# Where to get started in Loop Quantum Gravity?

1. Feb 28, 2013

### Leucippus

Hi, I wasn't sure where to post this so I hope it's ok to post my questions here.

I would like to learn the basic fundamentals of Loop Quantum Gravity as efficiently as possible. I'm currently 63 years old. I majored in Physics and Chemistry eons ago, and so I'm rusty on everything but can potentially get back parts I need relatively quickly. The main thing I'm looking for is an overview that basically gives the main concepts of Loop Quantum Gravity. Another thing that might also be useful would be a course outline or syllabus intended for students who are interesting in focusing on Loop Quantum Gravity.

Some specific questions I have:

1. Are there any good books or lecture videos that are specifically designed to be an "Introduction" to Loop Quantum Gravity.

2. Is a knowledge of String Theory required or helpful in anyway?

3. Do there exist course outlines, or curriculum that is specifically tailored for students wishing to embark on this study.

Personal note: I can handle some graduate level materials, but I would really appreciate anything at all that is in the undergraduate realm that would specifically apply to, or be useful for, entering into the study of Loop Quantum Gravity.

I'm looking for materials that will give me a good understanding of the basic ideas from which I can then expand from.

Final note: I've just finished reading Lee Smolin's book, "Three Roads to Quantum Gravity". I was able to understand many of the ideas he presented. I'm planning on re-reading his book and taking notes this time to try to create my own framework.

So that's where I'm at right now. I'm just looking for a way to get best organized to gain a better understanding as efficiently as possible. What I'd like to gain is a "Bird's Eye View" of what it's all about, then I can know better where I need to zoom in and get my hands dirty in the details.

All suggestions will be appreciated.

Thank you.

2. Feb 28, 2013

### Staff: Mentor

Another book to read would be the Black Hole War by Susskind also like Lee Smolins book only more actionable. I'm reading it now and it touches on QG and the entropy of Black Holes.

Susskind also has two other books:

1) Physics, The Theoretical Minimum (Basic Physics complements his videos online)

https://www.amazon.com/The-Theoreti...=UTF8&qid=1362107101&sr=8-1&keywords=susskind

2) The Cosmic Landscape (String Theory, haven't read it yet)

https://www.amazon.com/Cosmic-Lands...=sr_1_3?s=books&ie=UTF8&qid=1362107127&sr=1-3

and lastly there's Roger Penrose book: The Road to Reality (a much harder book to read a lot more math and theory)

Im in a similar boat with a similar age and finally decided to review my Vector/Tensor Analysis using Schaums Outlines, the Arfken/Weber book and the book Div, Grad, Curl and All That figuring that vectors and differential equations are the key to relearning what I forgot or never really knew.

Last edited by a moderator: May 6, 2017
3. Feb 28, 2013

### Mute

[strike]You should perhaps send a moderator a message and ask that this thread be moved to the "Beyond the Standard Model" forum. That's where a lot of stuff like Loop Quantum Gravity is discussed here, and the folks who frequent that forum may have a lot more suggestions for you.[/strike] It's been moved!

I'm not sure if the books jedishrfu mentions really touch on Loop quantum gravity specifically. In the black hole book I can see it coming up, but I don't know how detailed a discussion it would be, so maybe try to rent that one from the library first. I seem to remember that Susskind isn't/wasn't a big fan of Loop Quantum Gravity, so if he mentions it he may not be favorable towards it.

Otherwise, I don't know much about the field myself, so this is all the advice I have.

Last edited: Feb 28, 2013
4. Feb 28, 2013

### atyy

http://arxiv.org/abs/1007.0402
Introductory lectures to loop quantum gravity
Pietro Doná, Simone Speziale
(Submitted on 2 Jul 2010 (v1), last revised 20 Sep 2010 (this version, v2))
We give a standard introduction to loop quantum gravity, from the ADM variables to spin network states. We include a discussion on quantum geometry on a fixed graph and its relation to a discrete approximation of general relativity.

String theory is not necessary to understand LQG, but since string theory is the only working theory of quantum gravity in some universes, I personally find it helpful to understand string theory in order to understand the issues in quantum gravity.

5. Feb 28, 2013

### marcus

Gambini and Pullin have an undergraduate textbook intended as an introduction to LQG.
A lot of it is review, of the version of General Relativity they are going to build on, for example.

I heard a rumor that Robert Littlejohn (a physics prof at UC Berkeley) liked the book enough that he was considering teaching a semester course, Intro to LQG, using it as the textbook.
He's first rate so that speaks well for the book. I have not looked at it so I only have secondhand impressions like that.

==============

You also asked about video lectures. There is a series of a dozen or so PIRSA video lectures intended as an Intro to LQG. It was made by Carlo Rovelli in Spring 2012 when he was visiting at Perimeter Institute. They are free online. With PIRSA talks you can usually download the slides PDF and have that on your desktop, and then start the video. So then if during the lecture you can't read what is on the blackboard or the projection screen you can refer to the PDF of stills.
If you google "pirsa rovelli" you should get links to all those lectures. Pirsa stands for perimeter institute recorded seminar archive.
==============

I'll think some more, at leisure, and maybe come up with other suggestions.

BTW I just looked up Gambini and Pullin's text.
https://www.amazon.com/First-Course-Loop-Quantum-Gravity/dp/0199590753
Gambini and Pullin are first rate LQG people. The book is 192 pages and the price is around 45 dollars. Christine Dantas, who reviewed it is knowledgeable and sensible. IIRC she does practical physics for her country's military, but is up on astrophysics and has done some QG research. I think she has taught some at university level. Had a QG blog for some years and knows the literature. So her favorable review carries some weight with me.
The amazon page does let one see the table of contents and browse a little. So one can get a bit of an idea.

Maybe it would be too elementary for you. I can't say. I should really walk over to the Physics Library on campus and spend some time with the book. then I'd have some firsthand impressions to go on.
===============
I googled "pirsa rovelli" and got this http://pirsa.org/C12012
There seem to be 14 lectures in the series. The blackboard is used rather than computer slides. Some stuff is written ahead of time on blackboard panels.

Last edited by a moderator: May 6, 2017
6. Mar 1, 2013

### Demystifier

It's a good book, but I have found an elementary conceptual error in it. The error is present in the first paragraph of Sec. 6.1. For pedagogic purposes, I invite the readers to find the error by themselves.

Hint - the crucial keywords are: two particles, superposition, joint probability

Last edited by a moderator: May 6, 2017
7. Mar 1, 2013

### Staff: Mentor

I don't know much about LQG other than the generalities of it. However, the OP mentioned that he had read Smollins LQG book so I started from there with books progressively moving in that direction. Susskinds book and videos are attempting to bring long-time fans of physics deeper into the math of it so that they'd be able to actually read heavier books on the subject.

Susskind did mention QG in his BH War book and so I figured its good to know where LQG is coming from. He has a discussion on Bekenstein's BH entropy, Hawking temperature and his battle about lost information not really being lost like Hawking thought. The collest thing so far (still reading) is the Bekenstein tought experiment of adding a single photon to the BH with wavelength = radius if the event horizon and discovering that it increases the BH event horizon area by one square planck unit.

Anyway, I hope yours, mine and other posters suggestions helps him.

8. Mar 1, 2013

### Mute

Yeah, the books you mentioned definitely sound very interesting, and will probably be a good read for Leucippus! I just wanted Leucippus to be aware of the differences between LQG and other theories of quantum gravity, and that some of these books may not discuss a whole lot about LQG itself, beyond saying something like the black hole entropy can also be calculated in LQG, up to a multiplicative factor called the Immirzi parameter, for example.

9. Mar 1, 2013

### atyy

Probably put it in to see if anyone was actually reading their book .... ;)

10. Mar 1, 2013

### Naty1

Susskind's BLACK HOLE WAR is just plain fascinating....unique perspectives....

I do not remember anything about LQG..

If THE ROAD TO REALITY {Penrose] has anything, I could not find it in the Index....

Try reading here for some overview and perspective....

http://en.wikipedia.org/wiki/Loop_quantum_gravity

I just went to youtube and searched Loop quantum gravity....beside the tv show BIG BANG THEORY [humor]
there are a number of hits...Rovelli, Smolin,etc....

11. Mar 1, 2013

### Staff: Mentor

Ahh, young Palawan you forgot to check the R2R table of contents. Chapter 31 covers String theory and 32 covers Einsteins Loop Variables the basis for LQG theory.

12. Mar 4, 2013

### Demystifier

If so, have you found the error?

13. Mar 4, 2013

### atyy

I didn't read most of it. I read a few pages while browsing in the library.

I found the error by searching on Amazon, using your keywords.

14. Mar 4, 2013

### Leucippus

Thanks to everyone for the suggestions.

Gambini and Pullin's book looks like a good place for me to start. I've been reading it online as a preview in Google Books. They preview quite a lot of it so just that preview will keep me busy for a while and when I'm ready for more details I'll try the library.

I also looked through the Introductory lectures to loop quantum gravity by Pietro Doná and Simone Speziale. Some of the stuff on knot theory is familiar to me. I recently took a course in topology and manifolds in n-dimensional space. So some aspects of it will be more readily accessible to me than others.

I'm particularly interested in how LQG achieves background independence, since this part of why I favor it over String Theory. I can already see that this has something to do with diffeomorphisms and can be explained intuitively in terms of knot theory which is pretty darn nice for me.

Another reason I'm drawn to LQG is because I have some ideas of how LQG might relate to entropy concepts associated with the event horizon of black holes. I actually have several theories about black holes that LQG appears to be compatible with. So entropy associated with the area of the horizon of black holes, as well as background independence are all important to me, and LQC appears to satisfy all these criteria where String Theory doesn't.

So anyway, the Introductory pdf is actually well over my head in general, but will be helpful in terms of helping me to better understand what areas I need to expand on.

I'm definitely going with Gambini and Pullin's book for an introductory course that will probably take me the better part of a year to complete. So that looks like the best place to start.

I was actually looking at that book previously, but I thought I better ask here first in case there are other suggestions. This Gambini and Pullin's book does appear to be the best Introductory approach at the current time though.

So thanks for the input. I'll go with Gambini and Pullin.

Oh, yeah, I'll read Susskind's black hole wars too. I've been meaning to buy that book. I've actually seen quite a view videos and documentaries that have talked about this topic, but I'd like to read Susskind's own views. I like Susskind, he's a good guy even if he does favor String Theory over LQG. After all, he was practically the father of String Theory, he can hardly abandon his own baby now.

I might go for his book on The Theoretical Minimum, too. I realize it's all classical stuff, but it sounds like he really hits the nail on the head with the important ideas that underlying each concept, so it might be a good book to read just for review.

15. Mar 11, 2015

### qftalpha

I did find the error. Pretty elementary ( i'm 14)

16. Mar 11, 2015

### Demystifier

Recently Jorge Pullin informed me that the error I found is removed in the new edition of the book, which should appear soon.

17. Mar 11, 2015

### martinbn

So what's the famous mistake? I didn't see anything about two particles in that paragraph.

18. Mar 11, 2015

### Demystifier

They say "If one has two particles, represented by wavefunctions $\Psi_1(x)$ and $\Psi_2(x)$, the combined probability is ..."
Do you still don't see it?

19. Mar 11, 2015

### Demystifier

Good, so you can explain the error to the others.

20. Mar 11, 2015

### martinbn

Ah, yes. I was looking at the wrong section. That's obvious, I don't think anyone was confused by it.