19, 2015

### marcus

I want to see what examining the structure of the conference can teach me
http://www.gravity.physik.fau.de/events/loops15/program.shtml
On Monday the four invited speakers in the morning are
Astrid Eichhorn
Wolfgang Wieland
Benjamin Bahr
Then there will be other speakers in the afternoon parallel sessions (that happens every day except Wednesday. Then in the evening there is panel discussion on continuum limit (panelists: Dittrich, Percacci, Rovelli, Varadarajan)
The recent work of Eichhorn, Laddha, Wieland, Bahr (all differing in their approach, IIRC no two are co-authors) must contain pointers to how the continuum limit issue is being addressed. I associate both Eichhorn and Percacci with Asymptotic Safety QG research. Laddha and Varadarajan share an interest in variations of LQG achieving closure of Hamiltonian constraint algebra. Bahr's work is closely allied to that of Dittrich, who has a longstanding interest in deriving the continuum limit. I'm having difficulty integrating all this and getting a focused picture. I see Wieland's recent work as original and potentially transformative.

On Tuesday morning the four are
Anna Ijjas
Parampreet Singh
Mercedes Martin-Benito
Edward Wilson-Ewing
And the panel topic is Loop quantum cosmology. Martin-Benito has been writing about the different channels of information we have flowing from the early universe, and the specific relevance to quantum cosmology. It's interesting that the recent works both of Ijjas and Wilson-Ewing involve a serious challenge to the supposition of inflation. The inflation paradigm has recently become more vulnerable as people discover other ways that known features of the early universe may have arisen. See video talks on this, and panel discussion, from the December 2014 Paris conference "Primordial Universe after Planck" http://webcast.in2p3.fr/events-primordial_universe_after_planck
Param Singh has been a central figure in LQC for at least the past 10 years, with a special interest in connecting the LQC bounce model to observations.

On Thursday morning the four speakers are
Hal Haggard
Aristide Baratin
Matteo Smerlak
Javier Olmedo
all of whom have recent work on black hole issues, different ways to resolve the information loss problem, to replace the singularity, to outline the progress of evaporation. Quantum Black Holes is the topic of the evening panel discussion ( Eugenio Bianchi, Steve Carlip, Alejandro Perez, Jorge Pullin)

On Friday the morning speakers are
Wojciech Kaminski
Muxin Han
Emanuele Alesci
with a conference summary presented by Ashtekar, Rovelli, Lewandowski
In this case I don't immediately see a connection with the evening panel topic, which is the relation between canonical and covariant LQG (Dynamics)
Emanuele Alesci has been hard at work showing the connection between Loop cosmology and the main LQG theory. I believe he has also contributed to unifying canonical LQG with the spin foam path integral version, collaborating with Lewandowski on this, so that would fit the panel theme. But I think of Han and Kaminski as currently involved in the exciting development of a way to build the cosmological curvature constant into simplicial QG by using uniformly curved simplexes rather than flat ones.

It's harder to identify a single main theme to the Wednesday morning talks. and there is no evening panel discussion that day ( a free afternoon and the conference dinner is scheduled for Wednesday). The speakers are:
Stefano Liberati
Norbert Bodendorfer
Karim Noui
Jonathan Engle.

I'll look up the recent papers and in some cases recent talks by some of these morning speakers and see if anything more comes to mind. I'd be interested in other people's reactions. What are especially interesting recent papers by any of the 19 plenary speakers, which could give us clues as to what might come up and be discussed at the conference?

Last edited: Apr 19, 2015
8. Apr 19, 2015

### marcus

Of the Monday speakers I particularly want to have a couple of Wieland abstracts for easy reference:
http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. This comes as a surprise. In the continuum theory, the reality conditions are preserved in time, only if the torsionless condition (a secondary constraint) holds true. Studying an additional conservation law for each spinfoam vertex, we discuss the issue of torsion and argue that spinfoam gravity may indeed miss an additional constraint. Next, we canonically quantise. Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures

Also on the Monday topic, one can argue that the continuum limit issue has been already resolved and some people just didn't notice What do you want the limit to be? Well the GR equation. But the GR equation is not something to quantize, it is the thermodynamic equation of state of some microscopic geometry degrees of freedom. And it was already shown that the Loop geometric degrees of freedom have the correct behavior and collectively obey that equation of state. Air has sound waves, water has water waves, but neither the air molecules or water molecules have those things. One does not try to quantize such large-scale collective behavior, the goal is not to quantize an equation of state but to understand what underlies it.

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(Submitted on 21 Jan 2014)
A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation. This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity. We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.
12 pages, 1 figure

So general covariant statistical mechanics is where progress has to be made. We don't yet have a diff-invariant (reparametrization invariant) stat mech or thermodynamics. The classical theories involve a pre-ordained fixed time parameter (unknown in nature and a no-no in GR).

http://arxiv.org/abs/1503.08725
Statistical mechanics of reparametrization invariant systems. Takes Three to Tango
Thibaut Josset, Goffredo Chirco, Carlo Rovelli
(Submitted on 30 Mar 2015)
It is notoriously difficult to apply statistical mechanics to generally covariant systems, because the notions of time, energy and equilibrium are seriously modified in this context. We discuss the conditions under which weaker versions of these notions can be defined, sufficient for statistical mechanics. We focus on reparametrization invariant systems without additional gauges. The key idea is to reconstruct statistical mechanics from the ergodic theorem. We find that a suitable split of the system into two non-interacting components is sufficient for generalizing statistical mechanics. While equilibrium acquires sense only when the system admits a suitable split into three weakly interacting components ---roughly: a clock and two systems among which a generalization of energy is equi-partitioned. The key property that allows the application of statistical mechanics and thermodynamics is an additivity condition of such generalized energy.
9 pages, 2 figures

So I guess that one of the speakers on Monday afternoon will be Goffredo Chirco, because the work he is involved with is related, in a somewhat surprising way, to the issue of the "continuum limit". I cannot check that because the schedule for the parallel sessions is not yet posted.

Last edited: Apr 19, 2015
9. Apr 19, 2015

### atyy

Reading too fast. I first read Wieland's title as "New simplicial action for dummies".

10. Apr 19, 2015

### wabbit

This sounds like a very strong argument to me. I must admit I have been confused for a while about this point. What would be a good reference about the emergence of GR from LQG in the thermodynamic perspective ?

Also this is naive and comes from my ignorance of how things are done usually in QM, but how else could GR possibly emerge other than as collective behaviour ? (If there is a simple non technical answer that is, anything else would be beyond my grasp)

Last edited: Apr 19, 2015
11. Apr 19, 2015

### marcus

I think a good reference (not specifically from LQG perspective) would be Jacobson's 1995 paper "The Einstein Equation of State".

12. Apr 19, 2015

### wabbit

Right, I read this one (though perhaps not closely enough), the part about which I'm unclear is the proof that LQG gives the correct equation of state when going from quantum to classical limit.

13. Apr 19, 2015

### marcus

I don't think that part is all wrapped up yet. It might be, it should be discussed at Loops 2015 in July. The next step in the argument is the paper I mentioned by Chirco, Haggard, Riello, Rovelli. We can look at that and see if there are any missing steps or big "ifs".

The last Loops conference was in 2013, before the CHRR paper.

14. Apr 19, 2015

### atyy

I think there are two continuum limits
(1) the "low energy" limit, which should be GR
(2) the "high energy" limit which should be a continuum theory in order to link it with the Hamiltonian formulation.

Yes, are there missing steps and big "ifs"? Here's the CHRR paper: http://arxiv.org/abs/1401.5262.

15. Apr 19, 2015

### wabbit

Ah yes missed that, thanks !

16. Apr 19, 2015

### wabbit

But at high energy you wouldn't expect a continuum, more something like a disconnected phase, no ? Perhaps I am misunderstanding what you say here.[/size]

17. Apr 19, 2015

### marcus

It may help to read the qualifications in the CHRR paper. In the introduction they seem to stress that Jacobson's is a local result, valid in Minkowski space, that would be in a local neighborhood of GR spacetime. I'm not sure that implies that GR is recovered globally. It may. I'm just not sure.

For whatever reason in subsequent work Chirco and Rovelli seem focused on finding how to formulate statistical mechanics and thermodynamics in a general covariant way. Maybe this is needed to complete the recovery of GR from QG?

What they show in CHRR is that QG degrees of freedom can serve Jacobson's turn. No need to postulate some unknown molecules of geometry, we have them. But is Jacobson's result fully adequate? It uses thermodynamics but thermodynamics cannot as yet be formulated in a global general covariant way.

Last edited: Apr 19, 2015
18. Apr 19, 2015

### atyy

In a QG theory in general, there need not be a fundamental continuum. In LQG, if spin foams are to be connected to canonical or Hamiltonian LQG, which was their original point, then there must be a fundamental continuum because canonical or Hamiltonian LQG is formulated with a fundamental continuum.

This is, I believe, why Kaminiski, Wieland's work is interesting.

But there are people who think spin foams point towards a QG theory that is fine without connecting to canonical LQG, eg. some of the GFT people like Rivasseau and Oriti. I think Freidel, Livine, Smolin, Krasnov are all also sympathetic to this possibility.

Last edited: Apr 19, 2015
19. Apr 19, 2015

### atyy

I'm not sure this question makes sense, because I'm always a bit confused about space versus spacetime, but here it is anyway. For CHRR to work, doesn't it have to be shown that the LQG dynamics takes Hadamard states to Hadamard states?

20. Apr 21, 2015

### marcus

Likewise. I'm not sure your question makes sense either, for somewhat the same reason. I don't see any place where their argument requires a "hadamard" state defined at a particular time to evolve into another "hadamard" state at a later time.
However it could depend on that and I simply fail to find the logical step.

I think it is fascinating that the CHRR authors have defined a Hadamard-like property in the QG context. they use the term "Hadamard-like" and make it clear they are exploring something that is not the hadamard property of Quantum Field Theory on curved spacetime, but something new in QG which they see as analogous. And they use " " quotes to make sure we understand it is an analog.

There is a 1995 paper by Robert Wald which is in part an instructive survey of QFT on curved spacetime, rich in intuition. I think it's not a bad introduction to the hadamard property in the previous context. I'd like to understand the analogy better. Formally there is no visible connection.
http://arxiv.org/abs/gr-qc/9509057
Quantum Field Theory in Curved Spacetime
Robert M. Wald
(Submitted on 29 Sep 1995)
We review the mathematically rigorous formulation of the quantum theory of a linear field propagating in a globally hyperbolic spacetime. This formulation is accomplished via the algebraic approach, which, in essence, simultaneously admits all states in all possible (unitarily inequivalent) Hilbert space constructions. The physically nonsingular states are restricted by the requirement that their two-point function satisfy the Hadamard condition, which insures that the ultra-violet behavior of the state be similar to that of the vacuum state in Minkowski spacetime, and that the expected stress-energy tensor in the state be finite. We briefly review the Unruh and Hawking effects from the perspective of the theoretical framework adopted here. A brief discussion also is given of several open issues and questions in quantum field theory in curved spacetime regarding the treatment of back-reaction", the validity of some version of the averaged null energy condition'', and the formulation and properties of quantum field theory in causality violating spacetimes.
18 pages. Write-up of plenary talk given at GR14

Here's a sample excerpt to suggest the flavor, it is on the whole somewhat C* and observable *-algebra A, and distributions:
==Wald page 12==
The main results of the analysis of < Tab > are the following (see [4] for more details): (i) < Tab >ω can be defined only for states, ω, that satisfy the Hadamard condition, which, in essence, states that the “ultra-violet” behavior of the state – as measured by the short distance behavior of the two point distribution < φ(x)φ(x′) >ω – is similar in nature to the short distance behavior of the two-point distribution for the vacuum state in Minkowski spacetime. (A precise definition of the “global Hadamard condition” can be found in [8]; its equivalence to a “local Hadamard condition” was proven in [9].) States which fail to satisfy the Hadamard condition are to be viewed as “physically singular”, in that their stress-energy is infinite (or otherwise ill defined). The Hadamard condition thus provides an important additional restriction on the class of states which otherwise would be admissible when only the fundamental observables are considered...
==endquote==

For comparison, here's an excerpt from CHRR:
==CHRR page 7==
Let us now consider a particular family of states |Φ0⟩ such that the associated reduced density matrix takes the the form
ρf = e−2πKf .
where Kf = K⃗f⋅ñ is the boost generator in the direction normal to the facet (the notation ρf = ρS = ρT indicates the symmetry of the reduced density matrix). We call these states “Hadamard states” for a reason that will be clear below.
...
To the ensemble of single facet states given by (54), the observer can effectively associate an absolute tem- perature, via the general definition
T = δE/δS = aħ/2π
because if it interacts with a large number of these, this is the temperature determining the transition probabilities between its eigenstates. This is the Unruh temperature.1 Therefore all the ingredients for Jacobson’s derivation follow (see also [27]).
==endquote==

Last edited: Apr 21, 2015