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A question about gravitons&photons

  1. Feb 10, 2004 #1
    Hi. i have a question.Hope i could get some answers.

    Say i have 2 identical light emitting bodies placed next to each other, side by side, somewhere in space. Will the photons and gravitons given out by both the bodies collide in mid space? If so, what are the quantum mechanical(since both photons&gravitons are so small)consequences of such an even happending? Also, will the gravitons and photons pass through each other if they are about to experience a head-on collision?
     
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  3. Feb 10, 2004 #2

    turin

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    As far as I know, photons do not carry charge. If they do not, then they should not interact with each other, since photons interact with charge. I'm pretty sure about this. E&M radiation (photons) obey the superposition principle, which basically results in nothing happening to "interacting" photons.

    I'm not so sure about gravitons. I think that gravitons interact with mass energy. I seem to have gotten conflicting ideas regarding whether or not gravitons are energy. I have read in books such as Ohanian's "Gravitation and Spacetime," MTW's "Gravitation," and Rindler's "Relativity: Special, General, and Cosmological," as well as a few papers, that the gravitational field itself gravitates. However, I have been hearing from a few people, whose oppinions I do respect, that this is not the case. Or maybe I'm just confused.
     
  4. Feb 10, 2004 #3
    Gravitons and photons will interact only if there is a gravity-EM crossterm in the Lagrangian (i.e. a non-zero scattering amplitude).

    However, more to the point: we don't know if there is such a thing as a graviton, and current theories of quantum gravity are shaky at best.

    I'm not aware of the possible results from M-theory or LQG offhand.
     
  5. Feb 10, 2004 #4
    To GRQC, "Gravitons and photons will interact only if there is a gravity-EM crossterm in the Lagrangian (i.e. a non-zero scattering amplitude)."

    Put in layman's term...what does it mean?

    I don't think i'm getting an answer from the replies. My question is, what happends when a photon and a gravitnon are on a head-on collision? Just like two billiard balls, when they collide you could tell me they are deflected away in some directions set by the initial conditions.
     
  6. Feb 10, 2004 #5

    Stingray

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    He's basically stating a more specific version of "they interact if the theory allows an interaction between them. Since we don't have a theory for gravitons, who knows."
     
  7. Feb 11, 2004 #6

    turin

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    I would bet that they do not act very much like two billiard balls, just because they are wave-quanta. I think that a graviton and a photon would interact (my rather uneducated opinion) because the graviton interacts with energy (I think) and the photon is energy.

    Here's my guess:
    The photon and graviton are on a collision course. They meet. The graviton is an amount of gravitational field, which is basically a geometrical disturbance. Since the photon must behave geometrically, then the graviton essentially gets to tell the photon what to do. So, in the aftermath (overly dramatic term), there would be a photon and no graviton. The path of the photon would be diverted loyal to the graviton's last dying wish (the thing about bosons, as I understand it, is that they only get to tell other particles what to do once, and then they die, kind of like a bee sting).
     
    Last edited: Feb 11, 2004
  8. Feb 15, 2004 #7
    I think that photons and gravitons only exist as plankt-like quantifications that make it easier to study waves of light or gravity. The photon is the quantum approach to measuring and experimenting with light and the graviton, so far, is the quantum approach to measuring and experimenting with gravity. Someone please correct me if I am mistaken!
     
  9. Feb 16, 2004 #8
    I was wondering...Maybe gravitons and photons are composed of radeically totally different "substances"...If not for that, there'd be chaos in the world should light and gravity get jumbled around, somehow. Take for instance, the light i'm shining at you suddenly becomes a gravity wave thus pulling you toward me.

    What do you all think?
     
  10. Feb 16, 2004 #9

    Nereid

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    analogies

    Another way to think about it is 'which of the four fundamental forces does X feel?' The photon is the carrier of the EM force, and it's unaffected by the weak or strong nuclear force. A beam of photons would not interact with a beam of particles whose only interactions were weak or strong force ones, for example neutrinos.

    As GRQC said, since there's no theory of quantum gravity, we can only speculate on the properties of the graviton (indeed, it may not even exist). The only things limiting your speculation are the results of experiments and observations; if your speculation requires that photons exhibit frequency-dependent gravitational lensing (for example), then we can be pretty sure your speculations are wrong (within the limits of what we've observed, of course).

    Where might any photon-graviton interactions be observed? In colliding/coalescing neutron stars; in the merger of two galactic nuclei (supermassive black holes); other examples?

    What about the gravitational effects of a photon? (not to be confused with the effects on a photon of a gravitational field). How much energy does a photon need to have before we could detect the gravitational influence of this photon? Certainly far, far more energy than even the most energetic photons from the LHC. However, nature regularly produces photons with as much energy as a baseball pitched by a Yankee, and we've long been able to measure the gravitational influence of a baseball-sized mass.

    Anyone care to guess when we'll see the first reports of observations of the gravitational effects of very energetic photons?
     
  11. Feb 17, 2004 #10
    It is a popular observation that light bends when it come in contact with gravity. Gravity remains unaffected by light or in as much as we have not studied the effects of light on gravity, all things being equal, the two forces, light and gravity, can be, and by all accounts are, equal.
     
  12. Feb 18, 2004 #11

    TeV

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    What do you mean by term "collision" of photon and graviton at all?
    Particle interaction is one thing,effects of G fields another.
     
  13. Feb 18, 2004 #12

    TeV

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    Simply put yes,they will pass through each other if they are to experience a "head- on collision".
    Interaction is another thing.Photons have zero rest mass and can interact only with particles having charge and nonzero rest mass (can't interact with neutrons or neutrinos).
     
  14. Feb 18, 2004 #13
    Thank you TeV for your response.

    Could you substantiate on what you've posted? From where did you learn about what you'd typed? For eg. , "Simply put yes,they will pass through each other if they are to experience a "head- on collision. Interaction is another thing.Photons have zero rest mass and can interact only with particles having charge and nonzero rest mass..."

    It would help in my understanding if you could elaborate more on what you have typed.
     
  15. Feb 18, 2004 #14
    what he is saying is that a photon will have no effect whatsoever on the graviton. A photon, which has no mass, when at rest, therefore no rest mass, will only interact with a particle with a particle with mass at rest, and has a charge (i.e.-electron has a (-) charge). He is saying, also, that a graviton has no mass at rest, and has not charge, since he says they will not interact. I hope this explains what he is saying a little better. Now, if he is correct or not, I dont know.

    Now I was wondering, on the topic about how gravity bends light. Can there be a recipricol effect? Can bending a beam of light create a gravitational field? Just a question.

    Paden Roder.
     
  16. Feb 18, 2004 #15

    Nereid

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    Think of a high energy photon, say 1 TeV (1012 eV). If it were a massive particle at rest, what would its mass be? Could such a particle bend light? Now, to recap, we have a photon with an energy of 1 TeV; what is the magnitude of the gravitational field it creates?

    Now you're an experimenter. What's the smallest mass you can detect, using only the mass's gravitational influence? How energetic would a photon need to be before you could detect its gravitational field (in principle)?

    [Edit: fixed typo]
     
  17. Feb 18, 2004 #16

    Stingray

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    You usually don't have just one photon. Its probably better to ask whether any conceivable beam of light (a collection of many low energy photons) could have a measurable gravitational field. I think a few grams per cubic centimeter is technically detectable now.

    The energy density of an electromagnetic field goes as E^2. Having the equivalent of a gram of matter in a cubic centimeter would require E around 10^16 V/m if I didn't make a calculator error!

    In principle though, light does generate a gravitational field.
     
  18. Feb 19, 2004 #17

    Kit

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    can we put it in this way?[?]

    if graviton really exists, a photon will probably interact with a graviton coz light is bended by the gravtational field...

    but if they do not collide each other, how do they interact?

    moreover, does gravition interact with all things which have mass or energy?
     
  19. Feb 19, 2004 #18

    Nereid

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    Since no one has observed a graviton, and there are no solid theories from which to derive the properties of 'the graviton', you are free to speculate on how any such particle will (or will not) interact with photons.
    CANGAROO (http://icrhp9.icrr.u-tokyo.ac.jp/) is one project which counts cosmic ray photons one at a time; in particular TeV photons detected via the atmospheric Cherenkov technique. GLAST (http://glast.gsfc.nasa.gov/) will detect photons with energies up to 300 GeV directly.
     
  20. Feb 19, 2004 #19

    TeV

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    Both massless particles,free "boson" of G field and free boson of EM field,if they are to meet HEAD-ON vise will not interact,since they travel at speed of light in oposite directions (considered as stright line in Euclidian space).Any exchange of impulse or energy is impossible under these circumstances.Please,find me reference frame where interaction occurs in this case!
     
  21. Feb 19, 2004 #20

    TeV

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    Relativity predicts so called gravity waves.
    In any form of quantizied theory of gravity that would be just bunch of free gravitons.
    I undoubtly believe gravity waves exist.
    But G relativity doesn't need quanta to explain interaction of G field.You may say it is treated as sort of illusion there,not necessary to be treated as sort of field at all.
    It is a just local change of spacetime curvatore and objects do not fall,beams of light do not "bend",but they follow the shortest path in a curved higher dimensional space.
     
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