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Your Thoughts: speed of gravity

  1. Jul 13, 2004 #1
    I would like everyone to comment on this.

    What do you think is the speed of gravity?

    Some say it’s the same as the speed of light, others claim it might be 20 billion times faster than light.

    What is your idea? :)
     
  2. jcsd
  3. Jul 14, 2004 #2
    is that "20 billion times" a stated figure or was it off the top of your head
     
  4. Jul 14, 2004 #3
    speed of gravity??? what do u mean by that, i know of acceleration due to gravity, but i dont know what u mean by speed of gravity. Perhaps your talking about the interactions of gravitons between particles?? please give details about what you are talking about. :D
     
  5. Jul 14, 2004 #4
    The argument that gravity travels faster than light (20 billion times faster by some estimates) is that if it didn't, the gravity from the Sun on the Earth for example would have an 8 minute + lag so that the Earth would spiral out of orbit very quickly since it would be acting on the Earth's position 8 minutes ago, and vice versa. Some GR expert named Cliff something supposedly calculated that by some weird coincidence, gravity traveling at the speed of light from the sun does act on the instantaneous position of the EArth.
     
  6. Jul 14, 2004 #5
    If I remember correctly that calculation was debunked. To the best of my knowledge, the speed of gravity is the same as the speed of light.
     
  7. Jul 14, 2004 #6

    selfAdjoint

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    This is the prediction of general relativity. It hasn't been demostrated by observation yet, but big new experiments like LIGO and Gravity Probe B are closing in.
     
  8. Jul 14, 2004 #7
    I don't know about the technical stuff, but my guess is also that the speed of gravity is the same as the speed of light.
     
  9. Jul 14, 2004 #8

    BobG

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    The first argument would be equivalent to saying that each time you moved, the Sun would disappear until you came to a stop and remained stationary for 8 minutes. In reality, if you move, you run into light that left the Sun even before you started moving, so you don't see any difference unless you happen to be moving incredibly fast (you'd see a shift in the frequency of the light you're running into).

    Just because the distance is part of the equation for determining the force of gravity, doesn't mean that only the 'gravitrons' (or whatever) originating at that precise instant determine the gravitational attraction (the equation isn't the reality - it describes the reality). I would think any gravitational forces an object generates exist regardless of whether there is any other body there to interact with. If another body does exist and is in motion, it, too, has its own gravitational forces that are moving with it. The equation for determining the force of gravity describes the interaction occuring where the forces from the two bodies intersect each other and is independent of the speed either body is travelling - at least, unless you're travelling some very high speeds, at which point I'm not sure what kind of difference you'd perceive that would be equivalent to the frequency shift of light.

    Just out of curiosity, what kind of difference do LIGO and the Gravity Probe B expect to detect?
     
  10. Jul 14, 2004 #9
    the foce of gravity acts at the speed of light.
    if the sum just suddenly disapeared with would see and feel the effects simultaneously
     
  11. Jul 14, 2004 #10

    LURCH

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    I have probably said this before, but if I understand general relativity correctly, the term "speed of gravity" is not exactly valid. The LIGO experiment is attempting to detect the speed of gravity waves, which are predicted to travel at lightspeed. However, this is not exactly the same thing as the "speed of gravity" it is merely the speed at which changes in gravity propagate.

    I have always described with the following analogy;

    Suppose we are sitting in two small boats on the surface of a lake. There is water covering all the distance between our two boats. If I began knocking my boat, I will generate waves which will travel to your boat. You could measure the speed at which those waves are going past your boat (if you were inclined to do so), but this would not give you the "speed of water", or the "speed of lake". The lake exists throughout all the space between our boats at all times. Likewise, gravity is a curvature of space that fills up a certain area and occupies that area, not a force that propagates through or travel across that area.
     
  12. Jul 14, 2004 #11

    BobG

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    Good analogy.
     
  13. Jul 14, 2004 #12
  14. Jul 16, 2004 #13
    Why would the earth spiral out of orbit? Gravity is a field and it acts on anything surrounding it, even if there is nothing there, there is still gravity, so if the earth moves, there is gravity acting on it from the sun at that position keeping it in orbit. The only time there would be lag is when new matter is produced, lets say instaintaniously, the effect of gravity would kick in at speed of light.
     
  15. Jul 16, 2004 #14
    Gravity is a pretty difficult subject so I refer you to those experts on those links. The arguments revolve around the conservation of angular momentum.

    An analogy would be like swinging a ball in a circle attached to a string that is as long as the distance to the sun- if the tension on the string travelled at the speed of light the ball would not travel in a regular orbit like an ellipse, especially if you started moving as well. If you think gravity is like the presence of mass curving a rubber sheet then it gets pretty complicated I guess. In your argument you seem to be suggesting that the rubber sheet stays curved even after the mass moves away? I was thinking more like as the mass rolls down the rubber sheet, the curvature-indentation on the sheet moves with it.
     
    Last edited: Jul 16, 2004
  16. Jul 17, 2004 #15
    What you're saying kind of makes sense but the experts apparently think it's a problem- they even have a name for it- they call it retardation. They make different assumptions then you apparently when doing the calculations. The way I think of it is like you were flying an airplane in zero visibility and had to rely on a GPS mapping device to fly, but unfortunately the GPS satellite is orbiting the Sun and not the Earth so that it locates where you were 8 minutes ago- using this delayed information, you will surely crash.
     
    Last edited: Jul 17, 2004
  17. Jul 17, 2004 #16

    BobG

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    I think the "8-minute" analogy was kind of unfortunate and obscures the issue. The real issue is that, if gravity propagates at the speed of light, an accelerating object of mass wouldn't be at the center of its own gravitational field, especially if the object's rate of acceleration is constantly changing (which is true for any object orbiting in an ellipse).

    It's kind of difficult to visualize how two fields, both offset from their source, would interact. I think Carlip's explanation is at least plausible, but I'd tend towards believing Van Flandern's point of view (maybe at least partly because its better written).

    Definitely an interesting problem.
     
  18. Jul 17, 2004 #17

    Nereid

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    LIGO test of speed of gravity

    One possible test is if LIGO detects a core collapse supernova event, or an inspiral neutron star pair event, then analysing any differences in when neutrino bursts, light/gammas/other EM were detected should yield limits on the difference between c and the 'speed of gravity'.

    The 'retardation' etc stuff is 'just' math; IF GR, THEN how do objects (e.g. solar system planets) orbit serenely as if Newtonian instantaneous action at a distance were the real thing? At first glance, there might seem to be some contradiction; deeper analysis shows all is calm and bright.

    IIRC, there's a thread or two in SR&GR which goes into this at some length ...
     
  19. Jul 18, 2004 #18
    Gravitation has not delay to start an action.
    But its action forces any object to move with the speed of light at the limit


    Michael
     
  20. Jul 19, 2004 #19

    LURCH

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    The 8-minute argument does not prove that the gravity from the Sun would get weaker any more than it proves that the light from the Sun must get constantly dimmer.
     
  21. Jul 22, 2004 #20
    This topic is rather confusing so let me sum up what i think is the best 2 options. One will be true, one will be false, but i do not know which one is which.

    The Sun is 1 AU away. Light takes 8 minutes to get to us. Earth is pulled towards where the Sun looks like it is. In reality, the Sun is 8 minutes ahead. If the Sun is "turned off" we would not notice the loss of light, or the loss of gravity pulling us, for another 8 minutes.

    The Sun is 1 AU away. Light takes 8 minutes to get to us. Earth is pulled towards where the Sun looks like it will be in 8 minutes time. If the Sun is "turned off" we would notice we are no longer being pulled towards the Sun, and then 8 minutes later, it would go dark.

    Personally, I think it is like electrons in a wire. Even if they move at only 3 cm per hour, the effect on the other end is "almost" instant.
     
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