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Why do we use gravitons without a corresponding theory?

  1. Jul 30, 2003 #1

    Ivan Seeking

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    I hear many models for particle interactions described that use gravitons, virtual particles, and that reference many concepts that I thought were highly controversial; but these models don't seem to be treated as such.

    Mainly though the question of gravitons is what attracted me. If we have no quantum theory of gravity, then how can we effectively use these models? How can we know the nature of a graviton? Clearly any theories of Quantum Gravity fail thus far. Aren't we virtually assured that our models are incorrect since we can't produce a theory?
     
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  3. Jul 30, 2003 #2
    I thought there was a pretty good theory about gravitons, but we just cannot observe them because of their low energy or something.

    The classical attitude towards gravitons is that they SHOULD exist, no?
     
  4. Jul 30, 2003 #3

    Ivan Seeking

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    I thought that until we have a unified theory of GR and QM, it is all speculative at best.

    This is a comment posted by Jeff. I recognize Jeff as highly accomplished and I treat what he says with the highest regard. But I don't understand the apparent certitude in examples such as this one and hundreds of others.
     
    Last edited: Jul 30, 2003
  5. Jul 30, 2003 #4
    Just out of curiosity, but aren't gravitons considered
    THEORETICAL Particles?

    They are "The Theory".
     
  6. Jul 30, 2003 #5

    jeff

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    A theory of a self-interacting massless helicity-2 field is obtained in the weak field limit of GR in which the metric is approximated as a sum of a flat background part and the aforementioned field living on it. In a QGT the corresponding quantized field would represent the graviton, just like the potential in maxwells equations represents the photon in QED.

    At present string theory is the only quantum theory we know of whose spectrum includes a massless helicity-2 particle whose behaviour is governed by GR in the classical limit. In this sense string theory - whether or not it's correct - is our only known QGT.
     
  7. Jul 30, 2003 #6

    selfAdjoint

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    And string theory, at least so far as I have seen, is not prepared to calculate the phenomonolgy to support all these conjectures about gravitons. GR, although only an effective theory, remains our only source of usable calculations of gravitic effects.
     
  8. Jul 30, 2003 #7

    jeff

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    That string theory contains GR is not a conjecture, it's a fact. GR emerges from string theory as a consistency requirement that the theory on the world-sheet be weyl-invariant. This requirement takes the form of the vanishing of the beta-function, which to 1st order yields GR.
     
  9. Jul 30, 2003 #8

    LURCH

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    Re: Re: Why do we use gravitons without a corresponding theory?

    IOW; without knowing whether or not gravitons actually exist, we have found that the mathematical models used to represent them make predictions that match well with observation, right? So untill we can prove or disprove their existance, we will use the model because it works.
     
  10. Jul 30, 2003 #9

    Ivan Seeking

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    Is ST considered to be mostly correct but just incomplete? Is this considered to be only a matter of loose ends and exact solutions, or does ST itself qualify as speculative? What kind of consensus exists here? I know that Kaku thinks we have THE final theory in ST but that is just unfinished; ie it evades exact solutions for the moment. However, Kaku seems to come off as slightly towards to fringe so I don't tend to trust his claims entirely.
     
  11. Jul 30, 2003 #10

    Ivan Seeking

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    Re: Re: Re: Why do we use gravitons without a corresponding theory?

    I realize that this must be the case. I was alluding to more fundamental questions of certitude and context. Note also that many theories can be made to fit with a little nudging; which of course says nothing of correctness. The epi-cycles of Mars fit the observations for a time also, but they were of course completely without any basis in reality.
     
  12. Jul 30, 2003 #11

    jcsd

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    Gravitons are more of an educated guess at what a quantum field theory of gravitation would include, rather than part of a comphrehensive theory that explains gravitation.
     
  13. Jul 30, 2003 #12

    Ivan Seeking

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    As a related issue, and this has come up several times, I was taught that photons have a mass of hνC-2; that no distinction can be made in GR between energy and mass energy. Am I remembering this incorrectly? Is this a notion out of favor [if so by what argument?], or is this now known to be wrong and why? Obviously I don't mean to ask for a complete explanation, but a well defined pointer would be nice.
     
    Last edited: Jul 30, 2003
  14. Jul 30, 2003 #13

    jcsd

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    The quantity hvc-2 for a phton is what is known as it's relativistic mass, many moons ago this was the accepted defintion of mass but nowdays when a physicist talks about the mass of a particle he will almost certainly mean the rest mass unless otherwise qualified. The reason for this is simply it is a more convient definition. Obviously the two are not indistiungishable as by simply (at least in relativity)measuring the momentum and velocity of an object you can work out it's rest mass and kinetic energy and keeop the two terms seperate.
     
  15. Jul 30, 2003 #14
    Photons are massless:

    relativistic mass

    -wheeler

    -einstein

    you only find the concept of relativistic mass used in popular science books these days, and on internet physics boards or newsgroups. in real science textbooks, it is simply not found at all.

    photons have energy hν. and mass 0.
     
  16. Jul 30, 2003 #15

    Ivan Seeking

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    First, thanks everyone for your answers thus far.

    So the notion of mass is defined as meaning rest mass. What about gravity? Am I not safe to argue that by conservation of momentum, if a planet pulls on a photon, the photon pulls on the planet? If we have gravity then we must have mass - mass that agrees with the total momentum as mV, and mass that agrees with the forces observed between the planet and the photon? By this, can I argue that whatever we mean by the word mass, the thing represented by this concept is present within the photon?

    Edit: I almost did it again! The key question is one of gravity. By this reasoning, is it still safe to conclude that photons have a gravity field? Or has something considered "safe" now ruled this out as a proper interpretation?
     
    Last edited: Jul 30, 2003
  17. Jul 30, 2003 #16

    LURCH

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    I remember this idea being brought up a few times before. The example (IMHO) only proves that the photon has momentum, and the planet has gravity. It does not make a very good case for the photon having gravity.

    I actually started a Topic that addressed the question of relativistic mass having gravitational influence. The example was a neutron star with a mass of two Solar Masses, and a radius smaller than Earth's Moon. This object moves through space at such a valocity (relative to us) that it's relativistic mass is 20-30 SM. An object with a mass of 30 SM should be a Black Hole with an event horizon radius greater than the Moon's radius. So does the object become a black hole? The answer is of course, "no, it doesn't". After all, if it's not a black hole for objects that are stationary relative to it, then things inside the event horizon (as we perceive it) could climb out!

    So, if relativistic mass will not make a black hole out of a moving object, it cannot have a gravitational consequence.
     
  18. Jul 30, 2003 #17

    Ivan Seeking

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    But we do feel the gravity from the relativistic mass. So in this way the gravity would seem absolutely tied to relative mass.

    Also, how do we avoid the need for conservation of momentum with your statement about photons? If we observe a force between the photon and the planet, and if we use gravity [spacetime curvature] to explain the force on the photon, then what other mechanism is left for mediating the required momentum change to the planet? By all accounts the photon does experience a momentum change, so the planet must also...no?
     
  19. Aug 2, 2003 #18

    LURCH

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    Not following you here. How do we feel or measure the gravitational influence of relativistic mass?

    Or do you mean that there is attraction between the photon and the planet? What I was attempting to say with my earlier post was that the attraction between the photon and the planet can take place without the photon exerting gravitational influence on the planet, so long as the planet exerts gravitational influence on the photon.

    Oh no, I am not proposing that we "avoid the need for conservation of momentum", on the contrary; I completely agree that the planet must experience a change in momentum as a result of this interaction. However, you say, "If we observe a force between the photon and the planet, and if we use gravity (space-time curvature) to explain the force on the photon, then what other mechanism is left for mediating the required momentum change to the planet? ". My point is, this model of curved space around the planet altering the course of the photon works equally well whether or not the photon curves space. The curvature of space caused by the planet will alter the course of the photon, and will also compose the medium by which conservation of momentum is transferred, with or without any curvature of space being caused by the photon itself.
     
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