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Can gravitons bend space-time?

  1. Dec 17, 2009 #1
    Can gravitons bend space-time? How can it be shown experimentally? Ideas?
     
  2. jcsd
  3. Dec 18, 2009 #2
    What does your intuition tell you concerning this question?
    Space-time is bent by mass, and gravity (and thus, i suppose, the hypothetical gravitons) exists through this sloping of space-time. Gravitons are said to have no mass (if they exist), so intuitively, how do you suppose they could affect space-time? Secondly, no one is sure if gravitons exist, so maybe the question should initially be "how can we find out if gravitons exist?" and then ask what do they do and how do they behave.
     
  4. Dec 19, 2009 #3
    I thought the curvature of spacetime was curved by both mass and energy (along with other stuff)? As gravitons have energy doesn't it follow they would scatter off each other? The crossection must be pretty small? Would the simplest Feynman diagram of the scattering of two real gravitons include two, not one, virtual gravitons?

    In the attempts to formulate a quantum theory of gravity is there a "quantum" of spacetime change that moves at the speed of light?

    Thanks for any clarification!
     
  5. Dec 19, 2009 #4
    Of course I mean assuming gravitons exist. Ya, doesn't energy also bend space-time?

    Ya I know it's far sketched, but I'm still curious on the implications. I don't know, but somehow I have a feeling that self interaction will relate to some fundamental symmetry.

    What type of mathematics is needed to analyze gravitons if it can bend space-time? (Is there even any?)
     
  6. Dec 19, 2009 #5

    jtbell

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    As I understand it, a quantum theory of gravity that includes gravitons is supposed to supersede general relativity and its "classical" picture of space-time curvature, analogously to the way quantum electrodynamics and its photons supersede classical electrodynamics and its electric and magnetic fields.

    In this view, a graviton would be a quantum of spacetime curvature, similarly to the way a photon is a quantum of the electromagnetic field. To put it another way, gravitons would not "cause" spacetime curvature, but rather, they would "be" spacetime curvature in some sense.
     
  7. Dec 19, 2009 #6

    Haelfix

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    Gravitons represent a perturbation of spacetime, the quantum of the gravitational waves from GR.

    Its easier to think of what a gravitational wave is. They represent a sort of propagating change of the geometry, a jiggling of the matter/energy if you will: http://en.wikipedia.org/wiki/Gravitational_wave

    In much the same way, a graviton is basically just that, but at the quantum level. The subtlety is that they are basically a single term in an infinite expansion that represents this effect. The full classical gravitational wave is then a superposition of many of these gravitons which are all in whats called a coherent state.
     
  8. Dec 19, 2009 #7
    Can gravitons bend space-time?
    theoretically yes since they are packets of energy.
    anything with mass or energy exhibits gravitational effects. Black holes exhibit residual gravitational effects and affect spacetime from certain frames.

    How can it be shown experimentally? Ideas?

    We can't even detect gravitions yet.

    I'm not positive we have absolutely unambiguous "proof" that any massless "particle" exists, including the photon....
    this gets back to quantum theory and the likelyhood there is no clear distinction between wave and particle.
     
    Last edited: Dec 19, 2009
  9. Dec 24, 2009 #8
    A graviton must posses a self-energy . And doesn't a magnetic field create a gravitational field .
     
  10. Dec 25, 2009 #9

    Chronos

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    LIGO is still the gold standard experimentally. 2010 may be the breakthough year for this project.
     
  11. Dec 27, 2009 #10
    But a EM field doesn't create a EM field.

    Also strong force has the property of self creation.
     
  12. Dec 28, 2009 #11
    so then I am guessing that a gravitational field cannot create another gravitational field ?
     
  13. Dec 28, 2009 #12
    In what sense? Assuming that gravitons exist, a gravitational field would be an area in which the effects of a graviton would be exerted, therefore, to 'create' another gravitational field, one of two things might happen:

    • The gravitons replicate (?)
    • Some gravitons 'leave' the initial field (is it impossible? gravitons are particles, therefore they are also affected by gravity)
     
  14. Feb 2, 2010 #13
    interesting but , can a gravitational field keep creating another field and that field creates another field and so on , is that possible.
     
  15. Feb 2, 2010 #14
    Gravity IS a self interacting phenomena..that's why it took Einstein so long to develop general relativity...via Tensors, Riemann manifolds,etc. The interaction is described by the Einstein tensor.

    In rereading my post #7, I don't like it much now....because as in other posts here we are mixing two theories...GR says gravity is a warping of space, a geometrical description ...gravitons result from quantum mechanical (particle) considerations rather than geometrical ones....so describing either particle interactions or geometric explanations depends on which theory you are using....GR and QM don't mesh/mix all that well so far...that would be the result of grand unification combining all forces.

    I posted once before, but have forgotten the exact example, of the magntitude of graviton exchange....as derived from QM.....But the number of gravitons exchanged in QM theory is really,really tiny....like,for example, a pair of electrons might exchange a graviton once a year...don't quote me on that example but you get the idea....really really tiny...
     
  16. Feb 2, 2010 #15
  17. Feb 10, 2010 #16
    However, there is a perfect analogy between space warping and exchanging virtual particles. These are two mathematical ways to describe the same thing. So GR and QFT are not as far away of each other as it seems.
     
  18. Feb 10, 2010 #17

    tom.stoer

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    We had this discussion here for a coupleof times; I used to ask "what is a graviton?"
     
  19. Mar 12, 2010 #18
    The conclusion does not follow from the statement.
     
  20. Mar 12, 2010 #19

    tom.stoer

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    I have never seen a theory which describes exchange of virtual particles bending spacetime via a back-reaction. So I don't see this analogy. Where shall it come from?
     
  21. Mar 13, 2010 #20
    Both theories describe interacting bodies.

    First we have flat spacetime or free particles. We can do various mathematical operations, including differentiation.

    Then we turn on interactions. Our differential operator changes. In GR we have:
    [tex]D_{\mu} \phi_{\nu} = d_{\mu} \phi_{\nu} - \Gamma^{\alpha}_{\mu \nu} \phi_{\alpha}[/tex]
    while in QFT:
    [tex]D_{\mu} \phi(q) = d_{\mu} \phi(q) - i A_{\mu} q \phi(q)[/tex]
    .
    These formulas are pretty similar. Traditionaly we say that the first describes spacetime warping and the second that there are virtual quanta of [tex]A_{\mu}[/tex] field, but it is essentialy the same thing.
    The difference is that GR warps spacetime itself and QFT bends some abstract space of internal symmetry. Alternative description: in QFT we have quanta of the gauge field while in GR bodies interact with exchanging virtual quanta of spacetime deformations - the gravitons.

    So GR and QFT are not quite that different. The last thing is to find interpretations of gauge fields in GR and spacetime bending in QFT. Kaluza-Klein-like theories are most promising IMO.
     
  22. Mar 13, 2010 #21

    tom.stoer

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    The problem is that the concept of virtual particles is limited to perturbative regimes and is known to fail in many cases where non-perturbative methods are required; this is true even in standard QFT. There is chance to see color confinement based on virtual gluons. In GR you can calculate tree-level and one-loop level corrections, but you know that you construct this theory on flat (or at least non-dynamic) spacetime. And you will never get fully dynamics of spacetime based on static spacetime plus virtual particles.

    I agree that GR and QFT are mathematically rather similar (one can even describe GR as a gauge theory; in that case it shares many similarities like local gauge symmetry in tangent space, Gauss law constraint, etc.), but that does not mean that the concept of virtual particles really makes sense.

    What I am wondering is that so many people know about the similarities between QFT and GR and at the same time try to use concepts which are known to fail even in many areas of QFT
     
  23. Mar 13, 2010 #22
    I don't say that QFT-like approach to GR will lead us to Truth. I just say, that both GR and QFT describe... something and that "something" is similar in these theories both physically and mathematically. We can call it spacetime warping from GR standpoint or virtual particles from other side, but it is the same. What I say: GR and QFT are not that incompatible as most people think. That is, we can "mix" GR and QFT to some extent, say introduce gravitons.
     
  24. Mar 13, 2010 #23

    tom.stoer

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    I agree with everything except for the "virtual particles from other side". They are a very limited concept and I don't think that they will help to understand QG.
     
  25. Mar 13, 2010 #24
    (1) both have infinities of their own;Smolin lists combining Gr and quantum theory one of the five great problems in theoretical physics, (2) rather unlikely, I think, but stranger things have happened, (3) Well multi dimensional theories have no far proven to be the panacea anticipated...again we'll have to see...seems more likely to me we need a new approach, maybe entropic gravity origins, Verlinde like....
     
  26. Mar 14, 2010 #25

    tom.stoer

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    The big difference is that ART/GQ is diff.-inv.
     
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