What is Loop quantum gravity: Definition and 107 Discussions

Loop quantum gravity (LQG) is a theory of quantum gravity, which aims to merge quantum mechanics and general relativity, incorporating matter of the Standard Model into the framework established for the pure quantum gravity case. As a candidate for quantum gravity, LQG competes with string theory.Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometric formulation rather than the treatment of gravity as a force. To do this, in LQG theory space and time are quantized analogously to the way quantities like energy and momentum are quantized in quantum mechanics. The theory gives a physical picture of spacetime where space and time are granular and discrete directly because of quantization just like photons in the quantum theory of electromagnetism and the discrete energy levels of atoms. An implication of a quantized space is that a minimum distance exists.
LQG postulates that the structure of space is composed of finite loops woven into an extremely fine fabric or network. These networks of loops are called spin networks. The evolution of a spin network, or spin foam, has a scale on the order of a Planck length, approximately 10−35 metres, and smaller scales are meaningless. Consequently, not just matter, but space itself, prefers an atomic structure.
The areas of research, which involves about 30 research groups worldwide, share the basic physical assumptions and the mathematical description of quantum space. Research has evolved in two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, called spin foam theory.
The most well-developed theory that has been advanced as a direct result of loop quantum gravity is called loop quantum cosmology (LQC). LQC advances the study of the early universe, incorporating the concept of the Big Bang into the broader theory of the Big Bounce, which envisions the Big Bang as the beginning of a period of expansion that follows a period of contraction, which one could talk of as the Big Crunch.

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  1. P

    Perturbative approach in Loop Quantum Gravity

    I'm a novice in Loop Quantum Gravity, so please bear with me if this is a trivial question. As far as I know, LQG is usually formulated non-perturbatively, but is there also a perturbative approach in the sense that we use a series expansion in order to calculate vacuum expectation values? Are...
  2. N

    What exactly is loop quantum gravity?

    I don't quite understand the properties of loop quantum gravity. I have searched around and have not come up with anything very helpful. I'm pretty good when it comes to understanding quantum mechanics and string theory, but please, NO MATH! I'm only in high school! thanks
  3. S

    Where Can I Study for a PhD in Loop Quantum Gravity?

    Hi everyone, I will soon finish my B.Sc. in mathematics & physics, and I'm currently reviewing options for PhD. One of the subjects that interest me the most is LQG, but most places I've checked (MIT, Caltech, Harvard etc.) don't seem to acknowledge its existence... Please, recommend places...
  4. Physics Monkey

    Meaning of low energy in loop quantum gravity

    Meaning of "low energy" in loop quantum gravity In the recent Immirzi parameter thread we accumulated some evidence that to resolve and/or clarify the issues raised there, it would be necessary to have an understanding of the flow to low energy or the flow from micro to macro in loopy models of...
  5. D

    Physics Career studying loop quantum gravity

    What are the chances that a guy can get a graduate position studying LQG, and a job after that?
  6. marcus

    Twistors to Twisted Geometries: Exploring Loop Quantum Gravity

    http://arxiv.org/abs/1006.0199 From twistors to twisted geometries Laurent Freidel, Simone Speziale 9 pages (Submitted on 1 Jun 2010) "In a previous paper we showed that the phase space of loop quantum gravity on a fixed graph can be parametrized in terms of twisted geometries, quantities...
  7. marcus

    A new look at loop quantum gravity

    http://arxiv.org/abs/1004.1780 A new look at loop quantum gravity Carlo Rovelli 15 pages, 5 figures (Submitted on 11 Apr 2010) "I describe a possible perspective on the current state of loop quantum gravity, at the light of the developments of the last years. I point out that a theory is...
  8. Y

    Exploring Experimental Evidence of Loop Quantum Gravity

    Hello everyone, does anybody of you know, whether there are some publications/blog entries/ideas/anything on possibilities of experimental verification/falsification of Loop Quantum Gravity at collider experiments? I know, there are predictions that can be tested with astrophysical...
  9. J

    History of Loop Quantum Gravity

    Since there are several people here who intently follow the developements of LQG, I was wondering if someone could give some overview to help understand the 'historical path' of research on this topic better. What methods were developed, which died off, what was learned, where they are going...
  10. E

    Michio Kaku on loop quantum gravity

    Kaku is famous for promoting string/M-theory in a variety of books from hyperspace to physics of impossible, to name a few. He also popularizes string theory in radio and sci-fi documentaries. He was asked what he thought of loop quantum gravity: Antimatter How much closer are we at this...
  11. marcus

    Loop Quantum Gravity: Zurück vor den Urknall" by Martin Bojowald

    Has anyone seen this? Can you say how well it fits into the popularization of 4D Quantum Gravity picture? Whether it presents a clear popular account of Loop Cosmology? I just checked German Amazon and found this: Zurück vor den Urknall: Die ganze Geschichte des Universums von Martin...
  12. Spinnor

    Does loop quantum gravity hint at the Standard model?

    Have loop quantum theorists looked forward to how one might fit the standard model into loop quantum gravity? Thank you for any help.
  13. J

    Loop Quantum Gravity for newbies

    Hi there, I am currently doing a project on LQG and I am having troubles grasping the basics. I am just wondering if anyone here could help answer a couple of questions that I have during my study. 1.) How to use Roger Penrose's original spin-network? I read in his book that they are used to...
  14. G

    Level of Learning Theories: String Theory & Loop Quantum Gravity

    At what level (graduate, PhD, etc...) do people start learning theories, such as String Theory and Loop Quantum Gravity?
  15. G

    Trying to understand Loop Quantum Gravity in layman's terms

    Ok, I've been trying to follow what you guys have written about this. Is it sort of like protein folding in biology, only instead of proteins you have space time, and you spin it around, and most of it is quantum foam vacuum stuff, but in some places it clumps in sort of oscillating ways...
  16. F

    LQG Theory: Is Universe Closed or Infinite?

    In the LQG theory does the universe have to be closed, or can space be infinite in size? What I mean is, is the LQG theory similar to the quasi steady state theory in which the universe does collapse, and then bounces back, but is still infinite in size.
  17. marcus

    Covariant loop quantum gravity and its low-energy limit (throwing the eagle)

    Rovelli has given his team and himself just seven months to advance LQG to a new stage. Why do I call this "throwing the eagle"? Because a Roman general would on occasion hurl his legion's eagle standard into the opposing army's midst, confident his side could rout their foes the recover it...
  18. atyy

    Philosophy of Loop Quantum Gravity

    I came across an interesting discussion about LQG's philosophy (through Googling "Cauchy surface" and "loop quantum gravity"): Approaching the Planck Scale From a Relativistic Point of View: A Philosophical Appraisal of Loop Quantum Gravity Christian Wüthrich...
  19. A

    Three Roads to Quantum Gravity: String Theory, Loop Quantum Gravity & ?

    I have just read the book Three Roads to Quantum Grabity... but my mind has gone blank, the three methods are string theory (M-Theory), loop quantum gravity and one other. I can't remember the last :(:( Any help would be great.,..
  20. P

    A quick question about Loop Quantum Gravity

    There's an article about Loop Quantum Gravity in this month's Scientific American, and I have a quick question. They assume space itself is made out of "atoms" with definite size (Planck's length if I'm not mistaken). So how exactly does it work with the expansion of the Universe? Do these...
  21. R

    Loop Quantum Gravity: Is the Speed of Light a Function of Wavelength?

    Re the article by Bojowald in the October issue of Scientific American about Loop Quantum Gravity. He states that one of the consequences of the theory is that the speed of light is a function of wavelength. Any experimental evidence, or even hint of evidence, that anyone knows of?
  22. M

    Predictions from loop quantum gravity

    It is always said, that string theory makes no testable predictions. At least there are some qualitative predictions like supersymmetry and extra dimensions. One prediction of loop quantum gravity is a violation of Lorentz invariance. Are there other predictions of loop quantum gravity? I...
  23. S

    Loop Quantum Gravity Resources

    I have made a resource page for loop quantum gravity . I have made a list of books, papers and online tutorials. Check it out. It should be helpful http://mushfiq.net/loop-quantum-gravity/ ....... :wink:MUSHFIQ
  24. G

    Can loop quantum gravity see before the Bang?

    The May 20 issue of the journal Physical Review Letters will reveal the results of a National Science Foundation study by Penn State Eberly College of Science grad student, Victo Raveras and Madhavan Varadarajan, a professor at the Raman Research Institute in India that challenge the idea that...
  25. U

    Loop Quantum Gravity Explained: Free Online Lectures & Resources

    I understand that it is a quantum take on space-time. I just don't actually understand IT. Can anyone help? Any good online lectures or explanations? Thanks.
  26. B

    World-line lengths in loop quantum gravity?

    Hello everyone, I'm currently occupying myself with Loop Quantum Gravity and wonder about some question. In LQG, space is built up from a spin network. However, how is movement of material particles realized in this network? One could tend to the idea, particles are simply hopping from one...
  27. J

    Looking for a Course in Loop Quantum Gravity

    I am reading Barton Zwiebach's 'A First Course in String Theory'. I expect I will finish up in about a month. Not that I understand it that well. I will put it aside and perhaps read it again in a couple of years. I took a peek at the book 'String Theory in a Nutshell' by Elias Kiritsis and...
  28. jal

    Topological order - string-net condensation + loop quantum gravity

    I've just read Quantum field theory of many-body systems Xiao-Gang Wen His web page http://dao.mit.edu/~wen/ I thought that his book might be easier than his papers. hehehe It's a textbook. I did get to learn a few things. Here is what wiki says about the subject. In physics...
  29. marcus

    Entanglement entropy in Loop Quantum Gravity

    I downloaded William Donnelly's talk, given at Loops'07 Entanglement Entropy in Loop Quantum Gravity http://www.matmor.unam.mx/eventos/loops07/talks/7B/Donnelly.pdf maybe he will discuss it with us I just had a look
  30. S

    Loop Quantum Gravity and String Theory

    I'm having a little bit of trouble distinguishing between the two. Basically LQG states that the quantum structure of spacetime is composed of spin-networks which are composed of strings just with their ends tied, I guess you could say. LQG is also background independent. It says that these...
  31. A

    Loop Quantum Gravity is a noncommutative geometry?

    I was reading somewhere (I think wikipedia on quantum geometry?) that Loop Quantum Gravity is "noncommutative"...but I'm trying to figure this out on my own (naturally, there are no citations for this claim!). Now, my reasoning is that one would try to express various differential forms in...
  32. M

    The role of time in Loop Quantum gravity

    As far as I understand, loop quantum gravity treats space and time on very different footings. What I get is that in LQG time acts like a CPU clock in a computer representing a counter for the subsequent changes in the descrete structure of space. What worries me about this picture is, when...
  33. R

    Loop Quantum Gravity: Space Infinitely Divisible, Matter Not

    In loop quantum gravity it is assumed that space is not infinitely divisible. The nodes are not space . Actually they are the infinitely divisible part of a matter. The distance between the nodes are not discrete. The distance between the nodes may be 1.5 Planck's length or 2.5 Planck's length...
  34. jal

    Loop quantum gravity and Planck-size black hole entropy

    Loop quantum gravity and Planck-size black hole entropy http://arxiv.org/abs/gr-qc/0703116 Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja Unfortunately ... this non-expert ... did not see the light. I did do some research into what is expected to happen at CERN. I'll just...
  35. T

    Prerequisite mathematics for string theory and loop quantum gravity

    Here is a list of the mathematics one needs to know for string theory (i'm skipping the simple 1st and 2nd year math courses). http://superstringtheory.com/math/index.html Real analysis In real analysis, students learn abstract properties of real functions as mappings, isomorphism, fixed...
  36. Demystifier

    Covariant Loop Quantum Gravity: Experts' Views

    I find the covariant version of loop quantum gravity http://arxiv.org/abs/gr-qc?papernum=0608135 more appealing than the usual LQG approach. What the experts think?
  37. kakarukeys

    Exploring the Implications of Loop Quantum Gravity for S^4

    It seems to me that Loop Quantum Gravity has already ruled out the possibility that the spacetime could be a manifold like S^4, by making the assumption that the tangent bundle is trivial, or M = R x Sigma. TS^4 is non-trivial implies we cannot find a basis of tetrad fields that span the module...
  38. S

    Second-Order Graviton Propagator in Rovelli-Smolin's Loop Quantum Gravity

    I have finished a quick first reading of the rovelli at all paper ingraviton propagator http://arxiv.org/gr-qc/0604044 I couldn´t dive too much into the details because spin foams was a prerequisite and i only could do an equally fast reading of the alejandro perez review of the subjecto...
  39. S

    The Carlo Rovelli book on loop quantum gravity

    I had studied LQG using the thieman articles (mostly the brief of around 80 pages) and some other brief reviews (the one on living reviews on gnernal relativity for example) and also i had readed some articles announced in the threads which maintains Marcus. But i wanted a most serious...
  40. marcus

    Wang: Towards Conformal Loop Quantum Gravity

    https://www.physicsforums.com/showpost.php?p=847769&postcount=426 what is wrong with Wang's approach? or even simpler, what IS Wang's approach, what is the essential, what makes it different? how can one even consider using conformal diffeomorphisms instead of ordinary? Is this what he...
  41. S

    Getting into Loop Quantum Gravity: What You Need to Know

    Could somebody here tell me how somebody gets into Loop Quantum Gravity. I've seen that the CASM and Glasgow and a lot of other universities have String Theory courses, but I've never seen a LQG module. Is it simply a matter of applying to some group after your Ph.D.? Just curious...
  42. wolram

    Exploring Matter in Loop Quantum Gravity

    I think in LQG matter has only been tentatively considered, but if matter is reduced to a," field", or wave function, could one transform into the other?
  43. H

    Exploring the Meaning of Length in Loop Quantum Gravity

    Short question: is the concept of length meaningful in LQG?
  44. M

    Quantized Space & Lorentz Contraction in Loop Quantum Gravity

    Does anyone know how in LQG they avoid the contradiction of a smallest unit of quantized space and Lorentz contraction?
  45. marlon

    Introduction To Loop Quantum Gravity

    INTRODUCTION TO LOOP QUANTUM GRAVITY, everything you ever wanted to know... In Loop Quantum Gravity, also referred to as LQG, the attempt is made to introduce the concept of quantum gravity. This is the unification of the General Theory of Relativity and the Quantummechanics. It is a very...
  46. G

    Good Book for Loop Quantum Gravity

    i have the elegant universe and am about half way through. after this, i would like to read about string theory's rival, loop quantum gravity. does anyone know of a good intro book to loop quantum gravity?
  47. marlon

    Introducing Loop Quantum Gravity: Differences with String Theory

    hello, can anyone give me some introductory notions of what loop quantum gravity (i hope it is in the right order) and what are the differences with string theory. I know my QFT and i have some introductory notions on strings, so i don't mind a deeply elaborated physical answer. Feel free...
  48. marcus

    Separable Hilbert Space in Loop Quantum Gravity

    Yesterday Meteor found this new paper of Rovelli's and added it to the "surrogate sticky" collection of links. In case there is need for discussion, it should probably have its own thread as well. The paper expands a key assertion made on page 173 of the on-line draft of Rovelli's book...
  49. Nibles

    Loop Quantum Gravity: Explained for Physics Laymen

    I read about the Loop Quantum Gravity theory in some science magazine during my break at work. I wasn't reading attentively enough to make anything out of it. All I know is it is another theory to unite General Relativity and Quantum Mechanics, I think. Anyone care to share their knowledge on...
  50. M

    Lack of Books on Loop Quantum Gravity (LQG)

    I was wondering, why are there no books on LQG? For string/M- theory you have "The Elegant Universe", "Hyperspace", "Beyond Einstein", and numerous others. But, for LQG, the closest I may have found (and I'm not even sure about this, since I just got the book, and haven't really started...
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