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Does gravity have speed ?

  1. Jun 17, 2010 #1
    I was just wondering.

    If you imagine that there will be created an very vast and heavy obejct somewhere in the universe, just like that, and you imagine that the only thing that existed in the universe was the solar system. The object is created 1.000.000 light years away from the solar system. Would we then experience the gravity from this object instantaneously, or does the gravity have some sort of speed, and therefor a timeperiod before it reaches us ?

    I know that every object bend time and space some how. But is this instantaneously, or does it take some time, like dropping a ball in pool fx. ?
     
  2. jcsd
  3. Jun 17, 2010 #2

    Jonathan Scott

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    According to Einstein's General Theory of Relativity, changes in gravitational fields propagate at the speed of light. Experimental results are consistent with the theoretical predictions, although as far as I know there has been no experiment which directly verifies this result without relying on additional theoretical assumptions.

    It's very difficult to create a significant gravitational change. Not only do the laws of physics rule out spontaneous creation of massive objects, but they also mean that you can't do anything sudden to move a mass without moving a corresponding amount of mass the other way at the same time, cancelling out the effect. The only sort of thing you can do which has an effect at a distance is to change the arrangement of mass as quickly as possible, for example as if the sun exploded into two equal parts which moved in opposite directions, and even the effect of that is slow to take effect as seen from a distance.
     
  4. Jun 18, 2010 #3

    Chronos

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    A thought experiment might be in order - like the earth orbiting the sun and orbital perturbations observed in the motion of the moon.
     
  5. Jun 19, 2010 #4
    Hello.
    I used to be a newbe in this forum, but this questione is easily answerable to me.
    someone allready answered to you that you shoul read the 2nd theory by Albert Einstein - general theory of relativity.

    the trick about this is that gravity is being emited,created, effected by every atom that make up our earth, a car a ship or a man they are all pulling other objects towards themselves, since earth is bigger an closer than everything else than it is pulling the strongest.

    if i had a very sensitive aparatus i could gravitationally sense you that you are standing next to me.

    now the trick is that if gravity would travel faster than speed of light (e.g. instantaniously) than this sensitive gravimeter could feel something big moving on the serfice of earth from a very big distance, lets say from couple of lightyears away. and that means that by something big mooving in a prearranged order you could communicate to others far far away disobaying the speedlimit - speed of light.

    unfortunatelly this is not possible.
     
  6. Jun 19, 2010 #5
    Ah, but entangled particles act faster than light, and we cannot gain information from them without Classical verification. What if the same is true for gravity? I don't know that it is, in fact the research I've seen on gravitational waves seem to indicate that they do in fact, propagate at c. That being said, you cannot use your description as reasoning for a particular "speed of gravity".
     
  7. Jun 20, 2010 #6

    Nabeshin

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    If you've ever studied electromagnetism, the situation in general relativity is completely analogous (in the linearized gravitational wave case, at least!). The changes in the "gravitational field" propagate at the speed of light. The analogy even extends further than the propagation of gravitational waves: the "gravitational potential" pancakes up perpendicular to the motion of a boosted kerr-schild black hole, just as the same thing happens for a moving point charge.
     
  8. Jun 20, 2010 #7
    That makes sense to me.
     
  9. Jun 21, 2010 #8

    Chronos

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    Hulse proved the existence of gravitational waves several years ago - e.g., http://en.wikipedia.org/wiki/PSR_B1913+16
    I fail to perceive the crux of your objection, jonathan scott. It looks cranky to me.
     
  10. Jun 21, 2010 #9
    I think this video would also
    answer your question. You'll see that at the end, they make the sun suddenly disappear
    and it shows what would happen then.

    It is part from The Elegant Universe by Brian Greene, you can find the whole documentary
    on youtube, which explains a lot of things. Other people here may say if this is accurate
    or not.
     
    Last edited by a moderator: Sep 25, 2014
  11. Jun 21, 2010 #10

    Jonathan Scott

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    I'm sure that gravitational changes propagate at c, and the rate of loss of energy of the Hulse binary pulsar, matching the predictions of gravitational wave theory, is one of the strong pieces of evidence for that. However, I think it's a bit misleading to simply state that the speed of gravity has been experimentally verified, in that as far as I know it has not been directly measured in a way which doesn't rely on other theoretical assumptions. In particular Kopeikin and Fomalont claimed to have measured the speed of gravity from effects on the apparent position of Jupiter, but others reckon that their logic was effectively circular because of the theoretical assumptions they used in the calculation.
     
  12. Jun 22, 2010 #11

    Chronos

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    What experimentally derived direct measurements do you have in mind that do not rely on any theoretical assumptions?
     
  13. Jun 22, 2010 #12

    Jonathan Scott

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    Come on - it's a matter of degree! I mainly had Kopeikin in mind - he claims to have measured the speed of gravity, but others (including Clifford Will) have suggested that his reasoning was circular because of being based on using theory which effectively assumes that result.

    The fact that the rate of loss of energy in the Hulse binary pulsar matches the GR prediction very accurately confirms that GR gives the right result, and since the GR calculation depends on gravity changes propagating at c, it suggests that is correct, but it seems quite indirect to me. If in contrast we were to observe some violent event via light or other E/M and detect a corresponding gravity effect in LIGO or similar at the same time then that would be direct evidence (although if it were close enough to be detected we might have other things to think about).
     
  14. Jun 22, 2010 #13
    Well, that's a cheery thought. *cry*

    I think that the periodic nature of emissions of EM radiation and gravity waves can be matched and confirmed with LISA, if not LIGO. It might take a while, but as theoretical assumptions go, it's a bet I'd take... Chronos?
     
  15. Jun 23, 2010 #14

    Chronos

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    It is easy to criticize any experiment, and, in fact, a necessary part of the scientific process, jonathan. I think it is misleading to criticize experimental results because they rely on assumptions. it would be pointless to conduct any observation without them.
     
  16. Jun 23, 2010 #15

    Jonathan Scott

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    Of course experiments rely on assumptions. The important point is that the strength of the experimental result is constrained by the strength of the theoretical assumptions on which it is based.
     
  17. Jun 23, 2010 #16
    These are some very strong theoretical assumptions grounded in a highly successful and predictive theory (GR).
     
  18. Jun 23, 2010 #17

    Jonathan Scott

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    Of course. And the indirect evidence that the speed of gravity is c because of the success of GR is quite overwhelming.

    But if you want to directly measure the speed of gravity, as a test of GR (as Kopeikin and Fomalont claim to have done), you can't rely on GR theory for your calculations, as GR already assumes it is c. It's very tricky to work out what you can and cannot assume to make it a meaningful experiment. It seems quite plausible that they have succeeded, but I believe their theoretical assumptions are still being disputed.
     
  19. Jun 23, 2010 #18
    I understand that, but I think Chronos does too, and you're being needlessly precise in this case, or at least a bit harsh in your precision.
     
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