Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is Loop Quntuam gravity plausible formulation of quantum gravity

  1. Apr 16, 2015 #1
    what about loop quantum gravity? plausible formulation of quantum gravity in some universe. not necessarily ours.
     
  2. jcsd
  3. Apr 16, 2015 #2

    atyy

    User Avatar
    Science Advisor

    No, LQG is in a much weaker position regarding the emergence of general relativity, and a non-perturbative formulation.
     
  4. Apr 16, 2015 #3

    wabbit

    User Avatar
    Gold Member

    This is as far as I know a debatable position. LQG is a plausible approach just as much as ST and neither is a fully developped theory nor has made a successful prediction yet. There are other approaches too and the proof is in the pudding.
     
  5. Apr 16, 2015 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    A number of researchers do see it as a plausible formulation of QG, in this universe.
     
  6. Apr 16, 2015 #5
    i didn't mention spinfoam but now i will. lqg is non-perturbative formulation
     
  7. Apr 16, 2015 #6

    atyy

    User Avatar
    Science Advisor

    Can you show a single paper where LQG is non-perturbatively formulated and gives general relativity?
     
  8. Apr 16, 2015 #7
    currently what is the status of "the emergence of general relativity" from LQG?
     
  9. Apr 16, 2015 #8

    atyy

    User Avatar
    Science Advisor

    LQG does not even have an agreed upon non-perturabtive formulation, and there is evidence against GR emerging from EPRL which is the most "current" proposal for LQG.

    http://arxiv.org/abs/1111.2865
    http://arxiv.org/abs/1502.04640
     
  10. Apr 16, 2015 #9
    If Marcus can produce a paper(s) that (1) has a non-perturabtive formulation, and (2) shows GR emerging, would this satisfy the criteria you outline for string theory, that it is a theory of gravity in some universes not necessarily ours?
     
  11. Apr 16, 2015 #10

    atyy

    User Avatar
    Science Advisor

    Yes. But I'm pretty sure he cannot :) The reason I'm pretty sure is that the bulk of recent work has been on EPRL, and there certainly are papers indicating that it is a non-perturbative formulation, and that GR emerges from it. However, the two papers by Engle linked above indicate that other positive indications notwithstanding, there are problems with EPRL regarding the emergence of GR.

    I believe the non-perturbative definition of EPRL is also problematic, because the existence of the continuum limit is unknown. For example, http://arxiv.org/abs/1205.2019 states "In the general case, the definition of the refinement limit has recently been studied and formalized in [327]. There is no evidence that the amplitudes of any of the present models for 4D converge in such a limit."
     
  12. Apr 16, 2015 #11

    martinbn

    User Avatar
    Science Advisor

    What's weaker than none? Or do you mean to say that string theory has these to features?

    Can you do that for string theory?
     
  13. Apr 16, 2015 #12

    atyy

    User Avatar
    Science Advisor

    Yes: AdS/CFT.
     
  14. Apr 16, 2015 #13

    wabbit

    User Avatar
    Gold Member

    Please. Neither theory is complete and neither has a single successful prediction to its credit. Can we wait till they do intead of speculating on which one might or might not eventually do so or comparing their looks?
     
  15. Apr 16, 2015 #14

    martinbn

    User Avatar
    Science Advisor

    Can you clarify? In what sense is the AdS/CFT conjecture a formulation of string theory that has GR as a classical limit? Can you show the papers?
     
  16. Apr 16, 2015 #15

    atyy

    User Avatar
    Science Advisor

    You have to remember my original point was made in the context of an argument for stopping all research on string theory, because it was diverting focus from other good research on quantum gravity. It is undeniable that string theory has the best foothold at the moment, and not LQG. I don't think putting down string theory is a good way to advance the interests of LQG research, which I do support.

    Here is the thread in which I originally made my point: https://www.physicsforums.com/threads/evidence-against-string-theory-paper.808802/#post-5077563.
     
  17. Apr 16, 2015 #16

    atyy

    User Avatar
    Science Advisor

    Lots of them, just google.
     
  18. Apr 16, 2015 #17

    martinbn

    User Avatar
    Science Advisor

    In that case I have no more comments!
     
  19. Apr 16, 2015 #18
    does that work in 4 AdS dimensions and 3 dimensions CFT?
     
  20. Apr 16, 2015 #19

    atyy

    User Avatar
    Science Advisor

  21. Apr 16, 2015 #20
  22. Apr 16, 2015 #21

    atyy

    User Avatar
    Science Advisor

    There's some work on 4 also. But the main problem is that AdS itself is probably unphysical.
     
  23. Apr 16, 2015 #22
    what about LQG and the Kodama wave function? might be unphysical, but it is a theory of QG in some universes.
     
  24. Apr 16, 2015 #23

    atyy

    User Avatar
    Science Advisor

    Hmm, I don't know what the status of that is in LQG. Off the top of my head, I think it was proposed for canonical LQG. But canonical LQG in those days was not a theory, and the spin foam approach was meant to help complete canonical LQG. The most promising proposals of the last few years have been the FK and EPRL spin foam formulations. I don't know the status of FK, but as the papers above show EPRL seems to have problems.
     
  25. Apr 16, 2015 #24
    what about CDT, GFT, NCG, twistor theory ?
     
  26. Apr 16, 2015 #25

    atyy

    User Avatar
    Science Advisor

    I think they are not even close to LQG. Actually, GFT is closely related to LQG, but it has the potential to develop independently also.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook