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Coexistence of LQG and String Theory?

  1. Jul 16, 2012 #1
    Can Loop Quantum Gravity and String Theory coexist? What if String theory was correct in its description of matter and 3 of the 4 forces, with the only difference from modern M theory being that there is simply no real particle called the graviton, and instead, gravity is as is described by LQG: curvature of the space-time continuum, which is composed of spin networks?

    Probably a bit unlikely that BOTH of theories are partially correct, but could they theoretically be unified?
  2. jcsd
  3. Jul 16, 2012 #2
    This has already been discussed in PF, see this tread:

    If I can add just a small word: yes, you can combine them, there are of course interesting mathematical tool that you can use from string to LQG and from LQG to string. But in physics you should carefully consider the question that you want to answer. If you want to unify forces, there is string theory and LQG has few to say, maybe can be the quantum theory of gravity that you want to unify with the other quantum theories via string theory. If you want to quantize gravity, you do LQG, and for the moment string theory has not proven to have something to say about quantum gravity (if you do AdS/CFT, you can use a gravitational system to say something about a quantum theory, but people have not yet been able to do the viceversa).
    Also, string theory uses perturbation theory, while LQG is a non-pertubative theory of gravity, so they simply address different regimes. By the way, you can have the graviton in LQG, of course: it appears in the perturbative regime!
    If by string theory you have in mind just extra dimensions, of course there is such a mathematical construction also in LQG (but then, to do physics, I would like to describe our usual 4 dimensional spacetime).
    If by LQG you have in mind just the quantization based on Wilson loop, of course this is also present in QFT and therefore could maybe be implemented in string theory.
    Last edited: Jul 16, 2012
  4. Jul 16, 2012 #3
    Thanks for the reference! I'll check it out.

    I guess I want to quantize gravity. I don't particularly care if it is unified with the other three forces or not. It seems to me that it could be possible to quantize gravity using LQG, or something like it, but that String Theory could still be the correct description for all particles in the universe.

    Though I think, in that case, having 11 dimensions would be a little silly, as that was mainly motivated by the question of why gravity is so much weaker than the other forces; an obstacle to unification which is not needed in LQG. I guess to unify the two theories, String theory would have to be modified in this regard. But I wonder whether the unification of gravity into QFT via graviton follows NECESSARILY from the basic foundation of String Theory, or whether it was added in as a way to nicely unify the forces, and could just as easily be left out. My assumption is the latter, but I don't know enough about string theory (who does?? :tongue:)
  5. Jul 16, 2012 #4


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    But what is the evidence that LQG has gravity?

    In http://arxiv.org/abs/1109.0499, the authors conclude "The present work gives explicitly the critical congurations of the spinfoam amplitude and their geometrical interpretations. However we didn't answer the question such as whether or not the nondegenerate critical congurations are dominating the large-j asymptotic behavior, although we expect the Lorentzian nondegenerate congurations are dominating when the Barbero-Immirzi parameter is small.", so it seems that it is still unknown if LQG can produce the right semiclassical limit.

    Is there an update, or am I interpreting Han and Zhang wrongly?

    Also, isn't it still unknown if LQG is a reasonable quantum theory?
    Last edited: Jul 16, 2012
  6. Jul 16, 2012 #5
    Last edited: Jul 16, 2012
  7. Jul 16, 2012 #6


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    Thanks very much for the replies! It's great to have some professionals answer questions of laymen (I'm a biologist) like me!

    What I understand very poorly is - what are the criteria for saying that a spin foam theory is a quantum theory? Eg. satisfies some criterion that is analogous to say, unitarity? I don't know what that criterion is in spin foams. I thought maybe the criterion should be that it solves the hamiltonian constraint of the canonical formalism - but isn't the link between spin foams and the canonical formalism still murky?

    Also, it seems that Rovelli wants to do something like a "summing=refining" step to define full quantum gravity. In the Zakopane lectures, on p21, he indicates the C→∞ limit is full quantum gravity. So without that limit, spin foams are not a theory of quantum gravity. However, it seems the limit may not exist?
  8. Jul 16, 2012 #7
    That wouldn't work, string theory always comes with a graviton. It's one of it's defining features.
    No, that's not why string theory has extra dimensions. It requires extra dimensions to be mathematically consistent. Otherwise, it has negative norm states. In order to rid the theory of these negative norm states, you introduce terms called Virasoro operators. These require that Bosonic string theories have 26 spacetine dimensions. For superstring theories, it's only 10. M-theory needs one more.
  9. Jul 16, 2012 #8
    You welcome. It's a pleasure to know that there so good biologists around :biggrin:
    Do you have any advice for "bioforums" so that I can try the flux in the opposite direction?
    The EPRL vertex has been constructed so that it satisfies these 3 criteria (credits Eugenio Bianchi):
    1. locality (each term of the vertex expansion acts locally)
    2. Lorentz invariance (yes, the theory is locally Lorentz invariant)
    3. the Hamiltonian should be a density
    Be careful, unitary could mean different things... In LQG, the theory is unitary in the sense that it is built with unitary representations of the Lorentz group. But there are other sense, in which it is not unitary...
    There are indications, not a final prove. It's a tremendous task, because of the non-linearity of the theory. But if such a final prove would exist, this would be equivalent to solve the full theory! In fact, one should not prove it just for one vertex, but for al the vertex of the full amplitude... but people never work with withe the full amplitude, in LQG as in QED, see answer below.
    This limit correspond to consider all the order in the vertex amplitude. This limit exists in the same sense that it exists the limit for a QED with all imaginable Feynman diagrams... but who cares about this limit? If you compute QED at some order bigger than the 5th - I may be wrong to the 5th, it could be the 8th, but in every case: - the theory is already more accurate in its prediction than our possibilities to check it: the theory works. The same spirit should be assumed for quantum gravity. This does not mean that the limit should not be better understood, it has to, but I feel it is more a job for mathematicians than for physicists :wink:

    But we are a bit out of the topic of the tread, so I stop here :tongue:
  10. Jul 16, 2012 #9


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    This is dangerous what you are saying!

    We are talking about the Hamiltonian H; and we agree that one has to prove that H annihilates physical states. This is not just an eigenvalue equation like (H-E)|phys> = 0, it is a remnant of the fundamental symmetry, i.e. the local constraint algebra of the theory. This constraint algebra must hold in any circumstances.

    Compare it to QED or QCD: unitarity is guarantueed in all order of perturbation theory; it's not that unitarity violations converge to zero with αn; the same holds for gauge the symmetry. There are canonical formulations of QED and QCD which are completely gauge fixed and which are therefore gauge invariant; no approximation (of H) can ever spoil gauge invariance b/c it's formulated in physical d.o.f.; the difference is that gauge inv. in QED and QCD comes from Gauss law only, whereas in LQG the constraint algebra contains H as well.

    Now if you say that implementing H as a constraint on physical states approximately may indeed affect the consistency of the constraint algebra then this brings the whole LQG project in danger!
  11. Jul 16, 2012 #10
    Thanks, Mark. I've been wondering that.
  12. Jul 18, 2012 #11


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    I know that at a certain order in peturbative QED things start to go wrong and what were assumed to be small corrections turn out to be huge - apparently this happens early on in peturbative QCD. Plus I'd be careful of comapring other theories to background dependent field theories like QED and QCD which mathematically dont exist.
    Last edited: Jul 18, 2012
  13. Jul 19, 2012 #12
    Consider a simple quantum mechanical system, and try to describe it in the covariant formalism. The simplest thing could be a hydrogen atom, but this is a special case because it can be solved in a closed form. So, just to fix the ideas, think of an helium atom. In this case one would have a space of "non-physical" states given by the wave functions [itex]\psi(x,t)[/itex] that satisfy the analog of the Wheeler-deWit equation [itex]C \Psi=0[/itex], where [itex]C[/itex] is the constraint [itex]C=i \hbar\,d/dt - H(q_i, i \hbar\, d/dq_i)[/itex]. If we would "agree that one has to prove that H annihilates physical states", then we should also believe that the quantum mechanics of the helium atom can be formulated if and only if we can prove that such operator annihilates the physical states. But the physical states, thought in this way, are not known: nobody knows the exact solutions of the quantum dynamics of the interaction between the electrons and between the electrons and the nucleus. Therefore even usual quantum mechanics of an helium atom would be in danger if we follow this kind of logic. On the other hand, we can treat the helium atom by considering a perturbative expansion, calculating order by order the transitions amplitudes. Note that in this case the constraint express a fundamental symmetry of the system: this is an invariance under reparametrization of the covariant formulation of the dynamics of the helium atom.

    So, as Julian was stressing, it is important to understand the expansion we are working with. But I would not ask to my quantum gravity theory something that I would not even ask for a simple quantum mechanical system.
  14. Jul 19, 2012 #13


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    I don't think the comparison with helium is the same. If the full quantum gravity does not exist, then there is no diffeomorphism invariance, and canonical LQG does not exist. So it is not a matter of the theory existing, but we can only calculate approximately. I understand that physics has historically not proceeded rigourously, but that's because there have been observations and experiments to help theorists out. Too bad experimentalists are so incompetent nowadays - still stuck in 1964 :tongue:
  15. Jul 19, 2012 #14
    Physics is always a matter of calculating approximately, from the spherical cows to the helium atom to quantum gravity. Indipendently of what would be your favorite theory about human access to Nature, a theory does not exist if it satisfy some abstract laws that we have assumed as rules of the game (so quantum gravity works even if we have not solved the full theory, as tom.stoer was asking): rather, a theory exists if we can write down a coherent formulation that captures physical reality and we can compute. In this sense, what we have in quantum gravity follows what we often have in the physical quantum theories that we know. Maybe canonical quantum gravity does not exist, I do not know. But covariant quantum gravity exists and is defined by the amplitudes.
    I agree that historically the new step in physics has not be done starting from rigorous mathematics, otherwise we would not have Einstein's general relativity or Feynman's path integral. But I disagree that this happened because there were experiments, on the contrary. From Copernicus to Newton, to Einstein, we have had big steps in the understanding of Nature with no new experiments to support them, but solving a teoretical problem, like combining previous successful theories. In fact, search for rigor might be misleading. When Richard Feynman went to a general relativity conference, the 1957 Chapel Hill Conference on General Relativity, he famously said to the relativists (about quantum gravity!): "Don't be so rigorous or you will not succeed!" Feynman had an amazing grasp on how Nature works, so better keep in mind his advice :wink: But careful: the fact that one does an approximation, does not mean that "it is not rigorous" in the sense that there is no physical guidance of what we do, it is exactly the contrary.
    Last edited: Jul 19, 2012
  16. Jul 19, 2012 #15
    ps: experimentalists are not so bad! it's great time for people at Cern, it's nice to see all this excitation... I would not blame them if we don't have yet experiments in quantum gravity, actually it's a theoretician's job to propose some phenomenology to observe... Higgs was crying saying that he would not have believed to see such a day in his life, I would be happy even being less lucky than him :tongue:
  17. Jul 19, 2012 #16


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    this is not really my point.

    What I am saying is "that implementing H as a constraint on physical states approximately may indeed affect the consistency of the constraint algebra then this brings the whole LQG project in danger!"

    In QCD I have something like

    (H-E)|proton> = 0

    If H is completely gauge fixed this is a physical Hamiltonian H. What I do not have is the solution |proton>. Now I can make an approximation H'; this affects |proton>, so the new eigenstate |proton'> will be an approximation as well. But b/c H is gauge fixed I can make any approximation H' I like! The |proton'> will somewhow be wrong (or a bad approx.) but I cannot break gauge invariance.

    But if H in LQG cannot be fully 'gauge fixed' and if I now make an approx. H' this does not only affect the state (the solution) but the consistency condition H~0 which could introduce an inconsistency.

    What I do not fully understand is the problem with anomalies. If both Gauss and Diffeo-constraints are implemented H is the only constraint left. If H is invariant w.r.t. to Gauss and Diffeo I am in the same situation as described for QCD and I can't break any symmetry - besides H~0 itself (I can't introduce any anomaly b/c Gauss and Diffeo do no longer exist in physical variables).

    But if some gauge symmetry is still left I cannot expect to make an approx H' w/o introducing an anomaly.

    So what I don't understand is at which point an anomaly may enter LQG.

    Perhaps the main problem is the step-wise fixing of constraints.

    btw.: my feeling is that covariant LQG does exist but that it cannot be related to canonical LQG - and that this may be an even bigger problem b/c of the simplicity constraints not being taken into account appropriately; I am with Alexandrov that
    1) either you use Dirac brackets which modify the symplectic structure classically such that nobody is able to quantize the theory (constrint algebra with structure functions, ...)
    2) or that you use Poisson brackets + simplicity constraints w/o changing to Dirac brackets which is not allowed b/c the simplicity constraints are second-class so the resulting theory is not defined in a sound manner. I don't think that this problem has been fully addressed, neither in canonical LQG where H is not known, nor in spin foams where the measure cannot be fixed and where the amplitudes should be related to some well-defined H.
    Last edited: Jul 20, 2012
  18. Jul 19, 2012 #17


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    I personaly think that mathematical rigour is the guiding force in the absence of experimental results.

    But, as a tentative example of the force of physics+maths! - the infrared catastrophe of QED says that the renormalised charge will tend to zero regardless of what the bare charge is assumed to be. Landau et al concluded that QED is wrong!! But the actual resolution is Electro-Weak theory. You need a higgs type QFT to get out of this physical prediction. One of my favorate quotes of theoretic physics..."Veltman: I do not care what or how, but we must have is at least one renormalizable theory with massive charged vector bosons, and whether that looks like nature is of no concern, those are details that will be fixed later by some model freak...
    t'Hoof't: I can do that.
    Veltman: What did you say?
    't'Hooft:I can do that."

    Apparently some people have found the Higgs particle underneath the sofa.
  19. Jul 20, 2012 #18


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    what's your conclusion for LQG?
  20. Jul 20, 2012 #19
    I do not think that this is correct. There is nothing wrong in solving equations approximately. This is also true for the gauge. For example, in doing cosmological perturbation theory one has to solve constraints, of course, but does so only order by order in the expansion.
    QCD is different than gravity. In QCD, the hard part is the dynamics, which is a priori something separated from the constraints. In gravity, the dynamics is is intertwined with the gauge symmetries. It is a good idea to copy techniques from QCD, but it is a bad idea to oblige oneself to do exactly as in QCD.
    This is possible. If this is the case, we should follow the path that leads to a computable theory, and forget the other. It is always good to follow different paths, and see which one works.
    Here there is some confusion between canonical and covariant. It is not true that in the canonical theory H is not known. And it is not true that in the covariant theory the measure cannot be fixed. There exist a well defined Hamiltonian, and there exist a well defined measure, which is fixed by locality requirements (see for instance http://arxiv.org/abs/1005.0764). Of course maybe these theories can be improved, but for the moment they work, and no inconsistency has been found. Regarding "the amplitudes should be related to some well-defined H", I think I disagree with the "should": it would be great if somebody succeeded in doing this, but if one of the two versions of the theory is correct, it does not need the other to be interesting and possibly physically viable. Everybody is free to have fun quibbling about what would be good having in a perfect world (me too, possibly with some good whisky) but then as physicists we should come back to this world and find a theory that works...
  21. Jul 20, 2012 #20


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    No; approximations in gauge theory breaking gauge invariance are disastrous.

    I don't agree; all what I see is that the related problems (simplicity constraints, reality conditions, second class constraints and Dirac brackets, measure, ... H, vertex amplitude, ...) show up in both approaches; there are fundamental problems looking different depending on the perspective (covariant, canonical) but I think they are related.


    Anyway - you are the expert, so I stop insisting; I don't want to be impolite.

    ??? But that's what the experts say. There are candidate hamiltonians suffering from the usual quantization and regularization ambiguities w/o any hint for uniqueness, fundamental principle or something like that. The well-known Thiemann Hamiltonian does not "create volume"; it has been "cured" a couple of times, but I have never seen "the" Hamiltonian. Is there a reference with a construction?
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