How do you calculate the speed of gravity?

Discussion in 'Physics & Math' started by Joeman, May 9, 2002.

  1. Joeman Eviiiiiiiil Clown Registered Senior Member

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    First of all does gravitational pull has speed? If our sun disappear all in a sudden, would the gravitational pull stops at the exact instant? If not how long does it take?

    Second question, is gravity considered a field or a force?
     
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  3. thed IT Gopher Registered Senior Member

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    Have to admit I've never seen a calculation for the speed of gravity. <b>James R</b> probably knows more than I on that score. It is generally assumed that the affects of gravity propagate at light speed. So if the sun where to magically disappear it would take a little over 8 minutes before the Earth noticed and took off on a tangential path to it's orbit.
     
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  5. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    I want to see the results of an experiment involving one of those gravity detector thingies and the sun. Point it at sun, see if it lines up with the known position of the sun given that what we see is actually 8.3 minutes late. See if they are the same. Shouldn't be too hard, I guess.
     
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  7. James R Just this guy, you know? Staff Member

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    According to general relativity, gravitational waves travel at the speed of light. This suggests that most changes to the configuration of spacetime propagate at the speed of light. Therefore, if the sun disappeared now, we wouldn't know about it for 8 minutes. This also explains why the direction of the pull of the sun appears to come from where we see the sun now rather than from where we'll see it in 8 minutes time (as it would if gravity was instantaneous).

    Some changes to spacetime can travel faster than light, however. For example, in the inflationary period the early universe expanded faster than the speed of light.
     
  8. empennage Soccer King Registered Senior Member

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    Is that you Fah Que??
     
  9. Joeman Eviiiiiiiil Clown Registered Senior Member

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    Re: Re: How do you calculate the speed of gravity?

    No but I live with him unfortunately

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    I found this forum and I am trying to get him to post here. When he does he can have his avatar back.
     
  10. empennage Soccer King Registered Senior Member

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    Re: Re: Re: How do you calculate the speed of gravity?

    LOL that's funny stuff

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  11. huh??? Registered Senior Member

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    Despite what some people say about g being instantanious, according to albert Einstein gravity travles at C, the speed of light. There's your answer short and sweet.
     
  12. Jthomas Registered Member

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    Suns "gravitational" direction

    -----------------------
    "This also explains why the direction of the pull of the sun appears to come from where we see the sun now rather than from where we'll see it in 8 minutes time (as it would if gravity was instantaneous). "
    --------------------------------------


    JAMESR......

    Was curious, has there been an experiment that measures the local direction of the G-field that points to a different location than the suns current position?? seems to me an impossible measurement. The Earth moves very slightly in a basically circular arc in 8 minutes. If it was completely circular such a measurement would always point to the same place regardless of the amount of time. The Earth's path deviates from circular in 8 mintues by an extremely miniscule amount. I cant imagine this being detectable. Also, the G-field should point to the center of mass of the sun which is not neccesarily the center of the sun. The suns differential rotation and our poor understanding of the distribution of solar material internally would certainly thwart our efforts I would think.
     
  13. Tre Registered Member

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    But, if the sun sudden disappeared, the space-time diagram for a large mass would no longer exist at all. Say that it still takes the same amount of time for the fold to completely disappear, allowing the earth to travel off at a tangent: what would happen if the sun were twice as massive? Would the time be slowed, due to the deeper effect in space-time, or would it be the same?
     
  14. allant Version 1.0 Registered Senior Member

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    Strange as it seems we have not (yet <grin>) measured the speed of gravity. However there are multiple experiments going on to try and detect Gravity Waves. These will acording to theory only exist if gravity has a speed.

    Spinning black holes give the best evidence to date. There have been some results (not accurate enough yet to be sure) that support the idea that these black holes are spining/dragging space around them, which again depends on gravity having a speed.

    By the way the space-time curvature says that the curve will propogate at the speed of light. So even though you may think the curavture would be instantaneous, the propogation of changes in the curve are not.
     
  15. propagation of gravity

    the truth is gravity has never been proven (nor disproven) to have velocity, no one actually knows yet.

    it's possible that gravity is instantaneous, so if the sun were to implode suddenly, we would instantly know about it...

    even though we wouldn't see it for 500 seconds.
     
  16. Enqrypzion Registered Senior Member

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    speed of gravity and mass of light

    How about combining the question of the speed of gravity with the question about the mass of light (elsewhere here on the forum)?
    IF light has a mass AND gravity moves at lightspeed, every photon would seem to me to have a gravitational equivalent of the sonic boom

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    Also every photon will have no forward directed gravity

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    that probably won't be it then

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    IF light has no mass AND gravity has the speed of light, there will be no problems with gravitational booms or whatever, as the 2 don't interact.

    IF light has got a mass AND gravity moves instantanuous, there also would be no problem, although it would mean that space itself is one big 'whirlpool' of small gravitational fields 'created by' photons.. It actually doesn't seem likely to me either.

    of course there are more options, but it seems to me that thinking about it this way shows light has no mass and gravity most likely travels at the speed of light.

    Btw, hia'all!
     
  17. Enqrypzion Registered Senior Member

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    what's up with this radio silence? Respond ppl respond!

    anyone
     
  18. Stryder Keeper of "good" ideas. Valued Senior Member

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    Generally from what I remember Gravity can not go beyond the threshold of light speed.
    This means that you can continue accelerating until either:
    A: You reach light speed
    B: You impact into some surface.

    If you had a bottomless pit then you would stop accelerating at lightspeed.

    There is also an equation for the acceleration, of course this can be different depending on atmospheric pressure. I know it exists, otherwise parachute jumpers wouldn't say 1000-2000-3000 Check while jumping to make sure they are safely away.

    (Namely working out the distance of acceleration away from the craft they just left to know the distance tha they can open their chute.)

    I'm sure you would find this if you were to look at Galileo and his law of uniformed acceleration conducted at the leaning tower of Pisa.
     
  19. c'est moi all is energy and entropy Registered Senior Member

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    ""For example, in the inflationary period the early universe expanded faster than the speed of light.""

    but since that is just a hypothese ...
     
  20. thed IT Gopher Registered Senior Member

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    But a very important hypothesis.
     
  21. c'est moi all is energy and entropy Registered Senior Member

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    yes, but what do you call 'important'? it is a luxury to be able to develop theories about the universe and spent millions of dollars on this

    and btw, it is still just a hypothesis, it does not even merit the title of theory in my eyes (a small detail: background radiation has been predicted by others BEFORE the bigbangers, something that many don't seem to know, and they were almost right about the temperature)

    it really irritates me hearing people talkin' 'bout these things like somehow they were "facts", they aren't
     
  22. thed IT Gopher Registered Senior Member

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    Coupla things

    Almost right does not cut it.

    As you can not see the importance of this allow me to explain. The original models of the Big Bang based on General Realtivity made a number of testable predictions. These where tested and found to be sound. Hence competing theories where ditched. On further analysis this model was found to have serious shortcomings. One of which is the Horizon Problem listed in the link I posted above. The other problems are just as important.

    To fix all these problems, Guth et al proposed the Inflationary model of the Big Bang as a fix. Again it makes testable predictions.

    If experiments fail to verify these tests it means the Inflationary 'fix' to the Big Bang is flawed. Which brings us back to the original problems. Which means the whole model is flawed. Which means it is wrong. Dead flat wrong. A pile of dingo's kidneys. So wrong it is as wrong as a wrong thing. Which also casts serious doubt on Relativity as that spawned the model.

    Is that important enough for you?

    Oh, and yes, I did say it was a hypothesis, read my post again. Definitely has the phrase "important hypothesis" and not "important fact" or "important theory".
     
  23. Gravity

    Gravity has never been shown to have a velocity

    lightspeed or otherwise

    none has been detected at all

    so to say that gravity cannot travel faster than light is like saying
    everything einstein says is true, so we don't need to test it.

    that's not science.

    Gravity may have a velocity, or it may not..

    .. no one knows
     

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