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Light under light gravitational force

  1. Aug 1, 2007 #1

    We can see light emitted by a star which is effectively behind a gravitational mass. Ok! The light path will curve. Or the light path will be as straight as the space surrounding the gravitational mass. Ok!

    My question is:
    When two parallel beams are emitted in the same parallel direction, will these beams will interact? Will they go straight? Will the reach eachother? Will they swirl and stay distant? Will they swirl and reach eachother?
    For me, on earth! Or for me, on one of the beams!

    In fact, what will happen to these beams?
  2. jcsd
  3. Aug 1, 2007 #2
    What do you think of this? My guess (with my limited knowledge) is this is what will happen.

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  4. Aug 1, 2007 #3


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    No, I think the original poster is asking about whether two light beams will attract each other, without any massive object entering into the picture. There was a discussion about this here not too long ago:

  5. Aug 2, 2007 #4
    I believe parallel beams traveling in the same direction will not interact, while parallel beams travelling in opposite directions will tend to bend towards one another
  6. Aug 2, 2007 #5
    Merci for the other thread! Now, I have to be more specific about my interrogations!

    Suppose, in free space with no other gravitational field, we have a photon and a photon's stream which is a succession of photons as near to be considered as a continuous line of photons.

    First, at the image of bullets launched from guns, we could consider two photons, launched parallel at the same time, travelling at light speed.

    Q: What will be photon's trajectories? Are they supposed to interact with each other? If so, will the gravitational field travelling at light speed transform into another field which creates swirl paths?

    Second, at the image of bullet's streams launched from machine guns, we could consider two photon's stream, parallel at a certain time. Individual photons are travelling at light speed but each photon's stream is "static".

    Q: What will be stream's trajectories? Are they supposed to interact, (differently from standalone photon) with each other? If so, will the radial gravitational field travelling at light speed transform into another rotational field which creates swirl paths?

  7. Aug 2, 2007 #6


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    I think I can say with considerabl certainty that two photons travelling side-buy-side could not interact with each other, as such an interaction would require information to move faster than c to get from one to the other.
    The other situation seems to be answered in the thread jtbell linked, since the stream of photons litteraly is a "pencil" of light.
  8. Aug 2, 2007 #7


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    The photons do not attract - nor do parallel beams of light attract.

    An oversimplified and non-rigorous description of why not might consider replacing the photon by a massive particle, and taking the limit as the velocity of the massive particle approaches c while keeping the energy constant and equal to the energy of the photon.

    The mass of the massive particle approaches zero. Unlike the case of the photon, the massive particle has a rest frame, so we can see that the force in that rest frame approaches zero. No attraction in one frame implies no attraction in all frames.

    The "gravitational field" associated with a light beam or pulse would not be a "swirl", but would be a pp wave.

    Some references for PP waves:


    see http://adsabs.harvard.edu/abs/1998gr.qc....11052F for the publication history of the arxiv article
  9. Aug 10, 2007 #8

    why light bend towards eachother when it trevels antiparallel?
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