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Quantum loop gravity question

  1. Dec 3, 2014 #1
    How much of the standard model do we have to change/forget in order for quantum loop gravity to work?
     
  2. jcsd
  3. Dec 3, 2014 #2

    haushofer

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    Nothing, I'd say. This topic would suit better in the Beyond The SM subforum,btw :)
     
  4. Dec 3, 2014 #3

    Demystifier

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    It may be useful to forget thinking in terms of Fock basis, i.e. states with a definite number of particles.
     
  5. Dec 3, 2014 #4
    Why is that exactly? There are some protocols in for example quantum key distribution that rely on definite number particle states, are you saying they won't work with quantum loop gravity or? Sounds strange.
     
  6. Dec 3, 2014 #5

    Demystifier

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    I'm not saying that Fock basis is wrong on LQG, only that it is not very useful. For instance, I think nobody is studying quantum key distribution in LQG, but it doesn't mean that it is impossible to study it.
     
  7. Dec 3, 2014 #6

    haushofer

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    You mean that the notion of "particle" changes on curved backgrounds in QG?
     
  8. Dec 3, 2014 #7

    Demystifier

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    No, I mean papers and books on LQG usually do not even talk about classical curved backgrounds.
     
  9. Dec 3, 2014 #8

    jtbell

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    Agreed and moved.
     
  10. Dec 3, 2014 #9

    marcus

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    You might be interested in this:
    http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf
    It is a draft of an intro text book which has quite a bit of intuitive non-technical discussion at the beginning of some of the early chapters.

    The draft is preliminary, from a year or so before the final edit and the published version. Someone teaching an introductory Loop Quantum Gravity course would want to use the complete final version, available on Amazon. But this free, online preliminary version of the book can be very useful!

    Not only is there overview, perspective, intuitive non-technical explanation but for people who want to learn the symbol manipulation side of LQG there are a lot of elementary exercises, some at the beginning of the book quite easy, and a lot of equations to work through, if that's what you want.

    There is quite a lot of discussion of the relation between General Relativity (classical and quantum versions) and Quantum Field Theory (the basis of the standard particle model)---which you were asking about. Especially the first 10 pages of Chapter 1 (online draft version) but more generally the whole first chapter is addressing that question.

    For completeness, I checked the Amazon page for the Kindle e-book edition. Apparently they are just now getting it on the market! They promise delivery by 31 December, which is still 4 weeks away.
    https://www.amazon.com/Covariant-Loop-Quantum-Gravity-Introduction-ebook/dp/B00N4PM81W/
     
    Last edited by a moderator: May 7, 2017
  11. Dec 4, 2014 #10

    julian

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    Last edited: Dec 4, 2014
  12. Dec 4, 2014 #11

    Demystifier

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    This is an excellent paper, but has nothing to do with LQG (despite of being written by Rovelli).
     
  13. Dec 4, 2014 #12

    julian

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    It's been a while since I went through it but I thought the motivation came from the use/fate of particles in curved spacetime and the more drastic case of background-independent quantum gravity such as LQG. I agree that the paper isn't directly addressing the notion of particles within the context of LQG. I wonder what work has been done on this following Rovelli's idea? I know that there are others trying to address the general problem of Quantum fields versus particles and his isn't the only line of investigation.

    Anyway my point in referring to the paper was to eludicate to problems with using the notion of particles in contexts outside normal Fock descriptions based on QFT on Minkowski spacetime.
     
    Last edited: Dec 4, 2014
  14. Dec 4, 2014 #13

    julian

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    They say

    "These difficulties become serious in a background-independent quantum context (see for instance [6]). For instance, in loop quantum gravity [6, 7] quantum states of the gravitational field are described in terms of a spin network basis. Can we talk about gravitons, or other particle states, in loop quantum gravity [8]? A common view among relativists is that we cannot, unless we consider the asymptotically flat context. But there should well be a way of describing what a finite-size detector detects, even in a local background-independent theory! Indeed, a recent line of development in loop quantum gravity aims at computing transition amplitudes between particle states [9], using only finite spacetime regions, using a formalism developed in [10] and in [6]. What are those particle states?"
     
  15. Dec 4, 2014 #14

    marcus

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    I agree with Demystifier about it's being an excellent paper, it clears up important issues, is widely cited in subsequent research, and it kind of MOTIVATES the development of the boundary formalism in LQG which was achieved over the next few years. You need some grip on the background geometry of a bounded region to say what a particle is, and in LQG you can get that by having the amplitudes depend on boundary data. I want to highlight part of what you quoted from their paper:
    I should check to see what references [9] and [10] are to be sure about what they are saying here.

    I think it is relevant to what the thread-starter was asking, so I'll first copy the abstract here:
    What is a particle?
    Daniele Colosi, Carlo Rovelli
    (Submitted on 14 Sep 2004 (v1), last revised 5 Nov 2008 (this version, v2))
    Theoretical developments related to the gravitational interaction have questioned the notion of particle in quantum field theory (QFT). For instance, uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity. These difficulties have lead some to suggest that in general QFT should not be interpreted in terms of particle states, but rather in terms of eigenstates of local operators. Still, it is not obvious how to reconcile this view with the empirically-observed ubiquitous particle-like behavior of quantum fields, apparent for instance in experimental high-energy physics, or "particle"-physics. Here we offer an element of clarification by observing that already in flat space there exist --strictly speaking-- two distinct notions of particles: globally defined n-particle Fock-states and *local particle states*. The last describe the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are appropriately large, global and local particle states converge in a weak topology (but not in norm). This observation has little relevance for flat-space theories --it amounts to a reminder that there are boundary effects in realistic detectors--; but is relevant for gravity. It reconciles the two points of view mentioned above. More importantly, it provides a definition of local particle state that remains well-defined even when the conventional global particle states are not defined. This definition plays an important role in quantum gravity.
    19 pages.

    References [10] and [9] are about the general boundary formalism, and about its application to Lqg to handle particles:

    [9] L Modesto, C Rovelli, “Particle scattering in loop quantum gravity”, Phys. Rev. Lett. 95, 191301 (2005). C Rovelli: “Graviton propagator from background-independent quantum gravity”, Phys Rev Lett 97 (2006) 151301. E Bianchi, L Modesto, C Rovelli, S Speziale: “Graviton propagator in loop quantum gravity”, Classical and Quantum Gravity 23 (2006) 6989-7028. E. Alesci and C. Rovelli, “The complete LQG propagator: I. Difficulties with the Barrett-Crane vertex,” Phys. Rev. D76, 104012 (2007). E Alesci, C Rovelli, “The Complete LQG propagator. II. Asymptotic behavior of the vertex”, Phys. Rev. D77, 044024 (2008). C Perini, C Rovelli, S Speziale, “Self-energy and vertex radiative corrections in LQG,” arXiv:0810.1714.

    [10] R Oeckl, “A ‘general boundary’ formulation for quantum mechanics and quantum gravity, Phys. Lett. B 575 (2003) 318–324. R Oeckl, “General boundary quantum field theory: Foundations and probability interpretation”, Adv. Theo. Math. Phys. to appear, hep-th/0509122. R Oeckl, “Probabilites in the general boundary formulation”, J. Phys.: Conf. Ser., 67 (2007) 012049. D Colosi, R Oeckl, “Spatially asymptotic S-matrix from general boundary formulation”, Phys. Rev., D 78 (2008) 025020. D Colosi, R Oeckl, “S-matrix at spatial infinity”, Phys. Lett. B 665 (2008) 310–313.
     
    Last edited: Dec 4, 2014
  16. Dec 5, 2014 #15

    tom.stoer

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    I recommend to forget perturbative quantization.
     
  17. Dec 5, 2014 #16

    Demystifier

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    23 citation is actually not that impressive. I think it deserves much more citations. In particular, I think this paper is better than some of my own papers which have more than 60 citations. :D
     
  18. Dec 5, 2014 #17

    Demystifier

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    Of gravity, or in general? oo)
     
  19. Dec 9, 2014 #18

    marcus

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    Dreed, I guess the short answer is there's less and less to "change/forget" as time goes on.

    As time goes on and more work is done on the theory it is shown to be able to include more of the rest of physics. This paper that just came out today is a good example.
    LambdaCDM is the standard cosmic model that cosmologists use essentially for all their work. It incorporates a positive cosmological constant (Λ), cold dark matter (CDM) together with ordinary matter and radiation.
    On the other hand during much of its early development Loop Quantum Cosmology was a simplified model with simplified matter contents that were basically place-holders for more realistic stuff to be put in later. Sometimes Λ was not even included.
    Now two postdocs (Cai at McGill U, Montreal and Wilson-Ewing at LSU) had taken a major step of working out the LQC model with more realistic contents. They include the Lambda curvature constant and Dark Matter as well as energy in the familiar form of electromagnetic radiation.
    http://arxiv.org/abs/1412.2914
    A ΛCDM bounce scenario
    Yi-Fu Cai, Edward Wilson-Ewing
    (Submitted on 9 Dec 2014)
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
    14 pages, 8 figures
     
    Last edited: Dec 9, 2014
  20. Dec 9, 2014 #19

    marcus

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