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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?
     
  2. jcsd
  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 here.
     
  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.
     
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