# Introduction To Loop Quantum Gravity

by marlon
Tags: loop quantum gravity
 P: 4,006 Marcus, thanks for the reply... It can only be a good thing that others contribute but i am convinced that we need to keep the level basic enough in this sense that i wanna move up the "difficulty-scale" gradually. It would be a bad thing if we were to discuss high-level papers because i think most of us (including myself) will not be able to follow this up and we would get discouraged and drop the subject. I will continue this matter and i would suggest that we follow the content of Rovelli's book which is online at his website.You have given the reference to it... regards marlon
Astronomy
PF Gold
P: 23,235
Introduction To Loop Quantum Gravity

 Quote by marlon I will continue this matter and i would suggest that we follow the content of Rovelli's book which is online at his website.You have given the reference to it... regards marlon
I am very much looking forward to your continuing the essay, marlon!
I will restrain my tendency to talk too much, so as not to crowd.
BTW just yesterday in the mail was delivered the copy of Rovelli "Quantum Gravity" which I ordered from Amazon. I am very happy with the book and

I am only sad that it is so expensive----70 dollars. You have to be rich, or be willing to splurge. Or you have to be in graduate school and need it for a class, as textbook. In US the textbooks are all very expensive, so 70 dollars is fairly normal.

Anyway Rovelli is a good writer and Cambridge Press did a good job, with the editing and just the physical production----nice paper, nice binding, nice feel, and printing. So it is a pleasure to own: at least for me.

But to save money it certainly makes sense to print off the free draft copy at Rovelli site. Even just the first 3 or 4 chapters and some appendices---or whatever you find the most accessible parts and most relevant for you.

Marlon, why not give some online bibliography yourself? It would be a refreshing change (I am always doing the librarian work) and I would enjoy seeing your picks and how you organize it. (If you do not want to, I will not shirk the job, but maybe you would like to list intro-level links?)
P: 660
 Quote by marcus yesterday in the mail was delivered the copy of Rovelli "Quantum Gravity"
I sincerely hope you enjoy your new book, which I know you will. I was leafing through it at the U of T bookstore. I want to point out two things carlo says in the introductory bit.

1) That any correct quantum gravity theory must be able to calculate amplitudes for graviton-graviton scattering, and that he hopes that lqg will one day lead to a theory that can.

2) That he knows that GR must almost certainly be an effective field theory that is modified at higher energies so that lqg can't be correct. Thus he says he views lqg basically as a laboratory for investigating certain fundamental issues in quantum gravity.

As far as your sticky goes, would you be bothered if I corrected it?
Astronomy
PF Gold
P: 23,235
 Quote by jeff ... I want to point out two things carlo says in the introductory bit. 1) That any correct quantum gravity theory must be able to calculate amplitudes for graviton-graviton scattering, and that he hopes that lqg will one day lead to a theory that can. 2) That he knows that GR must almost certainly be an effective field theory that is modified at higher energies so that lqg can't be correct. Thus he says he views lqg basically as a laboratory for investigating certain fundamental issues in quantum gravity. ...
I believe you are mistaken, jeff. Carlo does not say these things in the introductory bit.
At least I looked in the first part of the book, and used the index to search the rest, and could not find any statements of the kind.

It would be nice to have some page references, if you have any more would-be paraphrases from Rovelli----even sweller of you to provide actual quotes. Since a paraphrase can often mislead as to what was said in the original.

Thanks for your kind wish as to the book! Indeed it is surprising me. I was not expecting this much, since I had read much of the last year's draft version.

BTW if you pick up a copy either at library or store and can give me some actual page reference (whether or not in the first 50-or-so pages, anywhere in the book will do) where he says these things 1. and 2. that you state, that would be most helpful of you and I will be very interested to read the actual passages and think about it. If he does say something like that my eye somehow missed it.
P: 660
 Quote by marcus I believe you are mistaken, jeff. Carlo does not say these things in the introductory bit.
We'll, I don't have the book on hand, but...

In rovelli's dec 30 2003 draft, he says on page ix entitled "PREFACE"

"What we need is not just a technique for computing, say, graviton-graviton scattering amplitudes (although we certainly want to be able to do so, eventually)"

On page 5 of the same draft,

"The einstein-hilbert action might very well be a low energy approximation of something else. But the modification of the notions of space and time has to do with the diffeomorphism invariance and the background independence of the action, not with it's specific form."

Be this as it may, jim bjorken in the forward of carlo's book states quite plainly that effective field theory has taught us that GR must be viewed as just an effective field theory, and it's difficult to believe that carlo would've allowed such a statement if it fundamentally contradicted his position.

Btw, did you notice that carlo writes (probably in the preface) that thiemann is publishing a book on the more mathematical aspects of lqg?
 Astronomy Sci Advisor PF Gold P: 23,235 Oh I see. I thought you were talking about the actual book. that you said you were browsing in the bookstore. but you apparently meant the draft, from 2003, which is available online. there's been considerable up-dating and revision. so one should be specific which ============= Meanwhile, maybe readers of this thread would be interested in the Loop and String lineup of talks at the conference that just finished in Mexico (at the Quintana Roo beach resort in sight of the island of Cozumel) A lot of the lectures were by top people both string and loop, and they were rather much introductory. The conference aimed at being a "school" to bring more people in. And to introduce stringies to loop research and viceversa. I thought the lineup of who the organizers wanted to talk about the various hot topics was enlightening. So since it could be instructive, I will copy it here: http://www.nuclecu.unam.mx/~gravit/E...I/courses.html --quote-- COURSES AND INVITED TALKS Courses: A. Ashtekar (PSU, USA): Quantum Geometry A. P. Balachandran (Syracuse, USA): Quantum Physics with Time-Space Noncommutativity P. T. Chrusciel (Tours, France): Selected Problems in Classical Gravity R. Kallosh (Stanford, USA): De Sitter Vacua in String Theory and the String Landscape A. Peet (Toronto, Canada): Black Holes in String Theory C. Rovelli (Marseille, France): Loop Quantum Gravity and Spinfoams Plenary talks: J. D. Barrow (Cambridge, UK): Cosmological Constants and Variations M. Bojowald (AEI, Germany): Loop Quantum Cosmology A. Corichi (ICN-UNAM, Mexico): Black Holes and Quantum Gravity A. Linde (Stanford, USA): Inflation and String Theory O. Obregon (U. Guanajuato, Mexico): Noncommutativity in Gravity, Topological Gravity and Cosmology A. Perez (PSU, USA): Selected Topics on Spin Foams L. Smolin (PITP, Canada): Loops and Strings R. Wald (U. Chicago, USA): Topics on Quantum Field Theory Short talks: E. Caceres (CINVESTAV, Mexico): Wrapped D-branes and confining gauge theories A. Guijosa (ICN-UNAM, Mexico): Far-from-Extremal Black Holes from Branes and Antibranes H. Morales (UAM, Mexico): Semiclassical Aspects and Phenomenology of Loop Quantum Gravity D. Sudarsky (ICN-UNAM, Mexico): Spacetime Granularity and Lorentz Invariance L. Urrutia (ICN-UNAM, Mexico): Synchrotron Radiation in Lorentz-Violating Effective Electrodynamics ---endquote---
 P: 2 marlon...got any extra info on LQG??? great introduction btw... lola
P: 660
 Quote by marcus Oh I see. I thought you were talking about the actual book. that you said you were browsing in the bookstore. but you apparently meant the draft, from 2003, which is available online. there's been considerable up-dating and revision. so one should be specific which
You want to play games? Fine with me.
 P: n/a This is a project I've been working on, and I'd very much like to know what the participants on this thread think. Thanks, nc Abstract and prospectus, Spacetime at the Planck Scale This is an abstract and prospectus for additional research. The proposal would use computational techniques such as those described in Stephen Wolfram's New Kind of Science as an exploratory probe of events at the Planck scale. Authors are currently recruiting mathematicians and physicists to mentor and contribute to the work. We still need someone who can design the NKS experiments. In this work in progress, we describe a mechanism by which four space-time dimensions are reduced to the classical view of three space-like dimensions arrayed in the customary orthagonal basis with one time-like dimension which can be thought of as permeating the space-like dimensions. The time-like dimension is shown to appear to be unique to a moving observer, and preserves the appearance of freedom of choice as one perspective in a structure which can also be viewed from other perspectives as competely deterministic. The Einstein-Minkowski principle of space time equivalence taken in the strongest sense creates a powerful model for investigation of the relationship between general relitivity and quantum mechanics. We begin by defining the Planck Sphere (here named to be consistant with the Planck length and Planck time) as a three dimensional volume filled by a radient event at the speed of light in one Planck time. Thus the radius of the Planck Sphere is equal to one Planck length and is equal to one Planck time, making a three dimensional model which can be used in a perspective sense to portray events which occur at the Planck scale in four dimensions. After describing the features of the model, we go on to propose that computational graphing techniques similar to those used by Stephan Wolfram in his book A New Kind Of Science be developed to explore the evolution of the Planck Sphere in Kepler dense packed space up to the scale of the fine structure constant, thereby showing the geometric origins of mass and charge. The first step in this process is to define a viable space-time lattice structure, which we believe we have done by defining the Planck Sphere as an element in a Kepler stack. The next step in this process is to develop a rational algorithem to simulate events on the Planck scale. This may be accomplished by applying what we know of cosmogeny and of physics near singularities. As a first approximation we advance the conjecture that expansion from the Planck scale will recapitulate cosmogeny. We carry through the first steps in this approximation to demonstrate a mechanism for early inflation in the burgeoning universe. References: [PDF] On quantum nature of black hole space-time: A Possible new source of intense radiation DV Ahluwalia - View as HTML - Cited by 11 ... spheres of fluctua- tions. The one that may be called a Schwarzschild sphere, and the other a Planck sphere. The sizes of these ... International Journal of Modern Physics D, 1999 - arxiv.org - ejournals.wspc.com.sg - arxiv.org - adsabs.harvard.edu [PDF] The Quantum structure of space-time at the Planck scale and quantum fields S Doplicher, K Fredenhagen, JE Roberts, CM Phys - View as HTML - Cited by 242... In the classical limit where the Planck length goes to zero, our Quantum spacetime ...components are homeomorphic to the tangent bundle TS 2 of the 2–sphere. ... Communications in Mathematical Physics, 1995 - arxiv.org - arxiv.org - adsabs.harvard.edu [PDF] Inflationary theory and alternative cosmology L Kofman, A Linde, V Mukhanov - Cited by 9 ... the large scale structure observed today were generated at an epoch when the energy density of the hot universe was 10 95 times greater than the Planck density ... The Journal of High Energy Physics (JHEP) - iop.org - arxiv.org - physik.tu-muenchen.de - adsabs.harvard.edu - all 7 versions » [PDF] Physics, Cosmology and the New Creationism VJ Stenger - View as HTML ... 10. -43 second time interval around t. = 0, if it was confined within a Planck sphere as big bang cosmology implies. The. universe ... colorado.edu 200411290100GTC Richard T. Harbaugh Program Director Society for the Investigation of Prescience
 P: 1 Hello Marlon i will thank you for the nice clear introduction on loop quantum gravity. I am planning to do my thesis on this subject and i would like to keep in touch with all the specialists here in order to get more info. I am just starting to know this field... bye...Luco
 P: 215 The challenge for string theorists and LQG theorists is to explain why the vacuum energy exists at 10^120 J/m^3 ( there is no reason to think there is anything wrong with the QM calculation) but does not curve space-time.How can quantum gravity be proved if gravity is not understood on its own yet?
Astronomy
PF Gold
P: 23,235
 Quote by Rothiemurchus ( there is no reason to think there is anything wrong with the QM calculation)
!

gotta be something wrong with it
Astronomy
PF Gold
P: 23,235
 Quote by Rothiemurchus explain why the vacuum energy exists at 10^120 J/m^3 ...
beg your pardon Rothie but that is a crazy amount of energy
maybe QFT can come up with a mechanism that cancels all or most of it out, or find some reason to say that it doesnt really exist----maybe QFT already has.

but that density of energy, not canceled out and real enough to cause gravity, is simply incredible (at least to me). commonsense persuades me that there must be something wrong with any theory that predicts it

And there is some reason to be hopeful, because QFT is still formulated in an unrealistic way: using a fixed spacetime framework. Reformulating it in a background independent version might possibly get rid of that huge vacuum energy.

BTW just to have a basis for comparision, the astronomers' dark energy estimate is currently around 0.6 joule per cubic km. In joules per cubic meter (the units you were using) that comes to:

0.6 x 10-9 joule per cubic meter.
 P: 215 I am aware of the cosmological evidence.But the problem is this: the energy that can be experimentally associated with the Casimir force is greater than the cosmological observation (10^-6 Newtons/m^2 net force at 10^-7 m plate separation - i think but i'm not sure,that this is at least 10-7 J/m^3).So, the plates involved in measurements of the Casimir force must somehow, switch on vacuum energy,locally.And what sort of effect would a galaxy have on the vacuum energy?
Astronomy
PF Gold
P: 23,235
 Quote by Rothiemurchus I am aware of the cosmological evidence.But the problem is this: the energy that can be experimentally associated with the Casimir force ...
Rothie, I will try to respond---tell me if I am making a mistake. I do not believe that the experimental existence of the Cas. force proves that the
QFT calculation of a huge vacuum energy is correct.
what I think is true is that there is some normal vacuum energy density and that between two conducting plates it is LESS namely

$$\text{energy density betw. plates = usual vacuum energy density} -\frac{\hbar c \pi^2}{720 d^4}$$

the QFT calculation of the usual vacuum energy density is bad or dubious, but the Casimir effect does not depend on this, it depends on the fact that the energy density between plates is LESS by the amount shown, which QFT does calculate successfully!, and which depends on the inverse fourth power of the separation distance.

So I say that I believe the QFT calculation of the Casimir effect and I like the Casimir effect, and this is consistent with not believing the huge vacuum energy which QFT calculates, which is roughly 120 OOM wrong---or actually different people try to fix it different ways and say different things, but anyway wrong.
Astronomy
 Quote by marcus $$\text{energy density betw. plates = usual vacuum energy density} -\frac{\hbar c \pi^2}{720 d^4}$$
$$\text{force divided by area} = -\frac{\hbar c \pi^2}{240 d^4}$$