What are the best introductory resources for learning Loop Quantum Gravity?

[marcus]

Also, in Oeckl's http://arxiv.org/abs/gr-qc/0312081 (second bullet point at top of p3) he says it is not necessary to endorse a wave function of the universe, which is very much a Copenhagen point of view…

If anyone wants an up-to-date presentation of GBF by Oeckl, they should look at
http://arxiv.org/abs/1212.5571
This paper gives the axioms as of 2013 when the paper was published in Foundations of Physics. and also, in an appendix at the end, gives OLD axioms as of, say 2003, so that readers can compare the current version with the one of ten years back.

In neither set of axioms is there mention of a "classical observer". Neither the old or new formulation by Oeckl have much to do with "Copenhagen point of view" at least that I can see.

I think Atyy that you may just be determined to force your own interpretation on this stuff and there cannot be much to discuss for now

As for "endorsing a wave function of the universe", the first 15 minutes of Rovelli's Oxford talk explained his non-endorsement of such, but his POV was not Copenhagen. So what you say does not follow. It simply is not true that non-endorsement of "wave function of the universe" implies a "Copenhagen point of view".

 

[Demystifier]

As a Bohmian, my favourite interpretation is Copenhagen, ...

 

[Physics Monkey]

As a Bohmian, my favourite interpretation is Copenhagen,

Seconded! I'm quite curious what you mean, atyy.

 

[Demystifier]

Marcus and atyy, I don't think that the boundary formalism by itself prefers any particular interpretation of quantum mechanics (Copenhagen, relational, many-world, Bohmian, or whatever), even if the creators or users of this formalism do have some interpretational preferences. But whatever is the someone's preferred interpretation, I think the quantum measurement cannot be correctly understood without taking into account the phenomenon of decoherence.

 

[Demystifier]

Seconded! I'm quite curious what you mean, atyy.

Maybe he likes my solipsistic interpretation
http://lanl.arxiv.org/abs/1112.2034
the only interpretation I am aware that tries to reconcile Copenhagen with Bohm.

 

[marcus]

I don't think that the boundary formalism by itself prefers any particular interpretation of quantum mechanics (Copenhagen, relational, many-world, Bohmian, or whatever),...

I agree.
To say the obvious, GBF is a formalism and RQM is an interpretation. As far as I can see, GBF does not entail a preference for RQM.

It's a different question to ask if RQM entails a leaning towards some version of boundary formalism (not necessarily in the exact manner of Oeckl). I don't know that we are discussing that question, or have any need to in this thread, given that the topic is "good introductions to LQG".

But it interested me that the Oxford phil. of physics people invited Rovelli to present the Relational view at that Cosmology and Quantum Foundations conference last year, and in his first lecture he said he considers himself a Realist. I think Rovelli is in the course of defining or "staking out" a type of *Quantum Realism* (which is different from what was called strict Einstein realism in the 2007 Relational EPR paper with Smerlak).
I think when you put that type of Realism together with the Relational approach to quantum spacetime it becomes almost inevitable to focus on the basic entity which is the distinct process occurring in a definite region, interacting with other contiguous processes, similarly defined.

As I said, this is not necessarily germane to this thread and there's no special need to pursue it here, but I don't see how this particular nexus of ideas (Realism, Relationalism, quantum spacetime) can fail to have a "preference" (as you say) for separate interacting processes occurring in bounded regions.

 

[atyy]

Seconded! I'm quite curious what you mean, atyy.

I just mean that since Copenhagen is emergent from Bohmian mechanics, I can just use Copenhagen, since it is more user friendly.

I think it is analogous to someone believing that AdS/CFT can emerge from lattice models.

Maybe they are even related, since Bohmian mechanics is more natural in a non-relativistic environment.

So if: quantum equilibrium -> BM -> lattice models -> AdS/CFT -> quantum gravity in AdS.

Could it be: quantum non-equilibrium -> quantum gravity in non-AdS ?

 

[julian]

Rovelli and Vidotto have written a new introductory textbook on loop quantum gravity:
https://www.amazon.com/dp/1107069629/?tag=pfamazon01-20

It is not yet officially published, but a draft can be freely (and legally) downloaded from
http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf

I have started reading it, so I can tell its excellent.

A lot of introductory material on loop quantum gravity can also be found at the wikipedia page
http://en.wikipedia.org/wiki/Loop_quantum_gravity
where I have found the link to the draft above.

Thanks, I wrote up most of the wiki page on LQG - and most of it is still there - nobody has changed much - go figure. Not happy with it, incomplete, probably mistakes. Also related topics (to which I put links) incomplete. It covered mostly canonical LQG, not so much covariant LQG. Although have also started reading the Rovelli/Vidotto book and learning/reminding myself more about it (and reading papers about the connection between them - in particular a paper on the 3D curvature operator and references therein). Perhaps a new wiki page should be devoted to "Covariant Loop quantum gravity"?

Been looking at their book, excellent, very readable which they should be congratulated on - but also the feeling that this may be at the expense of presenting a picture of things that are maybe a bit too hunkydory. But then in the last exercise of the book they say "Show that the theory defined in this book is fully consistent; if it is not, correct it"...

There are certainly details left out...

Demystifier also said

"I would even go that far to propose that someone who never says anything silly, probably has nothing important to say at all." To which I say Thomas Thiemann.

People on this forum are capable of not just talking about this stuff but also writing articles for wiki for LQG related stuff...I know Marcus has much to contribute to "time" in quantum gravity and some other Rovelli type stuff I think.

I also had a go at wiki pages for

"Hamiltonian constraint" needs work.

"Hamiltonian constraint of LQG" - I'm missing stuff on matter coupling there and inclusion of their qunatization.

"Tetradic Palantini action"

"Self-dual Palatini action" - important proofs of stuff I could not find in any text and only came across in some quasi-paper, I have written up on wiki and checked it is correct.

Put up stuff on the Ashtekar variables page.

Spin connection.

Mean to include the derivation of Ashtekar's formalism from the Self-dual action.


THERE is a lack of any wiki pages on:

Dirac observables!

Problem of time.

Complete, partial observables.

Mandelstam identities to do with trace of SU(2)/SL(2,C) matrices generalized for LQG purposes (Mandelstam was quite big on identities in general and there are a few different identities with his name).

The loop representation.

Reduced phase space quantization.

Evolving constants of motion.

Consistent discretetizations.

momentum representation of QM! Erh? why not done yet?

Just to mention some.

 

[Physics Monkey]

I just mean that since Copenhagen is emergent from Bohmian mechanics, I can just use Copenhagen, since it is more user friendly.

I think it is analogous to someone believing that AdS/CFT can emerge from lattice models.

Maybe they are even related, since Bohmian mechanics is more natural in a non-relativistic environment.

So if: quantum equilibrium -> BM -> lattice models -> AdS/CFT -> quantum gravity in AdS.

Could it be: quantum non-equilibrium -> quantum gravity in non-AdS ?

I don't understand the first two implications between quantum equilibrium, BM, and lattice models.

(May I ask, why are you a Bohmian?)

However, I do think quantum non-equilibrium should have something to do with non-ads gravity, e.g. dynamical loss of entanglement could represent some kind of singularity.

 

[Demystifier]

So if: quantum equilibrium -> BM -> lattice models -> AdS/CFT -> quantum gravity in AdS.

Can you explain
BM -> lattice models
and
lattice models -> AdS/CFT?

 

[atyy]

I don't understand the first two implications between quantum equilibrium, BM, and lattice models.

(May I ask, why are you a Bohmian?)

However, I do think quantum non-equilibrium should have something to do with non-ads gravity, e.g. dynamical loss of entanglement could represent some kind of singularity.

I don't think we mean the same thing by quantum equilibrium. In BM, "quantum equilibrium" is a distribution of initial conditions. So BM is quantum equilibrium and deterministic dynamics of hidden variables. I was thinking that if AdS/CFT has a Bohmian version, then a generalization would be keeping the deterministic dynamics, but with initial conditions such that QM fails. I think QM still holds in what you mean by quantum nonequilibrium.

The relationship between BM and lattice models is that lattice models are nonrelativistic, with relativity being emergent. Since BM is more natural with nonrelativistic QM, it seems to me more natural from the BM point of view for relativity to be emergent.

I'm not really a Bohmian since I don't know if it's true. But since it allows a naive conception of reality, it accords with my intuition. Also it assuumes that QM fails at some level, so it shares with Copenhagen the possibility that QM is only an effective theory.

 

[atyy]

Can you explain
BM -> lattice models
and
lattice models -> AdS/CFT?

The idea is that with BM, it is more natural if relativity is not fundamental, but is instead emergent.

So I'm hoping there is a lattice model from which the relativistic supersymmetric CFT of AdS/CFT is emergent.

 

[Demystifier]

The idea is that with BM, it is more natural if relativity is not fundamental, but is instead emergent.

So I'm hoping there is a lattice model from which the relativistic supersymmetric CFT of AdS/CFT is emergent.

Makes sense to me.

 

[marcus]

Rovelli and Vidotto have written a new introductory textbook on loop quantum gravity:
https://www.amazon.com/dp/1107069629/?tag=pfamazon01-20

It is not yet officially published, but a draft can be freely (and legally) downloaded from
http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf

I have started reading it, so I can tell its excellent.

A lot of introductory material on loop quantum gravity can also be found at the wikipedia page
http://en.wikipedia.org/wiki/Loop_quantum_gravity
where I have found the link to the draft above.

Atyy, your examples demonstrate that the book should be read critically. There are claims in it which are controversial. But still, I find the book good because I find many illuminating explanations in it. Rovelli certainly has a talent to explain complicated technical stuff in a relatively simple and intuitve way, and that's what I like about this book.

Thanks again for the links! I didn't realize that a draft of the book "Covariant LQG: an Elementary Introduction" was online, free to download. I also think it is an excellent book.

Rovelli just gave a 5 lecture "minicourse" introduction with the same title "Covariant Loop Quantum Gravity" at the June 2014 SIGRAV school at a villa on Lago di Como. I understand the course went well! Students had the new introductory textbook available, and I think the course was based in part on portions of the text.
The other six lecturers at the June school were Abhay Ashtekar, John Barrett, Eugenio Bianchi, Alain Connes, Laurent Freidel, and Roberto Percacci. Typically each gave a 5 lecture mini course. Here are the topics and brief course outlines:
http://www.centrovolta.it/sigrav2014/Schedule.pdf

The brief outline for Rovelli's course at the villa was:
COVARIANT LOOP QUANTUM GRAVITY
1. Quanta of space
2. The quantum states of the geometry
3. Dynamics
4. Measurements and predictions in quantum gravity
5. Planck stars

I was interested to see that part of the course was devoted to the Planck star model of black holes, and that phenomenology (measurements and predictions) was covered. The subject of CLQG is not purely theoretical. Here is the website for the 2014 SIGRAV school
http://www.centrovolta.it/sigrav2014/

 

[marcus]

I've got a copy of the book and have been skipping around reading different chapters. It's great. It reminds me of a "Guide Bleu" travel guide to various lively research areas in quantum gravity, and cosmology. Written at the most elementary possible level, so as to make the different topics accessible.

Here's the link that Demystifier gave (see post #84), to the free online (earlier draft) version of the book, I just checked and it still works. People might want to download (save as PDF on your desktop for convenient reference).
http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf
I'll get the amazon link too:
https://www.amazon.com/dp/1107069629/?tag=pfamazon01-20
It is an updated version of the one Demystifier gave.

 

[marcus]

The book is loaded with pictures/diagrams which are really useful in understanding the material. At the moment my favorite sections of the book are Chapters 10 and 11. Two short chapters: one on Black Holes and one on Cosmology. They explain stuff that I recognize from recent research papers, but seen from a different angle and in a broader context making it easier to understand.

 

[julian]

Just ordered mine but got it from:

http://www.amazon.com/Covariant-Loop-Quantum-Gravity-Introduction/dp/1107069629/ref=sr_1_1?s=books&ie=UTF8&qid=1419776044&sr=1-1&keywords=loop quantum gravity

and bought it in pounds. :) "Very British".