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Quantum gravity vs. general relativity

  1. May 26, 2009 #1
    Let's suppose gravitons exist, and you have a machine that is 100% effective at detecting them. If you were in a room with no windows, and there is an apparent gravitational field, then would using this machine let you tell if you were in a gravitational field or accelerating?
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  3. May 27, 2009 #2


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    I don't know, because I don't know what a graviton really is.

    Counterquestion: Let's suppose photons exist, and you have a machine that is 100% effective at detecting them. If you were in a room with no windows, and there is an apparent electromagnetic field, then would using this machine let you tell if you were in a electromagnetic field or accelerating?

    This has been answered by the so-called Unruh-effect which states that an accelerating observer measures thermal photons (and other particles).
  4. May 27, 2009 #3


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    Steady acceleration would correspond to a constant gravitational field and
    that means virtual gravitons, not detectable as particles.


    As Tom.S says, same thing with photons. If you just have a static electric field in the room, say the ceiling is more negative and the floor more positive (to get something approximately analogous to gravity) then no matter how good your detector is it will not see photons.

    You see photons when there is an electromagnetic wave---a change in the field.
  5. May 27, 2009 #4
    So what you're saying is that gravitons only exist for gravitational waves not in a static gravitational field, right? That doesn't sound like a complete description of gravity, does it?
  6. May 27, 2009 #5


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    In my post I distinguished between gravitons and virtual gravitons, friend.

    The original poster, kashiark, imagined having a perfect graviton detector. My claim is that such a detector, no matter how perfect, would not pick up virtual gravitons. No more does a photon detector register the virtual photons which mediate a static electric field. This was by way of responding to kashiark's question.

    Could you detect the putative virtual gravitons mediating a static gravitational field? I believe even in a thought experiment you could not.

    The issue of giving a complete description of gravity is a different topic, I think.

    I believe that the graviton is a useful mental construct which appears in the mathematics when the background space is nearly flat, or has some other suitable fixed geometry. I don't think one would expect to use gravitons to analyze a highly dynamic, high curvature situation. Like the collapse of a star to form a black hole. I would not advise anyone to consider gravitons as providing a fundamental description of gravity's reality.

    That's just my personal viewpoint. I think of them as small ripples in a nearly flat geometry. The quanta associated with models of grav waves. Limited applicability but very useful for special purposes and valid within the appropriate context.

    No reason to suppose that individual gravitons will ever be detected, as Freeman Dyson has famously pointed out. But that's a completely separate issue.

    A fundamental description of gravity (according to a common view which I share) has to be a fundamental description of spacetime geometry. Useful as they might be in certain situations, the graviton does not offer a complete description of geometry. So, as you suggest, it cannot provide a complete description of gravity.


    But this wasn't what kashiark was asking about. He was talking about using a "graviton detector" to distinguish between grav and acceleration. (so they wouldn't really be equivalent!) Now he has been answered by various people. Let's see what he says, and what else, if anything, he wants to talk about :biggrin:

    BTW did you read what Janus said in that thread I linked to?
  7. May 27, 2009 #6
    ok so let's say it doesnt detect virtual gravitons then you could tell if you were accelerating or in a "real" gravitational field disproving the little adage of general relativity "no local experiment can differentiate between acceleration and a gravitational field"
  8. May 27, 2009 #7


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    I think you misunderstood the previous posts. Gravitions making up a static gravitational field would be virtual, so no, it wouldn't detect the difference. Real, non-virtual, gravitons would be present if you have a gravitational wave.
  9. May 27, 2009 #8
    ah yes you're right i knew i must have been missing something thanks :D
  10. May 27, 2009 #9


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    marcus, this is a dangerous concept. It seems to me that you are mixing the idea of gravitons as small ripples on spacetime with the detector's ability to detect these small ripples but not static fields. If your detector is not able to detect static fields that does not necessarily mean that static fields are not "gravitons" in some sense!

    Let's start with QED: QED in an Hamiltonian, gauge-fixed formalism contains a differential operator D. Inverting this operator leads to the Coulomb potential between charges and currents; that means the interaction term in the Hamiltonian density contains something like h(x) = j(x) V(x,y) j(y) which is formally V = 1/D; 1/D is a nonlocal object.

    Now let's continue with QCD. The formalism is much more involved, but the central step is the same. You have a differential operator D; inverting this operator gives you the "static" chromo-electric field in h(x). In QCD other interaction terms will appear, of course. Why QCD? Because due to its non-abelian nature D = D[A], that means D[A] depends on the gauge fields!!! That's why it is tricky to calculate 1/D[A], but formally this operator exists. This shows that the chromo-electric field consists of gluons, a fact that is hidden in QED due to its abelian nature.

    So what I am saying is that even static fields are to be derived from the dynamic gauge fields.

    Of course your restriction to small ripples leads to gauge fields as "plane waves plus interaction terms" and therefore this difficulty disappears. Nevertheless it does not explain what gravitons are. The perturbative concept of gravitons fails because of non-renormalizibility of GR. Non-perturbative concepts like LQG (you know this much better than me :-) are hardly able to tell you what gravitons are. So what are gravitons?
  11. May 27, 2009 #10
    Just a quick question regarding gravitons.

    If they do in fact exist, exactly how big would one be? It seems that a gravitational field is a steady state wave, unlike the point-to-point nature of something like a photon.

    Wouldn't the wavelength of the sun's gravitons be something like twice the size of the solar system?
  12. May 27, 2009 #11


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    As the sun's gravitational field is static, there are no waves at all.

    Regarding frequencies and wavelength I found the following data (w/o calculation):
    - two merging stellar black holes: ~1mHz
    - two circulating stellar black holes: ~10mHz
    - core collaps SN = SN II: ~10kHz

    You should check the GEO600, LISA and LIGO websites
  13. May 28, 2009 #12
    Are they not measurable because they are not moving? How could you measure particles if they don't move through your detector machine, right? We know that gravitons from graviational waves are moving with the wave. But static gravitons are not moving and so are not measurable and thus considered virtual, right?

    Like virtual electrons and positrons, they pop into existence and then almost immediately they disappear in the same place. So how would you detect them, they don't exist long enough to move through any machine that would register them. That's why we can not directly measure the zero point energy, right?

    But positrons and electrons annihilate each other as they pop into and then out of existence. Would virtual gravitons also pop into and then out of existence? What would they annihilate with to pop back out of existence?
  14. May 28, 2009 #13
    That's what I was trying to say. The Sun's gravity isn't propagating outward; it's just there. The curvature of space describes a waveform, but it's not going anywhere.

    Why should we expect to find gravitons at all? It seems like we would have a better chance at detecting "Magnetons" from refrigerator magnets.
  15. May 28, 2009 #14


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    You still have problems with the concept of gravitons.

    If you think of gravitons in terms of waves and fields, then the sun's "gravitons" are static and do not propagate; I doubt that many peoplewould call this "graviton".
    If you think of gravitons in terms of particles described by propagation and annihilation operators (second quantized gravity), then they are always propagating with the speed of light; unfortunately the theory becomes inconsistent!

    So everybody should specify what is meant by gravitons.

    Regarding the detector: it is made of atoms; the sun is made of atoms, too; two atoms are interchanging a graviton; this is a "virtual graviton" according to standard concepts of QFT; so the detector will detect virtual gravitons ONLY. What I am saying is that by construction asymptotic states are the real states, whereas exchanged particles are virtual (they are off-shell). As soon as a particle is absorbed and detected, it is no longer a real particle but a virtual particle.

    => I think this distinction is fine for certain mathematical concepts but missleading for the description of many physical processes, especially if static fields are involved.

    If you want to compare gravitons with other particles I would not use electrons. Photons and gluons are much more similar to gravitons, except for the fact that gravitons lead to inconsistent quantum theories.
  16. May 28, 2009 #15
    I do. I recognize that there is a need for something to fill in the gap of understanding gravity, but gravitons seem like a placeholder at best.

    Photons; they do something. Object A radiates energy outward, and when it hits object B, that quantized energy is registered as a photon. That's an idiot level simplification, but it is possible to talk about what photons actually do and how they behave in plain english.

    Gravitons sound like magic. What is the commonsense explanation for how object A emits a graviton towards object B, and when it gets there it says, "Hey, come over here." Especially when gravitational fields are for the most part stationary.

    Does anyone have a reasonable explanation of "how" gravitons mediate the gravitational force, or are they simply self-defined?
  17. May 28, 2009 #16


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    why is this more magical than a photon going from a positive charge to a positive charge and saying "hey, move away" or going from a positive charge to a negative charge ans saying "hey! come over here" ?
  18. May 28, 2009 #17
    Well, photons are affected by gravity, as demonstrated by gravitational lensing and black holes. Do photons exchange gravitons with whatever gravitational source they find themselves affected by? That sounds unlikely.

    Do black holes emit gravitons? If so, how? Light cannot escape. What is the mechanism for graviton emission from a black hole?
  19. May 28, 2009 #18


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    nrqed is right. The main problem with gravitons is not that they are more mystical than photons. The two main problems are simply:
    - they have not been detected (no gravitational wave has been detected directly, so far)
    - if used for quantization they lead an inconsistent theory

    If those two issues were absent gravitons and photons would be quite similar, except for the fact that a raviton is spin-2 and therefore the force is always attractive.

    You can compare "plane wave photons" with "plane wave gravitons" and you can compare "static photons", e.g. the Coulomb potential with "static gravitons". More or less the same idea.
  20. May 28, 2009 #19


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    Why does it sound unlikely?
    According to GR, any stress energy momentum tensor couples with gravity, so we would simply have the corresponding tensor of any theory (whether it is QED or anything else) coupled to gravitons.

    They don't emit gravitons anymore than an electron at rest emits photons (even though there is a static electric field). We have to wiggle the electron to get photons produced. We have to wiggle a black hole to get gravitational waves emitted (and not any wiggling will do)
  21. May 28, 2009 #20
    Okay, so how do gravitons reconcile with black holes again? How does the matter inside the event horizon of a black hole exert a gravitational influence on matter outside the event horizon of the black hole if gravitons are the mediating particle? How would such a particle escape? <--- The recursive nature of this question makes my brain hurt.
  22. May 28, 2009 #21
    I understand this, and it's exactly why I don't see how gravitons usefully explain anything about the nature of gravity. Sure, if you move a gravitational field relative to something else you get a waveform that can be quantized into a particle at some level.

    That still doesn't explain how gravity actually works. Isn't that the point of speculating the existence of these particles? Are we trying to figure out how the engine works, or just what color the paint happens to be?
  23. May 28, 2009 #22


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    I am not sure what you have in mind by "explaining the nature of gravity".
    In your language, do photon "explain" the nature of electromagnetism?

    What do you mean by "explaining how a force works"? What is your criterion?

    Gravitons are just a byproduct of quantizing the gravitational force using quantum field theory.

    Gravitons can be used to calculate quantum corrections to classical GR, that's all.
  24. May 28, 2009 #23


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    I agree. Gravitons do not really explain how gravity works. As we just figured out, gravitons are something like a concept taken from a perturbative treatment of conventional quantum field theory.

    This concept has failed. Nobody is able to recover full non-perturbative physics from perturbative concepts only, neither in gravity nor in QCD. In gravity it's even worse because the introduction of gravitons (perturbation theory) breaks background independence; that means the geometry is no longer able to evolve dynamically; only small ripples are able to propagate.

    If you would have a full understanding of QG, you might be able to go to a certain limit of that theory where gravitons turn out to be a useful approximation. The full theory may be LQG or string theory; neither starts with gravitons as fundamental objects.
  25. May 28, 2009 #24


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    In line with what you said, in LQG spinfoam papers where they derive and study the graviton propagator they must first restrict to the case where they have an approximately flat background. This is difficult because the theory is essentially backgroundless, so it has taken some ingenuity and effort to force restriction to a flat background case---and so get gravitons. Gravitons are somewhat artificial in the theory. Or they emerge only in the appropriate circumstances. I think this agrees with what you said.

    I will get some links to a sampling relevant papers. Rovelli co-authored of most, or if not he then Simone Speziale. I should be able to find a few just by listing articles by those authors since 2005. Here are a few:

    Graviton propagator in loop quantum gravity
    Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale
    (Submitted on 10 Apr 2006)
    We compute some components of the graviton propagator in loop quantum gravity, using the spinfoam formalism, up to some second order terms in the expansion parameter.
    Comments: 41 pages, 6 figures

    Group Integral Techniques for the Spinfoam Graviton Propagator
    Etera R. Livine, Simone Speziale
    (Submitted on 30 Aug 2006)
    We consider the proposal of gr-qc/0508124 for the extraction of the graviton propagator from the spinfoam formalism. We propose a new ansatz for the boundary state, using which we can write the propagator as an integral over SU(2). The perturbative expansion in the Planck length can be recast into the saddle point expansion of this integral. We compute the leading order and recover the behavior expected from low-energy physics. In particular, we prove that the degenerate spinfoam configurations are suppressed.
    Comments: 16 pages

    Graviton propagator from background-independent quantum gravity
    Carlo Rovelli
    Last edited: May 28, 2009
  26. May 28, 2009 #25


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    Gravity is certainly the most complex theory, especially because of diffeomorphism invariance the the missing PT expansion. Even in well-understood theoris like QCD the plane waves are a very poor approximation or essentially useless in studying non-perturbative effects like confinement and chiral symmetry breaking.

    The main benefit of plane wave states in conventional QFT was that calculation of scattering amplitudes was "simple" (ever tried to calculate a two-loop amplitude in QCD?). This benefit is lost in gravity as the result of the calculation is meaningless. The reason to study this approximation is lost, too, simply because of the missing graviton colider experiments.
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