Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Deflection of light and gravity

  1. Apr 30, 2014 #1
    Light is deflected in a gravitational field and this effect has been measured empirically, confirming the predictions of GR.

    Gravity is also deflected in a gravitational field according to GR, the geodesics for light and gravity being the same, but has any deflection of gravity ever been empirically detected? If so, has it been confirmed to be close to the deflection of light?

    (My guess is the answers are No and No, because of the difficulty of detecting deflection, but experimentalists are sometimes very clever in finding indirect confirmations of theoretical predictions.)
     
  2. jcsd
  3. Apr 30, 2014 #2

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    The closest thing to what you are describing would be deflection of gravitational waves (rather than gravity per se). Gravitational waves do not exactly follow null geodesics except in approximations (that are highly accurate for plausible scenarios). Anyway, no gravitational waves have been detected at all, and from there to detecting deflection would be mind bogglingly harder.
     
  4. Apr 30, 2014 #3
    Yes, but the existence of gravitational waves has at least been supported by observation of how fast certain binary star systems are losing energy, if I remember correctly. Aren't there any consequences of the deflection of gravity that can be detected in such an indirect way?
     
  5. Apr 30, 2014 #4

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Yes, that is strong indirect evidence of their existence, but you can't measure anything about deflection that way. You need to be able to detect precise direction of source as it passes behind some large mass. We are very, very far from being able to do that with gravitational waves.
     
    Last edited: May 1, 2014
  6. Apr 30, 2014 #5

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    At the risk of making this thread longer, but in the interest of clarity, I should make a few comments. If we consider first the case of electromagnetism, we see that light, or electromagnetic radiation, aberrates, as has been measured experimentally, but the coulomb force between charges does NOT aberrate in the same manner. To be more specific, see Steve Carlips paper http://arxiv.org/abs/gr-qc/9909087

    Similar results apply to gravity, though the details are more complex, and the absence of dipole gravity means that the propagation delay effects are cancelled to a higher order. This is discussed in the reference above. The main point is that one should not expect the "force" of gravity to aberrate in the same manner as waves would, while one should expect that gravity waves would aberrate in the same manner that light waves do.

    This "debate" historically occurs with respect to the "speed of gravity" rather than its "deflection".
     
  7. May 1, 2014 #6

    Bill_K

    User Avatar
    Science Advisor

    Doesn't the same qualification need to be made for electromagnetic waves?
     
  8. May 1, 2014 #7

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Strictly, yes, but I was referring to GW tails, which, so far as I know, are not associated with EM radiation.

    http://arxiv.org/abs/gr-qc/9710038
     
    Last edited: May 1, 2014
  9. May 4, 2014 #8
    I'm beginning to think that I have a major misunderstanding here. I assumed that in any configuration of masses, the deflection of gravity is going to have an effect, for example, in the earth and moon system, but any such effect has to be accounted for in the minimization of the action (as in Einstein-Hilbert action).

    Minimization of the action is minimization of the curvature of spacetime. There is no concept of deflection in GR. That exists only in coordinate space and time, where the image of a geodesic appears as a curving trajectory. To test it we'd have to find an exact solution, or a close approximation to one, in coordinate space and time, where we would see a deflection, and then we'd have to compare it with observation.

    For example, maybe this could be done with the Earth-Moon. You might then find an error in GR, but I don't see how that could be traced to deflection rather than some other problem.
     
    Last edited: May 4, 2014
  10. May 4, 2014 #9

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    The conservation of angular momentum means that the Earth and moon will essentially be attracted along the line joining their centers of mass, according to the instantaneous position and not the retarded position.

    A *very small* amount of angular momentum is carried away by gravitational radiation, so it's not exactly along the instantaneous line connecting the center of masses. But for all practical purposes the difference is negligible. The electromagnetic case is similar when there is no significant emission of electromagnetic radiation, the attraction is not retarded in that case either.
     
  11. May 4, 2014 #10
    I think my problem is that I didn't understand gravitational radiation. Suppose you have two rigid planets connected by a rigid bar of negligible weight, isolated in space. Then there would be no gravitational radiation, right? In general there is no gravitational radiation in any isolated static system, right?
     
  12. May 4, 2014 #11

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Correct, no GW in a static set up.
     
  13. May 4, 2014 #12
    Thank you.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Deflection of light and gravity
  1. Deflection of light (Replies: 5)

Loading...