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Basic question about gravity

  1. Jan 13, 2010 #1
    Greetings, community. I'd like to ask a very simple question about gravity and the bending of spacetime.

    What would happen if the sun disappeared at this very moment? Would the orbit of our planet be affected instantly and continue travelling across the space in a straight line, or is there some sort of speed limitation for gravity to take effect?

    Looking forward to read your replies, and excuse me if the question was too obvious!
  2. jcsd
  3. Jan 13, 2010 #2


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    The question is not obvious, though it has been asked many times here on PF.

    One caveat before I answer: the sun cannot simply disappear. The method by which the sun were removed from it position would also affect how the force of the sun's gravity would affect Earth.

    That being said:

    Gravitational waves travel at the speed of light. Any change in the sun's mass would take 8 minutes to affect Earth.
  4. Jan 14, 2010 #3
    Thank you for your reply. I've been trying to search for some information about the subject, and I've come up with this article. http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

    If this result was only within 1%, how can scientists be so sure about the speed of propagation of gravitational waves? Could you possibly name me another experiment that has been done so I could read about it?

    Thanks once again
  5. Jan 14, 2010 #4


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    1% isn't a bad start and in any case provides a very definitive answer to your question of if it could be instantaneous.
  6. Jan 14, 2010 #5
    I'm not very knowledgeable on the subject, but didn't Einstein's Theory of General Relativity predict the speed of gravity to be c?
  7. Jan 14, 2010 #6
  8. Jan 14, 2010 #7
    From what I understand, gravity is not an object pulling on another object. The earth doesn't pull a person to it, but rather the earths mass distorts space and a person located in the distorted space is PUSHED to the earth by the distorted space. If the sun were to vanish, space would no longer be distorted and all the planets would travel a path determined by the other bodies nearby. How quickly this would happen after the sun is no longer affecting the earth is puzzling to me as well. I'm not quite sure if I buy the "gravity waves travel at the speed of light" statement either.
  9. Jan 14, 2010 #8


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    Speaking only from my personal experience with GR, I know that it's fairly straightforward to show that linearized gravitational waves (the simplest kind) travel at c.
  10. Jan 14, 2010 #9
    I believe there's more too it; that perturbations of the Minkowski metric are small.
  11. Jan 14, 2010 #10
    I understand what you're saying, but what perplexes me is that if gravify is basically the distortion of space/time, then a traveling gravity wave will be disorting time as it moves along. So if time is being distorted by the gravitational wave, then how can it's speed be determined with any amount of accuracy?
  12. Jan 15, 2010 #11
    How do you measure time distance and velocity on the curved manifold of spacetime? The speed of light is, in general, only locally c. The same is true for gravity waves. If you project your local flat space over nonlocal space curved from gravity waves, you will not measure a velocity of c.

    This is why small perturbations travel at approximately c for small applitude waves, and larger perturbations depend on what you mean by velocity.

    In flat space-time the trace of the metric is (-1,1,1,1) and other terms are zero. A small perturbation would add huv to each term where all the huv<<1.
  13. Jan 16, 2010 #12


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    Good explanation! Makes me glad I didn't try.
  14. Jan 27, 2010 #13
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