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. K

    I Ads/CFT and Ashtekar variables or spinfoam

    there are theoretical problems with loop quantum gravity based on directly quantizing Ashtekar variables, for example, "One can pinpoint the technical error in LQG explicitly:To recall, the starting point of LQG is to encode the Riemannian metric in terms of the parallel transport of the...
  2. H

    A Spin networks with different intertwiners

    Hi Pfs Spin networks are defined by the way their links and their nodes are equipped with SU(2) representations and intertwiners. Could you give an example of two different spin networks with the same number of nodes, links between them, the same coloring of the links (and their orientations)...
  3. TheHeraclitus

    I Carlo Rovelli, loop quantum gravity and point particles

    I am reading a popular-science book Reality Is Not What It Seems by Carlo Rovelli, one of the founders of loop quantum gravity. He writes: and and So basically, space (spacetime) is just another quantum field like all the others, and the quanta of this field is the nod. Nods have volume...
  4. H

    A Loop quantum gravity and General relativity

    Hi PFs, I am reading this paper written by carlo Rovelli: https://arxiv.org/abs/1010.1939 there are many things that i fail to understand, but i would like to begin with a simple thing. Rovelli write that: It is locally Lorentz invariant at each vertex, in the sense that the vertex amplitude...
  5. M

    I Is quantum gravity still an immensely popular field?

    I was just wondering how much work is being done in the field of quantum gravity nowdays. Is there still a huge volume of research published on the topic? Are we closer to a "solution" nowdays than we were a few years ago? And also, what exactly would constitute a solution to such problem?
  6. A

    A Loop Quantum gravity or String theory?

    I understand that string theory has almost no testable predictions, however loop quantum gravity is an enticing candidate for only quantum gravity and it doesn't explain much of symmetry, constants, mixing angles etc in Standard model. There is obviously not enough evidence to create a full...
  7. AfonsoDeAlbuquerque

    A Triangulating Hamiltonian Constraint in LQG

    Im trying to obtain regularized (and triangulated) version of Hamiltonian constraint in the LQG. However, one step remains unclear to me. I am starting with the Euclidean Hamiltonian:$$H_E=\frac{2}{\kappa} \int_\Sigma d^3 x N(x)\epsilon^{abc} \text{Tr}(F_{ab},\{A_c,V\}) $$ Now i have to...
  8. Lutz-F

    B Theory of Loop Quantum Gravity

    From the german Version of Carlo Rovellis book "La realtà non è come ci appare. La struttura elementare delle cose" I have learned about the theory of Loop Quantum Gravity that space and time arise through the interactions of gravitational quanta, the space quanta have discrete volume spectra...
  9. H

    A Triangulation and Discrete Volume Spectrum in Covariant LQG

    What is often said in Covariant LQG is that the triangulation is a truncation, and is not what is responsible for the discrete volumes one ends up with in the theory. Rather, what is responsible is the discrete spectra of the volume operator acting on the nodes of a spin network. My confusion...
  10. ohwilleke

    I Trouble for Loop Quantum Gravity?

    A new preprint establishes that almost all models implementing loop quantum gravity violate the principle of general covariance, and can only satisfy a more limited condition. Is this the huge problem for loop quantum gravity theory that it appears to be? Or is the less strict condition that can...
  11. kroni

    A Simulating Quantum Loop Theory: Can S-Knots Be Represented Numerically?

    Hello, I am contacting you because I would like to know if there is a way to simulate quantum loop theory. Indeed, the S-Knots are much more complex objects than graphs because between the points there is a curve that can be knotted. S-Knots are graph embeddings in 3D and I do not see how such...
  12. K

    I Asymptotic safety and loop quantum gravity

    this paper https://arxiv.org/abs/1701.02311 Hypercuboidal renormalization in spin foam quantum gravity Benjamin Bahr, Sebastian Steinhaus (Submitted on 9 Jan 2017) In this article we apply background-independent renormalization group methods to spin foam quantum gravity. It is aimed at...
  13. N

    A Loop Quantum Gravity proven wrong?

    https://arxiv.org/abs/1411.1935 This paper says LQG does not produce the Unruh effect effect, and therefore must be wrong. What do you guys think. You can click into the PDF to see the entire paper.
  14. K

    I Tensor networks, spin networks and loop quantum gravity

    Loop Quantum Gravity, Exact Holographic Mapping, and Holographic Entanglement Entropy Muxin Han, Ling-Yan Hung (Submitted on 7 Oct 2016) The relation between Loop Quantum Gravity (LQG) and tensor network is explored from the perspectives of bulk-boundary duality and holographic entanglement...
  15. A

    I Spin Foam models in Loop Quantum Gravity

    Hi all, I fairly basic question about spin foam models in loop quantum gravity. I just want to verify that spin foams represent effectively represent all of spacetime (in a quantum form), and that the idea is that general relativity can be obtained in the classical limit? Not sure if that's...
  16. J

    I Loop Quantum Gravity and Virtual Particles....

    Hi all, Following up on another post - for a layman, can someone describe the status of virtual particles in Loop Quantum Gravity models? Since LQG avoid UV divergences, and has a different structure from field-based theories, are virtual particles still talked about? (in the context of closed...
  17. Ken Gallock

    B What kind of experiment is needed to quantize gravity?

    Hi everyone. I heard that there are several theories or hypotheses to quantize gravity such as superstring theory, loop quantum gravity, etc., and I believe none of them are conclusive (that's what I heard). So, here is my question. 1) Is it because there are no experimental results which prove...
  18. A. Neumaier

    A Loop quantum gravity and diffeomorphism group

    How is loop quantum gravity related to the principle of relativity, in particular to the diffeomorphism invariance of general relativity?
  19. B

    I Exploring the Popularity of Loop Quantum Gravity in Current Physics Research

    is Loop Quantum Gravity as popular as String Theory in current physics research?
  20. K

    Does Loop quantum gravity have an effective UV fixed point?

    loop quantum gravity and loop quantum cosmology gravity becomes weaker then repulsive instead of stronger, towards the Planck regime, due to the onset of quantum effects. gravity near Planck energies and densities, LQG/LQC becomes repulsive. at the inflection point where LQG gravity strength...
  21. K

    Towards self dual Loop Quantum Gravity Jibril Ben Achour

    interesting new paper on possible new research direction for LQG http://arxiv.org/abs/1511.07332 Towards self dual Loop Quantum Gravity Jibril Ben Achour (Submitted on 23 Nov 2015) In this PhD thesis, we introduced a new strategy to investigate the kinematical and physical predictions of self...
  22. K

    Where can I study (research) Loop Quantum Gravity?

    Thank you for opening this Thread I am kind of new here and this is my first time to post so I apologize if I did something wrong So my question is where can I study/ research Loop Quantum Gravity I have checked fair amount of grad school websites and compared to Super String, I have not yet...
  23. P

    What is the new Hamiltonian constraint operator for loop quantum gravity?

    A critique of LQG here: "The next step in the construction of LQG is to decide what the dynamics are. Technically, this is done either (A) by choosing a "Hamiltonian constraint" in parallel with the Hamiltonian formulation of GR, or (B) in the spin-foam formalism, by postulating some sort of...
  24. ohwilleke

    LQG has not been shown to violate the holographic principle

    Backreaction has the story: http://backreaction.blogspot.com/2015/09/no-loop-quantum-gravity-has-not-been.html debunking this article: Violation of the Holographic Principle in the Loop Quantum Gravity Ozan Sargın, Mir Faizal http://arxiv.org/abs/1509.00843 and also showing the authors behaving...
  25. Q

    Is Loop Quantum Gravity a Machian theory?

    Hi All! I have heard that loop quantum gravity is a "background independent" theory, in that there is no preferred coordinate system. Is LQG also a Machian theory?
  26. Georges Simenon

    Linear simplicity constraint in Loop Quantum Gravity

    Homework Statement Hi, I am studying covariant LQG from the recent book by Rovelli & Vidotto, and i am struggling with the linear simplicity constraint. My problems are not with its proof, that i understand, but rather with the physical interpretation in terms of boost generators. I will try to...
  27. L

    What is the right theory of quantum gravity?

    This question, at the moment, is ridiculously difficult or even impossible to answer. Some prefer string theory, and some think that it is overly speculative and LQG may be a better fit, and some say the opposite. I would like to ask; In your opinion, what could be the right theory of quantum...
  28. K

    Conformal gravity vs loop quantum gravity

    if conformal gravity is a successful research program, what would its implications be for loop quantum gravity? can conformal gravity be loop quantized? can loop quantum gravity be made to respect conformal invariance? or are the two mutually exclusive?
  29. ohwilleke

    Experimental Limits on Spin-Foam Effects From Quasars

    Both string theorists and loop quantum gravity theories have proposed that space-time may be something other than the perfectly smooth, perfectly local space of classical general relativity, which can potentially lead to path dependent phase alteration of light from a common source due to the...
  30. nuclearhead

    In what way is M-Theory different from Loop Quantum Gravity?

    I have been reading about M-Theory and LQG, and from what I read they are actually very similar. It seems that just a a string-world sheet is the same as a sum of an infinite number of Feynman diagrams, a membrane would be the sum of an infinite number of graphs or networks. Or as Susskind...
  31. K

    Is there a holographic principle in loop quantum gravity?

    for example, is loop quantum gravity/SF in 4 dimensions dual to a CFT in 3 dimensions? does a serious proposal of QG have to respect the holographic principle?
  32. R

    Loop Quantum Gravity and the Concept of the Universe as a Co

    Disclaimer: I'm just an amateur enthusiast who is a programmer during the day I suppose everyone who takes an interest in physics who doesn't have the mathematical tools to further their enquiries end up writing posts like mine, and that at the very least it's nice to see where the ideas reside...
  33. W

    Schools Looking for grad schools in string theory/quantum gravity

    I'm beginning to apply to grad schools (primarily in observational or computational cosmology and extragalactic astronomy) but have a deep interest in M-theory and quantum gravity although I haven't been able to study it. I very much enjoy theory and think that I would find studying M-theory or...
  34. 2

    Tests of loop quantum gravity?

    I am doing a project of theories of gravity, and am having some trouble understanding whether loop quantum gravity is testable or not. I had some questions which I cannot seem to find the answers to: 1. Is it theoretically possible to probe distances of 10^{-35} or would the concentration of...
  35. W

    String Theory Vs Loop Quantum Gravity

    This is a question currently pondered by many Quantum physicists and theoretical physicists. I was just wondering how many of you prefer ST(string theory) as opposed to LQG(loop quantum gravity.) Please provide points and reasons regarding your answer. I will provide an opinion when a few...
  36. _Daniel_

    PhD on Loop Quantum Gravity (at UK): What's the best option?

    Hello guys! I hope that you could help me. I want to do my PhD on LQG at the UK and I have the following options: Newcastle, with David Toms. (His research area is QFT on curved spacetime but he liked my project on LQG) Aberdeen, with Charles Wang Nottingham, with Kirill Krasnov or John...
  37. Q

    Introducing Loop Quantum Gravity: Where to Start?

    If you search major book retailers on the internet, you can find lots of new and very good books on string theory, for both novice and more advanced students. However, for loop quantum gravity, you can't really find anything. Is there an introductory book that takes you from QFT to LQG ? Where...
  38. Digitalism

    Loop Quantum Gravity and entropy

    How specifically does LQG explain how the universe can oscillate from big bang to big crunch ad infinitum? Wouldn't the total energy able to be used as work decrease after a couple of bounces? Am I simply misunderstanding or making a false assumption about what LQG's premises are? I've not...
  39. L

    Where to get started in Loop Quantum Gravity?

    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...
  40. haushofer

    Loop Quantum Gravity and Correspondence Principle

    Hi, I'm a non-expert on LQG, and have a question about it: What is the status of the correspondence principle applied to LQG? I.e., in which sense can we take the classical (h --> 0) or non-relativistic (c --> oo) limit in order to obtain classical GR or Newtonian gravity out of LQG...
  41. W

    Gravitons in Loop Quantum Gravity

    How do you understand about Gravitons in Loop Quantum Gravity? All I know about it is from what I heard that "LQG hopes that its predictions for experiments occurring far below the Planck scale will be almost identical to that of gravitons on flat spacetime". So do you consider it a pure...
  42. K

    What is the recent development of Loop Quantum Gravity

    Recently, I am very interested in Loop Quantum Gravity. But I hope I can know more about the recent development of Loop Quantum Gravity. I mean the development from 2000 to 2011. Any conceptual or practical or technical development in this realm? Further more, I do not know the relationship...
  43. tom.stoer

    Prospects of spin foam formalism in loop quantum gravity [Alexandrov, Roche]

    I would like to continue discussing SF (i.e. PI) models of LQG based on chapter 3 from http://arxiv.org/abs/1009.4475 Critical Overview of Loops and Foams Authors: Sergei Alexandrov, Philippe Roche (Submitted on 22 Sep 2010) Abstract: This is a review of the present status of loop and...
  44. tom.stoer

    Prospects of the canonical formalism in loop quantum gravity [Alexandrov, Roche]

    I would like to continue discussing canonical LQG based on chapter 2 from http://arxiv.org/abs/1009.4475 Critical Overview of Loops and Foams Authors: Sergei Alexandrov, Philippe Roche (Submitted on 22 Sep 2010) Abstract: This is a review of the present status of loop and spin foam...
  45. tom.stoer

    Prospects of the canonical formalism in loop quantum gravity

    There are still unsettled questions in loop quantum gravity, especially regarding uniqueness of the Hamiltonian constraint, constraint algebra, on-shell vs. off-shell closure, operator norm and convergence, ultra-locality, possibly quantization anomalies. These questions have been asked in...
  46. tom.stoer

    Thiemann on the relation between canonical and covariant loop quantum gravity

    http://arxiv.org/abs/1109.1290 [B]Linking covariant and canonical LQG: new solutions to the Euclidean Scalar Constraint[/B Authors: Emanuele Alesci, Thomas Thiemann, Antonia Zipfel (Submitted on 6 Sep 2011) Abstract: It is often emphasized that spin-foam models could realize a projection on...
  47. C

    The end of String Theory and Loop Quantum Gravity ?

    If spacetime is composed of tiny quantum "grains," the gamma-ray photons' polarization should change from random polarization (at the GRB source) to biased toward a certain polarization when received by the Integral spacecraft . The Integral polarization results depend on spacetime being...