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Speed Of Gravity?

  1. Jun 22, 2006 #1
    This is a strange question that has probably been asked before but I could not find it... An example of my question would be this: If the Sun suddenly disappeared how long would it take for the Earth to no longer be affected by the Suns gravity? I'm not sure about this and I don't quite know how this would be proved..

    Thanks,
    Helical
     
  2. jcsd
  3. Jun 22, 2006 #2
    If this were to happen, we should feel the effects of the lost gravity at the same time as we see darkness. The force of gravity travels at the speed of light. I'm sure a more detailed description will come soon...:tongue2: I just don't have much time...
     
  4. Jun 22, 2006 #3

    pervect

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    The sun can't suddenly disappear (at least not according to GR). The differential form of the conservation of energy and momentum is built into General Relativity, therfore the equations have no solution in which the sun disappears.

    You can, however, generate gravity waves by blowing up the sun asymmetrically, or better yet, by imploding the sun to form a black hole. The first gravity waves from the implosion or explosion event would reach the Earth at the speed of light.

    Implosion would generate a much stronger gravity wave signal than explosion, BTW. (The implosion generates gravity waves only because the sun is rotating).

    An explosion must not be spherically symmetrical if it is to generate gravity waves.

    If you just planted a big bomb at the center of the sun and blew it up in a spherically symmetrical manner, there would not be any gravity waves or other gravitational effects on the Earth's orbit until the debris actually reached the Earth's orbit.

    Try also reading the sci.physics.faq

    http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

    There have also been past discussions on this topic on PF.
     
    Last edited: Jun 22, 2006
  5. Jun 26, 2006 #4
    There are no good experiments for verifying the speed of gravity - the best we can do is determine it inferentially - GR asserts it is not faster than the velocity of light - One of Einstein's first tests for GR is based upon the use of a c velocity of gravity to explain the perihelion motion of the planet Mercury - but this same result can be due to solar oblateness. -there were some other tests that purportedly measured the velocity of gravity using accurately timed signals of light passing through the G field of Jupiter - but these have been analysed by others to cast doubt on the results. Most of the arguments favoring a higher than light speed for gravity are based upon the fact that the G field of the Sun is not parallel to the light rays eminating from the Solar center (aberration). But these are not too convincing because GR also provides an offsetting correction that almost exactly cancels the retardation effect of a G field that moves at velocity c. In summary, I think it is good to not be too sure in these matters - at least until better experimental data is available.
     
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