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Is there any exp. evid that massless photons cuves spacetime?

  1. Dec 10, 2008 #1
    Is there any experimental or observationa evidence that massless photons warp spacetime?

    Of course I know that in GR everything that has energy curves spacetime, including massless particles, but what-if this is wrong, and that particles must have mass to act as a "charge" for gravity? what if we modify gravity so that massless particles have no effect on spacetime itself. (but can be themselves affected by curved spacetime)
  2. jcsd
  3. Dec 10, 2008 #2
    In that case we have to look for a better theory. :smile:
  4. Dec 10, 2008 #3
    I don't believe there is experimental evidence for anything curving spacetime...it's a mathematical model. Nobody even knows what spacetime is.

    We do know light moves in a curved trajectory when passing the sun...that's been observed....but is that a "force" or curvature of spacetime....no way to prove one way or the other as far as I know. What has been proven is that general relativity works, is more accurate than Newtonian formulations, but does not work near extremes of curvature/energy...that is near singularities....
  5. Dec 10, 2008 #4


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    Science Advisor

    The solar deflection of light is some evidence for it. This is usually thought of as curvature affecting light, rather than light affecting curvature. However, the geodesic motion of free particles and photons doesn't have to postulated separately, and can be derived from Einstein's field equation that says that light curves spacetime.

    For comparison, there is actually a consistent relativistic theory that says that gravity is the curvature of spacetime, but light does not contribute to the curvature. This theory predicts no global bending of light.

    Another place to look is the standard model of cosmology. The model uses Einstein's field equation to postulates a certain relationship between matter and spacetime curvature. In the model, the matter at a certain era is radiation dominated, during which nucleosynthesis and helium formation occurred. To the extent that this model gives a good fit to present data, it is consistent with light causing spacetime curvature.
  6. Dec 10, 2008 #5


    Staff: Mentor

    This is incorrect, or at least a mischaracterization. Curved spacetime is part of a physical theory not just a mathematical model. As such it makes concrete testable predictions, many of which have been verified experimentally and thus relate the mathematical model to physical reality.
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