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

Is quantum gravity viable without gravitons?

  1. Jan 10, 2010 #1
    The logic of spin-2 gravitons is that GR predicts gravitational waves, and in QM, all waves are also particles.

    Gravitational waves are seen as waves of spacetime whereas gravitons are mediated as particles on fixed spacetime. Spin-2 gravitons can reproduce GR using QFT in the weak field approximation.

    But perhaps nature does not allow the existence of fundamental particles greater than spin 1, and that there are no gravitons associated with gravitational waves b/c the properties of spacetime which makes gravitational waves possible, do not allow the wave-particle duality a corresponding graviton particle.

    Is it possible then to have GR and GR waves, but no particles, (only example of wave-particle duality not applying) b/c spacetime can carry waves, but is not itself particle, and the quantum spacetime is some sort of fundamental quantum structure. Gravity is not a force mediated by gauge bosons, as EM, QCD, weak, but curvature and geometry.
     
  2. jcsd
  3. Jan 10, 2010 #2

    turbo

    User Avatar
    Gold Member

    Why should there be a particle to mediate every force? Indeed, why should we be looking for a particle that magically conveys mass to other particles? (Higgs)

    There are many doctorates to be earned and many papers to pad such honoraria, but I fear that there is little real science in that realm. There are billions of dollars and many thousands of careers spent on such pursuits...the best I can say about this is at least they are not using my tax money to kill people.
     
    Last edited: Jan 10, 2010
  4. Jan 10, 2010 #3

    tom.stoer

    User Avatar
    Science Advisor

    Let me ask a question: what do you thing is a graviton?

    It is unclear to me if a quantization of GR must lead to a theory where gravitons play a fundamental role. Quanta of a field are not only based on physical reasoning, but also on certain mathematical concepts - perhaps a different quantization approach like LQG is required.

    If you look at lattice gauge theory you will not find gluons in the usual sense. There is a gluon field, but no object like a "particle" based on e.g. weak field limit, plane wave etc.

    Why do you think that nature does not allow for spin 2 particles? In SUGRA spin 3/2 and spin 2 are quite natural.
     
  5. Jan 10, 2010 #4
    SUGRA has not been shown to apply to nature, and spin 3/2, spin 2 and even spin 0 fundamental particles have not yet been shown to exist. Particles with spin larger than 2 are believed to not exist. Nature may not allow spin 2 particles for the simple reason that gravity is the geometry of spacetime, not a quantum field defined over a background. Alternatively, perhaps the geometric interpretation of gravity is simply wrong, and that gravity is a quantum spin-2 field defined over flat spacetime, this spacetime does not change, does not distort, is infinitely continuous.

    A graviton is spin 2, massless particle described by standard QFT that is defined over a background metric like Minkowski space-time and attraction is mediated by exchange of virtual gravitons, not spacetime curvature.

    Any approaches to quantizing gravity along lattice gauge theory ?
     
  6. Jan 10, 2010 #5

    tom.stoer

    User Avatar
    Science Advisor

    I don't think so because separating gravity from some background structure violates background independence / diffeomorphism invariance.

    I don't think that the gravitons you are descrinbing are viable concepts in QG - but that's my personal opinion and I know that other colleagues here see things different.

    Yes, Causal Dynamical Triangulation (CDT) is something like a "sum over discretized spacetime"; see homepage of Renate Loll http://www.phys.uu.nl/~loll/Web/title/title.html and the publications of the group
     
  7. Jan 10, 2010 #6
    Hello,
    is it possible to have a theory of gravity that reproduces GR's predictions but (a) has more easily quantizable gauge structure and (b) does NOT have BI nor diff. invar?

    How would you describe gravitons? How does string-M theory?
     
  8. Jan 10, 2010 #7

    tom.stoer

    User Avatar
    Science Advisor

    The theories equivalent to GR (or Einstein-Cartan theory) with gauge structure which I know are all diff.inv. Here are some discussions regarding Horava gravity which has a restricted diff.inv. only; but I don't think that this applies to your question. Unfortunately LQG does not reproduce GR so far as the semiclassical limit is purely understood; this is one central issue with LQG (the other are the ambiguities in the Hamiltonian).

    I would describe gravitons just as you did - but that's exactly why I think that they are not a useful tool for quantizing gravity. String theory describes gravitons as a certain vibration mode of a string; in the low-energy limit it reduces essentially to your description (but I have to admit that I am not an expert in string theory)
     
  9. Jan 10, 2010 #8
    I know that gravitons and spin-2 field in QFT reproduces GR in the weak field limit. This theory is non-BI, does it have diff inv?
     
  10. Jan 10, 2010 #9

    Physics Monkey

    User Avatar
    Science Advisor
    Homework Helper

    Hi ensabah6,

    I too am unsure whether the quantization of GR must lead to gravitons. I suspect that the answer is yes, and that the concept of a graviton is meaningful as long as one sticks to the appropriate regime (asymptotically Minkowski space, for example). I doubt it has universal validity i.e. for strongly nonlinear quantum gravity.

    We do know that a quantization of gravity can lead to gravitons. String theory certainly predicts gravitons around Minkowski space. The non-perturbative AdS/CFT duality makes it quite plausible that gravitons are consistent with quantum gravity. While AdS isn't directly experimentally relevant, quantum field theory does certainly make sense, so nature appears to be ok with gravitons in an indirect way at least.

    Overall, I suspect this question is quite delicate and may depend strongly on what exactly one means by a graviton.

    Hope this helps.
     
  11. Jan 10, 2010 #10
    If as in GR gravity is geometry, while the other 3 forces are mediated by gauge bosons, while it does not prove gravitons do not exist, it does suggest that gravity is a different sort of category than the other 3 forces. I am aware of the argument that if gravitational waves exist, and gravity is quantized, then these waves must be also particles, but I wonder if the mistake is made in applying the quantum properties of the other 3 forces, with the geometric aspect of gravity. If this is right, there is no TOE, no unification of gravity with the other 3 forces as in GUT, as gravity is something entirely different.

    Another way to put it, gravity can be described geometrically. Can the other 3 forces, EM, EW, Strong, be described geometrically, as distortions of spacetime, and if so, what is the geometric quanta of geometrized 3 forces?
     
  12. Jan 11, 2010 #11

    tom.stoer

    User Avatar
    Science Advisor

    The problem is that as far as I can see these theories are not yet well-defined. It does not help if gravitons exist in the weak field limit, but if the theory breaks down beyond zeroth order perturbation theory.
     
  13. Jan 11, 2010 #12

    tom.stoer

    User Avatar
    Science Advisor

    Your conclusion is right. If a program like string theory failes entirely then it means that gravity cannot be unified with the other forces. If string theory succeeds then both questions are answered simultanously: gravity is unified with the other forces and they are all somehow described geometrically - not by geometry of spacetime but by geometry of the string world sheet.

    But keep in mind that AdS/CFT suggests that gravity may be an emergent phenomenon where the gauge principle seems to be more fundamental than the geometrical principle of GR. In addition you should look at theories like teleparallel gravity (or Einstein-Cartan which should be the classical limit of LQG) which is a gauge theory of gravity with force equations - and which is from the perspective of physical predictions 100% equivalent to GR. It does not use Riemann but Weitzenboeck manifolds, but that's a mathematical subtlety. LQG (or Einstein-Cartan theory in Ashtekar formulation) is something like a gauge theory (geometrically) with additional structures.

    So gravity based on geometry is not in contradiction with the gauge principle; it seems to me that the gauge principle is more fundamental and can be used to describe gravity as well.
     
  14. Jan 11, 2010 #13
    GR can be derived as the equations of motion from the Hilbert-Einstein action. As far as quantizing GR, I suppose the question is what justifies the Hilbert-Einstein action in the path integral. Right now we insert the H-E action in the path integral simply because that's our quantizing procedure. But I don't believe anyone is going to be comfortable with that until we know where quantizing procedures comes from and why they exists to begin with. Maybe once we know that, the H-E action may emerge naturally from that.
     
  15. Jan 11, 2010 #14

    tom.stoer

    User Avatar
    Science Advisor

    I am not comfortable with that, either:
    1) I strongly believe that every PI has to be derived (or at least backed up) by canonical quantization (Feynman dervided his PI from the Hamiltonian).
    2) I have never seen a PI in QFT that is well-defined. Even in QCD you have problems with Gribov horizons, i.e. the PI is defined (not in a mathematical sense :-) only in a restricted domain, perturbatively etc. (the Fadeev-Popov trick failes non-perturbatively)
    3) I don't believe that that the Einstein-Hilbert action is the right starting point. There are other variables where (e.g.) the gauge and constraint structure becomes more obvious; and the E-H action as it stands is not able to deal with fermions.

    I doubt that any reasonable quantization procedure of GR will reproduce the E-H action in the PI.
     
  16. Jan 11, 2010 #15
    I think the question comes down "to what is a particle?".

    In flat space the notion of a one, two, three, ...n particle state is clear because the vacuum can be well defined. But when we take into account gravity(semi-classically) or even accelerated observers the number of particles is now a observer dependent quantity.

    Really a graviton is no different. If we use effective field theory, which is well accepted to give leading order corrections for quantum gravity, and expand around empty space we can again define a graviton as well as any other particle. So I'd say an effective graviton description works at least at low energies. When we go to energies of order the planck mass this description must break down.

    But in a full theory of quantum gravity it may well be that there are no particles at all (e.g. string theory). If we define a particle as some localized energy then in theories such as strings or loops that are inherently non-local it may be that we need some other description.
     
  17. Jan 11, 2010 #16

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Extremely important point. I added emphasis. The number of particles in a given situation is observer-dependent. Therefore the conventional concept of particle is not a valid way to look at the universe---*which is not flat.*

    For instance an accelerated observer will see more particles (Unruh).

    http://arxiv.org/abs/gr-qc/0409054
    What is a particle?
    Daniele Colosi, Carlo Rovelli
    Classical and Quantum Gravity 26:025002, 2009
    19 pages. Revised version, with a new title, of the 2004 paper "Global particles, local particles"
    (revised 5 Nov 2008)
    "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."
     
    Last edited: Jan 11, 2010
  18. Jan 11, 2010 #17
    Nice paper. Might read it when I have time. So maybe particles do exist in LQG. I guess the point is that if we have a detector and it detects some event locally in which some quantities of momentum, angular momentum etc are transferred to the detector then one can say that we have detected a particle.
     
  19. Jan 11, 2010 #18

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You already have the basic perception by your own accord. I wish other people would read that paper! But in case anyone is interested there is also a later online lecture---slides+audio.

    For a 2009 conference presentation of What is a Particle? go here
    http://gravity.psu.edu/events/abhayfest/
    click on "conference proceedings"
    and select from 5 June talks:

    SLIDES http://gravity.psu.edu/events/abhayfest/talks/Rovelli.pdf
    AUDIO http://gravity.psu.edu/events/abhayfest/proceedings.shtml

    That 5 June Abhayfest conference was a collection of serious bigwigs, not just the top quantum gravity people but also people from superstring/M and from classical GR and classical cosmology! For example:

    Gary Horowitz (string)
    Robert Wald
    James Hartle (of Hawking-Hartle)
    Klaus Fredenhagen
    Badri Krishnan
    Roger Penrose

    I think it is significant that Rovelli chose "What is a Particle?" from among all his recent research to present to this group.
    I think that with a non-mediocre select group like that, they already know that at this time in physics the fundamental concepts are being reconsidered. It is not just shush-and-calculate. Research which is philosophically naive is apt to be futile.
     
    Last edited: Jan 12, 2010
  20. Jan 11, 2010 #19

    tom.stoer

    User Avatar
    Science Advisor

    I agree with this physical description. But we should also agree that according to this definition a "particle" as it is used in QFT (= a certain fock state) is not a particle in this physical sense as it is not localized.

    So there is a difference if we talk about particles according to the above mentioned physical description, or if talk about a mathematical object which we call "particle" as well. A graviton in the usual sense (e.g. perturbative QG, SUGRA) is such a mathematical object only.
     
  21. Jan 11, 2010 #20
    And in in QFT the number of particles is a "fuzzy" quantity that is not precisely defined unless you make an observation at which point the standard weird quantum things happen.

    My impression of QFT is that concept of a "particle" is merely a way of doing perturbation theory, which may not be that useful once you get outside of the weak field limit.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook