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

Rationale behind gravitons

  1. Jul 15, 2009 #1
    Can someone explain the rationale behind gravitons to me?

    My background is computational physics, and as such i may be biased towards physics that is actually computable, such as LQG and regge calc. I have some clue what this is all about, but i have some questions:

    Is there any reason (beyond aestetics which i disagree with anyway) to favor a particle over a geometric explanation? Any sort of empirical matter gravitons may help explain?

    How is something like gravitational lensing explained in a flat spacetime with gravitons? Are there force-carrier-to-force-carrier interactions in such a model? I have a hard time imagining how youd explain bending of light with gravitons. It seems likea pressing question to me, but noone else seems to care, as far as i can tell.

    If your response is anything like the reasons listed on the wikipedia page on gravitons, then please try a different angle. Perhaps there is something to these arguments, but the way they are stated, they seem like garbage to me.
  2. jcsd
  3. Jul 15, 2009 #2

    You'll find in any area of physics place for computations.

    The only reason to favor a particle explanation is the success of QED and QCD for describing Electromagnetic and weak and strong interactions, and is not a bad reason at all, don't you think so? Also, the aesthetical guidings are lead by knowledge and predjuices, so also a geometrical point of view for all forces could be at least as aesthetical as a particle/field explanation for gravity and the other interactions as well (is nice to see for example some discussion of Rovelli about aesthetics in http://arxiv.org/abs/gr-qc/0604045).

    The existence of gravitons were speculated very likely a little before than formalizing QED, as was growing QFT. That was just to associate a particle to the spin-2 wave that comes from general relativity in the linearized limit, so as can be associated the photon to the waves in Maxwell theory. The plane waves that results in linearized gravity are analogue of the plane waves in electromagnetism, and actually the same equations for linearized gravity can be obtained from the Fierz-Pauli action. As happens in electromagnetism that real phenomena is not described by plane waves but a superposition of infinite plane waves for instance, and the calculations can be made with perturbative expansions, one could take also more and more nonlinear terms in a expansion of the equations when dividing the metric in flat plus a perturbation. In this way the particle way was (and is) hopping that the bending of light, and in general the curvature of space-time observed will be an effect of the interactions of gravitons among them and with matter; continuing analogies it could be checked the scattering of light when passing throguh electric and magnetic fields, a point that reminds me also another recent post about a paper of Smolin.
    Last edited: Jul 15, 2009
  4. Jul 15, 2009 #3


    User Avatar
    Science Advisor

    Personally I don't like gravitons very much:
    - nobody has ever seen or measured a single graviton
    - any attempt to formulate a physically realistic theory based on gravitons failed (due to infinities etc.)
    - string theory contains something like gravitons - but the prize to pay is very high (extra dimensions and all that)

    So if "gravitons" are real physical entities they could be something completely different than ordinary quantized plane waves. Supergravity may we a way out - quantized gravity with a finite perturbation expansion in terms of gravitons - let's wait and see ...
  5. Jul 15, 2009 #4
    I saw a paper once where it was argued that they might not even be detectable in principle. Please note that gluons can not either.
    That is simplistic. There are many self consistent non-relativist calculations which can be done with gravitons. Anybody remembers references for non-trivial metrics calculated by graviton exchanges ?
  6. Jul 16, 2009 #5


    User Avatar
    Science Advisor

    So let's weaken the requirement to some sort of indirect effects like 3-jet events, a well-known signal for one hard gluon in deep inelastic scattering.

    There are many self consistent non-relativist calculations which can be done with gravitons. Anybody remembers references for non-trivial metrics calculated by graviton exchanges ?[/QUOTE]
    I haver no idea what a non-relativistic graviton exchange should be.

    What I want to say is that even if gravitons are not detectable in the sense mentioned above, they should be at least a tool for doing certain calculations. Then we could treat them as some physically relevant entity (compare it to virtual particles, where I see two different interpretations: a) either you take QFT seriously and talk about asymptotic states plus the exchange of virtual particles; then you should agree that virtual particles and asymptotic states are physical entities; or b) you say that there are no asymptotic states at all because in the very end you must do a measurement, so the asymptotic states are absorbed by a detector which turnes them into virtual particles exchanged between the event and the detector)

    So if you have a calculation in quantized gravity using plane wave gravitons to produce a finite and reasonable result, then I would again start to think about the existence of gravitons.

    I would accept a concept from QED and QCD: If you quantize QED in the hamiltonian formulation and do an quantum mechanical gauge fixing via resolving the Gauss law, you have to invert a certain differential operator D, which results in a non-local operator 1/D in momentum space; this is just the (static) Coulomb-potential between current densities. If you do the same calculation in QCD (you will need more paper to write it down :-) the operator becomes 1/D[A] where A are certain momentum space modes of the gluon field. So again you find a Coulomb-potential, which now depends on the full dynamic gluon field.

    I accept those kind of affects as something showing the relevance of photons and gluons as "physical entities", even if they are not measurable directly.

    Perhaps you can convince me if with finite supergravity theories ...
  7. Jul 16, 2009 #6
    Thank you for your replies; glad to see im not the only one why isnt too keen on gravitons.

    When i say that i disagree with the aesthetics, i mean to say that i think the geometrical explanation of curved space is more aestetically pleasing to me. Insofar such matters should factor into it at all, but thats another debate.

    There is computations and computations. Has someone ever computed something as simple as the bending of light around the sun by pertubative graviton calculations? Thats all im asking for. Pictures, or it didnt happen.

    I think thats a horrible reason. Things should be made as simple/unified as they can be, but no more. Should we also have a centripedal particle? After all, thats really the same thing. Gravity is a 'fictional' force in the same sense that centripedal force is. It is conceptually a very different thing than the other forces.

    You lost me there. Can you or someone else rephrase that?

    Id like to press my question: how does one explain refraction of light without either an interaction with other particles, or by an intrinsic curvature of space? Bonus points awarded for not getting lost in vague generalities.

    That i do not see this question getting asked can mean only two things: either it has a very simple, trivial answer. Or it has no answer at all. If that question has no answer, then how are gravitons anything other than bogus?
  8. Jul 16, 2009 #7
    Just remember there is a difference between calculating a scattering amplitude through (virtual) graviton exchange (which allows simple results) and a real graviton (which is the quantum of a gravitational wave). When I said "non-relativistic graviton" I was referring to the former, while it has been argued that the latter might never be detectable.
    I need to find the reference, but yes I have read at least one from arXiv.
  9. Jul 16, 2009 #8
    This thread seems relevant to my question:


    In the paper linked, it is claimed that Einsteins equations can not be recovered from any graviton theory. This claim is disputed, but it does confirm a suspicion of mine: im not the only one to whom this does not make sense: nobody has any clue whatsoever what he is talking about when it comes to these matters.

    The easiest person to fool are you yourself. As such, I dont care to hear your explanation of gravitons unless you can phrase it in physical terms: Regge calculus can give me a picture of spiraling black holes. Can pertubative graviton doohickies even account for the bending of light around the sun?

    I repeat: Pictures, or it didnt happen.

    Metaphorical pictures will do too, but arguments in terms of 'Hilbert actions' or somesuch need not apply, because unfortunately I can only interpret them as evidence that you are horribly confused.
  10. Jul 16, 2009 #9
    By the way, does garbage like this make anyone elses toes curl?

    I mean, of course it could be true, but id much prefer 'the flying spaghetti monster did it'. Its much more concise, way less pretentious, and it has exactly the same intellectual content!
  11. Jul 16, 2009 #10


    User Avatar
    Science Advisor

    Hawking radiation and the area-entropy law is a prediction of the graviton formalism. You can't have one without the other.
  12. Jul 16, 2009 #11
    Is you strategy to challenge people by being arrogant and insulting so that they contribute to your thread ? We agree on one thing : it easy to fool oneself, and he only person we should care to convince are ourselves. Have a good day.
  13. Jul 16, 2009 #12
    I am unable to make that connection myself, but it is something tangible, so thanks.

    Tangible in a limited sense though. Has anyone ever observed Hawking radiation? Nope.

    And where does the 'law' in area-entropy law come from? As far as i can tell, it is nothing but a rather daring cross-application of a concept far out of its verifiable domain.

    Any single shortcoming of gravitons to explain any actual observed phenomena, would brush these theoretical constructions right into the trash can as far as im concerned, and i wouldnt shed a tear.
  14. Jul 16, 2009 #13
    Thats an unorthodox interpretation of feynmans words; not at all what he intended, id say.

    Either way, I am asking a question, feel free to ignore it, but im trying to make clear what i do and do not consider to be answers. Make of that as you please, i just dont want to waste anyones time writing long epistels in a language in do not speak. If the conclusion should be that these matters are too complex to be understood by me, then fine.

    In summary: i am hoping to shortcut what could be a very long discussion. If you can refer me to a simulation, a picture, i will instantly believe that my question has an answer. If noone is even trying to make such a picture, i consider that to be a very bad signal indeed.

    Oh, and a good day to you as well. Mine is awesome, summer is finally arriving!
    Last edited: Jul 16, 2009
  15. Jul 16, 2009 #14
    I'm not sure that will convince you but I found the paper I was thinking of
    Quantum Gravitational Corrections to the Nonrelativistic Scattering Potential of Two Masses
    Phys.Rev. D67 (2003) 084033

    Also, I think it is worthwhile to glance at
    Can Gravitons Be Detected?
    Found.Phys. 36 (2006) 1801-1825
  16. Jul 16, 2009 #15
    From what i understand, most physicists are invested in graviton-ish theories. So maybe asking people to seriously inquire as to why their investment might be 'sub prime' is expecting too much of 'a special kind of honesty'. Its called special for a reason.

    I was hoping somone whos biases swing my way might comment, Marcus for instance. Someone who has his money on background independent, discrete geometrical theories of space, shouldnt have any inhibitions from putting his mind towards dealing some serious blows to gravitons.

    I havnt been able to find such a critique though, so that leads me to suspect i am misunderstanding gravitons. Am I? (or more accurately: to what degree am I?). How can gravitons, on a conceptual level, possibly account for the observations we already have, such a gravitational lensing?

    Or does the answer depend on obscure mathematics, that does not translate back into a physical mechanism? If so, shouldnt you want to simulate that, to have at least one check that you are not fooling yourself with all your fancy mathematical jargon? (im not trying to attack anyone here, that happens to me all the time!)
    Last edited: Jul 16, 2009
  17. Jul 16, 2009 #16
    Last edited: Jul 16, 2009
  18. Jul 16, 2009 #17


    User Avatar
    Science Advisor

    In classical general relativity, is there any reason to favor a gravity wave over a geometric explanation?

    We do have a correct quantum theory of gravitons at low energies:
    http://relativity.livingreviews.org/Articles/lrr-2004-5/ [Broken]

    So even LQG must have gravitons if it is to be correct:
    Last edited by a moderator: May 4, 2017
  19. Jul 16, 2009 #18

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    An even better reason is that it would provide a deeper answer, which ultimately is what physics is all about. Stopping at a geometrical explanation of curved space is aesthetically displeasing. It is akin to those "because I said so" answers that physicists received from their parents when they were kids after having asked "why?" one too many times.

    The classical Maxwell's equations provide a good enough answer for almost any problem that a power engineer will face. Invoking quantum mechanics to explain what happens in a high voltage transmission line is confounding rather than helpful. That doesn't mean that quantum mechanics is useless.

    Classical Newtonian gravity provides a good enough answer for almost any problem except for precision timing that a low Earth orbit aerospace engineer will face. Invoking general relativity to explain the motion of a satellite in low Earth orbit is confounding rather than helpful. The error from ignoring general relativity are orders of magnitude smaller than the uncertainties induced by the relatively poor knowledge of the characteristics of the upper atmosphere (one, maybe two decimal places of accuracy is about as good as it gets). That doesn't mean that general relativity is useless.

    Aside: How does one even model the non-spherical nature of the Earth in a general relativistic formulation? The typical approach in high precision satellite modeling or planetary ephemerides computations is to model general relativistic effects as a perturbative force in an otherwise Newtonian universe.
  20. Jul 16, 2009 #19
    The gravity wave is part of the geometric explanation. It seems we are wielding different terminology.

    'Its validity is implicitly assumed'

    Uhm, yeah. Thanks for reiterating my point.

    Reading things like that, this is really all I have left to say to any graviton-theorists:

    1) Give me a picture of a simulation of light deflected by gravitons, corresponding to an observed case: case closed, gravitons are a possibility
    2) Make an honest attempt at doing such a thing: ill suspend judgement
    3) Dont even try: I have lost all confidence this is going anywhere whatsoever. You are shamelessly shaping your physical theory to the constraints of real analysis, with complete disregard for experiment: the least you could do is admit it. Go learn to program a computer.

    Im leaning towards 3 right now.

    LQG is a dynamic field with many competing theories, so perhaps this is again a matter of terminology. LQG is not my point: my point is that there are competing theories without gravitons.
    Last edited by a moderator: May 4, 2017
  21. Jul 16, 2009 #20
    This doesnt make any sense.

    Why does matter emit gravitons?

    Its not about a race to the deepest answer. Its about finding a model that fits the data. If you have a model that fits all the data (and im not saying we do), then asking why once more, is simply a stupid question. Because, thats why. Maybe you can find an even simpler model. Thats cute, but no more of an answer to your 'why'. To say gravitons qualify as the simpler model seems bizzare to me, but whatever floats your boat.

    The important question is: how can gravitons ever possibly fit the data of gravitational lensing?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Rationale behind gravitons
I Geometry of GR v. Spin-2 Massless Graviton Interpretation