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Speed of gravity

  1. Apr 25, 2008 #1
    Does gravity travel at the speed of light? If so, how can a black holes suck in photons?
     
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  3. Apr 25, 2008 #2

    rbj

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    it is believed to and is assumed to in General Relativity.

    i believe that there is some astronomical measure they did (wasn't it when that asteroid or comet crashed into Jupiter?) that determined to within 20% that GR was correct about it.
     
  4. Apr 25, 2008 #3

    JesseM

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  5. Apr 25, 2008 #4

    George Jones

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    A quasar passed behind Jupiter. The announced results were controversial; see https://www.physicsforums.com/showpost.php?p=681816&postcount=12".
     
    Last edited by a moderator: Apr 23, 2017
  6. Apr 25, 2008 #5

    DaveC426913

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    Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.
     
  7. Apr 25, 2008 #6
    So the force of gravity is instantaneous, but changes in gravity move at the speed of light?
     
  8. Apr 25, 2008 #7

    rbj

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    think a bit about the meaningfulness of your question.
     
  9. Apr 26, 2008 #8
    Let me know if this sounds as dumb as my previous question. If an object was to spontaneously appear within the earth's atmosphere, would it feel gravity instantly? If the earth's mass was split in half after the object appeared, would it take time for the object to feel the difference?
     
  10. Apr 26, 2008 #9
    may be you could think about a curvature of spacetime instead force of gravity :)

    no if an object suddenly appear , it take some time that change in gravity will reach the earth.
     
  11. Apr 26, 2008 #10

    DaveC426913

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    Gravity is omnipresent; it is there in the first place (because it is is in fact the curvature of space-time).


    This is a dangerous analogy, but it's like a swimmer asking 'I see waves traveling at 100mph across the lake, but what is the speed of the lake'?
     
  12. Apr 30, 2008 #11
    Hi Nuby,
    Your spontaneously appearing object should feel the Earth's gravity instantly. This is easy to visualise using the (aprozimate) rubber sheet analogy with masses deforming the sheet to create gradients. You could call the map of those gradiants the spacetime "landscape". Your object would appear in this ready made landscape and instantly respond to the curvature of that landscape.

    Now if half the Earth was to disappear then, yes it would take time for the object to "notice" because the landscape deforms at the speed of light. There is a caveat here, that it would be impossible for anything to suddenly appear or dissappear. For example if the Sun was somehow anhilated by some antimatter, it would turn into a bunch of photons and the energy of those photons would still behave as a gravitational mass. It would take about 8 minutes for the photons to radiate outwards past the orbital radius of the Earth from the Sun, after which the Earth would no longer continue in its normal aproximately circular orbit.

    To more directly answer your original question "Does gravity travel at the speed of light? If so, how can a black holes suck in photons?" there is no requirement for something like gravitons from the black hole to "chase" after photons in order to draw them back. The photons simply move in the ready made spacetime landscape around the black hole.
     
    Last edited: Apr 30, 2008
  13. Apr 30, 2008 #12

    rbj

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    i really meant to say and should have said think about the meaning of the question. it's more that the meaning of the two were inconsistant.

    that's the direct answer i was groping for.

    it's a good analogy, though, me thinks. but we have to add, that no matter how hard the swimmer is swimming nor in what direction, the speed of the waves going across the lake is the same for that swimmer. and equal to any other swimmers' measurements. assuming they can accurately measure the wavespeed to a precision of much better than their swim speeds. and, they can't really measure their swim speeds except relative to each other. they can't measure their swim speeds against the water because they think they are drifting on it. so maybe a better analogy is that all of the swimmers are really just sitting around on very bouyant light rafts that skim across the water without any drag.
     
    Last edited: Apr 30, 2008
  14. May 1, 2008 #13
    *gravitation

    Gravity is localized gravitation.
     
  15. May 1, 2008 #14

    DaveC426913

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    This is where the analogy breaks down, so we don;t carry it that far. All analogies break down at a certain level of detail. If they didn't, they would be models!
     
  16. May 1, 2008 #15
    Gravity is an effect caused by a form of energy. The speed can vary anywhere between relative zero to infinity.
     
  17. May 1, 2008 #16

    DaveC426913

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    Really?

    So, I could set up a gravity-making machine and transmit singles across the diameter of the universe in zero time.

    Huh.

    Seems like one of those little facts that has escaped, oh say, the entire collective body of physicists on the planet Earth.:uhh:
     
  18. May 1, 2008 #17
    yes you could

    No you couldn't.

    'the entire collective body of physicists on the planet Earth' haven't come up with any idea of what causes gravity so it's not really relevant making that comment.
     
  19. May 1, 2008 #18
    Gravity is caused by mass. And mass is a potential form of energy, right? Interesting.
     
  20. May 1, 2008 #19

    DaveC426913

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    But they do know that it doesn't travel at infinite speed. Which is why what you said makes no sense.
     
  21. May 1, 2008 #20

    jambaugh

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    One must be very careful in GR of thought experiments where things "appear instantly out of no-where". Built into the theory is the continuity equations which prevent such from happening. The source of gravity is stress-energy. There's no way for energy (mass) to appear at a point without there being some energy-momentum current transmitting it there.

    If you violate such continuity you've stepped entirely outside the theory. You can suppose that you create for example monopolar and dipolar waves which cannot be done physically and the theory of how such should be have is pure theological speculation.

    Related to the OP's question about black holes consider also that throwing negative charges into the black hole will not prevent their charges from being felt outside. You will find that the black hole will manifest the same electrical field (and gravity) as a larger star of the same mass containing these charges.
     
  22. May 2, 2008 #21
    I may have gone over the top with the 'infinite' speed part but my point was that we shouldn't necessarily think of its speed being a universal constant or that it should be constrained by factors which dictate the speed of light.
     
  23. May 2, 2008 #22

    DaveC426913

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    Oh I see. You were saying something akin to 'scientists have narrowed down the speed of gravity to between zero and inifinity' - a tongue-in-cheek way of saying 'we just don't know'.
     
  24. May 2, 2008 #23

    rbj

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    that's fully incorrect. we should think of both EM and gravity as well as all other fundamental interactions as propagating through a vacuum with this same speed (it's not a function of which interaction, they're all "trying" to be instantaneous, it's a property of space and time that these ostensibly instantaneous interactions do not have effect instantly on distant object, as viewed by someone who is equi-distant from both source and destination object), and since we can only measure dimensionless quantities, there is no meaning to this speed being a different value. if it were'nt "constant', we wouldn't be able to tell.

    now, conceivably, if the speeds of propagation of different interactions were different, we could detect that (and they're trying, there was one astronomical measurement that claims to have measured the speeds of gravity vs. light to be equal to within 20%). but with other quantitative predictions of GR being confirmed in experiment, with the fact that we have measured G (in terms of our meter sticks, cesium clocks, and kilogram prototypes) to 5 digits, if the speed of gravity was much different than c, that constant in the Einstien field equation (G/c[sup]4[/sup]) would 4 times more different, and the quantitative results would be different than what was measured.

    whatever property of space and time that makes disturbances of EM propagate at a finite speed is what makes disturbances of gravity propagate at the same finite speed (as well as nuclear interactions, even though it doesn't make much difference for them, since the interaction is between particles very close to each other).
     
  25. May 2, 2008 #24
    Wow. How many non-answers can a single thread generate.

    Yes, gravitational forces propagate at the speed of light according to all accepted theory and known experiments. If the hypothesized graviton (gravity force carrier) exists, it will travel at the speed of light like a photon.

    You needn't even consider hypotheticals to understand this. Consider the planet Mercury. If you calculate the orbit using Newtonian mechanics (which assume gravitation is an instantaneous force) you will find that Mercury's true orbit is slightly different than what you calculate. The perihelion, the point at which the planet is closest to the Sun, actually moves over the course of time. This is explained by General Relativity due to the curvature of spacetime or, another way of looking at it, the fact that gravitational forces propagate at the speed of light.
     
  26. May 2, 2008 #25

    DaveC426913

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    While well-aware of the precession of Mercury, and its explanation via the curvature of space-time a la GR, I'm not sure it is proof positive of the speed of gravitational waves.
     
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