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Does light have its own gravity field?

  1. Jun 22, 2007 #1
    i.e. if you shoot off two parellel photons will they oscillate around each other or will they stay parallel? The main reason Im asking is because Im wondering what the correlation is between the higgs boson and gravity. If gravity only occurs in mass, it is caused by the higgs boson either directly or indirectly.
     
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  3. Jun 23, 2007 #2

    pervect

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    Parallel light beams won't attract according to GR, but anti-parallel ones will.

    I'm not sure where you're getting to with your question about the higgs, that probably belongs in another forum anyway (it's not a GR question).
     
  4. Jun 23, 2007 #3
    eh it was just a thought that made me think of this question. The question is a GR one tho. So what does that mean only anti-parellel beams attract? Photons dont have a symmetric gravitational field?
     
  5. Jun 23, 2007 #4
    Beams of light, yes. Whether that is true for parallel beams of particles is another matter. But in any case the beam of light does generate a gravitational field. This was first in the article On the Gravitational Field Produced by Light, Richard C. Tolman, Paul Ehrenfest and Boris Podolsky, Phys. Rev, 37, March 1931. The abstract reads
    In this article it shows that a ray parallel to the direction of radiation of the pencil of light will not attract nor deflect rays of light which run parallel to the beam. The opposite is true for anti-parallel beams. However in both situations the light is still affected by the pencil of light in that as the ray moves along, parallel, to the pencile it will become redshifted. By this I wanted to point out that the pencil of light generates a gravitational field. I placed the calculations done by these authors online at my website at

    http://www.geocities.com/physics_world/gr/grav_light.htm

    If you'd like I could place the entire article online. I can't do this for everything since there isn't a lot space on the web site or the other website I own in which the files of certain articles will be placed (there is only 25 Mb of room).

    Best regards

    Pete
     
  6. Jun 23, 2007 #5

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    GR is a classical theory, so we would talk about the field associated with a beam of light, not that of a photon.

    I'm not sure what you mean by "symmetric gravitational field". Rather than to attempt to get into what you might mean by that statement, I think it's clearer to just say that parallel light beams don't attract, while anti-parallel light beams do.

    Here's more detail

    Suppose you have two, small light beams with negligible energy, which play much the same role as "test particles" do except that they are test beams.

    -------------->
    -------------->

    If you put nothing between them and they are in empty space, the beams are parallel.

    Now, suppose you put a massive object (a planet or a star, say) in between them. The light beams converge due to gravitational lensing.

    Now, if you put even a very high energy light beam going in the same direction as the two "test" light beams, the beams do not converge - there is no gravitational lensing. But if you put in a very high energy light beam going in the opposite direction as the two test light beams, they will converge, though you will need extremely high energies to make this happen, i.e. to get an effect similar to a planet, you'd need beam energies of planetary masses * c^2, to get an effect similar to a star, you'd need beam energies of stellar masses * c^2. (That's a LOT of energy!).

    You're probably looking for some rough idea of why that happens - probably the best answer is gravitomagnetic effects become important at high velocities, and light has the highest velocity possible. See for instance http://en.wikipedia.org/w/index.php?title=Gravitomagnetism&oldid=140120977
     
    Last edited: Jun 23, 2007
  7. Jun 23, 2007 #6
    It seems to me that you can replace the photon with a localized bunch of energy directed in one direction. Thus you replace the photon with a wave packette. Do you disagree with such a model? I'm not 100% sure that this is true. I.e. I'm open to suggestions.

    Pete
     
  8. Jun 23, 2007 #7

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    The solutions I've seen (Tolman, etc) are actually "null dust" solutions, not an actual EM wave satisfying Maxwell's equations, so they are rather idealized.

    I think you get a pp wave http://en.wikipedia.org/w/index.php?title=Pp-wave_spacetime&oldid=114304502 if you assume a pulse rather than a continuous beam, but I'm not actually positive about that.
     
  9. Jun 28, 2007 #8
    I do not have the impression that the opening post refers to test beams.

    Is there any reason why we might not want to discuss the effect (if any) of the energy of those beams?

    It seems a rather limited approach to understand GR only in terms of test particles and test beams.
     
  10. Jun 28, 2007 #9
    To be exact the OP refered to if you shoot off two parellel photons

    Pete
     
  11. Jul 3, 2007 #10
    What happens if the third beam is orthogonal to the plane of the other two, between them?
     
  12. Jul 3, 2007 #11

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    I haven't seen that case discussed specifically (the parallel and anti-parallel cases are talked about a lot). I believe there should be a net focusing effect that isn't as large as the anti-parallel case. There will probably be frame dragging (aka gravitomagnetic) related effects too but I'm not quite sure what they are.
     
  13. Jul 3, 2007 #12
    Thanks for the answer.
     
  14. Jul 3, 2007 #13
    Do you know about the geon, invented by John Wheeler, that is an object made of a high-energy light beam captured in an orbit by its own gravitational field?
     
  15. Jul 4, 2007 #14
    It's a question for me?
    I don't know it.

    Related question: could I make such an high energy light beam on a very small circular path, so that it's held by its very gravitational field? In a circular path, two diametral opposite portions travel at opposite direction so, for what pervect said, they should attract each other.

    Can I create an elementary particle this way?
     
  16. Jul 5, 2007 #15
    You've described a geon. In principle you can indeed make an "elementary" particle that way. It would be quantized by the wavelength of the light and would have Planck mass and size. To see this, set the Schwarzchild radius equal to h/mc.
     
  17. Jul 5, 2007 #16

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    I'd expect that you could find an equilibrium solution for a ring of light, but I doubt very much that it would be a stable equilibrium.

    This is indeed a geon, Wikipedia has quite a bit about them, unfortunatley I haven't read the original sources (like Wheeler, or Brill-Hartle).

    As far as whether or not you could make a "particle" out of them, this would be a question for the quantum gravity folks, not GR which is a classical theory.
     
  18. Jul 6, 2007 #17
    This reminds me of the classical problem with the orbiting electron. The orbit should decay as the accelerating electron radiates energy. But in quantum mechanics the allowed orbits are stable. The classical geon does radiate, but what about the quantum geon...?
     
  19. Jul 6, 2007 #18

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    I'll give the same answer I gave before - you'll have to ask a quantum gravity person that question. GR is a classical theory, and can only answer the question of whether the classical geon is stable. You'd need a quantum theory of gravity to determine if the quantum geon is stable, this isn't a question that can be handled by semiclassical techniques.
     
  20. Jul 6, 2007 #19
    Hmm... But the question probably would not have been raised in th QM forum. The geon is a GR concept. Maybe you'll have to give your answer one more time... :smile:
     
  21. Jul 6, 2007 #20

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    We do have a beyond the standard model forum. Not that I'll guarantee that anyone there has a better answer. While I could move the thread there, because the thread has drifted a bit since the original post, it might be just as well to re-ask the question.

    At best you'll get an answer like "LQG predicts ...", "string theory predicts ...", "X" predicts, where X is some theory of quantum gravity.
     
    Last edited: Jul 6, 2007
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