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Please help me understand gravity.

  1. Jul 9, 2014 #1
    I am slowly learning more and more about physics, but one issue I've come across is understanding how gravity can have a speed. I was under the impression that gravity couldn't have a speed. I know gravity can be visualized as just curvature in spacetime, but how is it that something that is seemingly everywhere can even have a speed? What is propagation and how does it relate to gravity and gravitational waves?

    Thank you for your time and knowledge.
     
  2. jcsd
  3. Jul 9, 2014 #2

    A.T.

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    Right. With a static configuration of gravity sources nothing propagates. Gravity, described as spacetime geometry, is just there, everywhere.

    Changes in spacetime geometry propagate as gravitational waves, with a finite speed. Compare this to static E-fields vs. EM-waves which propagate with a finite speed.
     
  4. Jul 15, 2014 #3
    As you mentioned, gravity effects spacetime - just as a bowling ball does on a trampoline.
    What you wanted to know is about gravity having speed - what i want to correct is that gravity doesnt have a 'speed' as such. Gravity is an attraction - like a magnet. Magnets do not have a speed, but they do have a variable representing its magnetic strength.

    So, to explain how gravity works, imagine the bowling ball on a trampoline. If you 'place' a tennis ball, it rolls downhill towards the bowling ball. Just like will planets, the smaller mass is attracted to the larger.

    I hope this helps. This is how I envision gravitational attraction
     
  5. Jul 15, 2014 #4

    Drakkith

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    A minor correction. Gravity, the attraction of one object to another, is an effect. It is caused by the curvature of spacetime by mass and energy. Gravity doesn't affect spacetime, it is caused by spacetime.


    Agreed. The "speed of gravity" is actually the speed at which changes in the metric propagate.

    And the larger mass is attracted to the smaller one as well. :biggrin:
     
  6. Jul 15, 2014 #5

    Drakkith

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    Imagine throwing a pebble into a pond. The ripples in the water move outwards from the point of impact with a certain speed. We call this movement, propagation. IE, the wave propagates outward from the source. This is the case for all waves, including gravitational and EM waves.
     
  7. Jul 15, 2014 #6
    Ah, thank you for that! My mistake aha. You are right, gravity is the effect caused by the curvature of space time just like how the speed at which the tennis ball moves towards the bowling ball if due to the curvature of the trampoline!
     
  8. Jul 15, 2014 #7

    Bandersnatch

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    The trampoline(aka rubber sheet) analogy is flawed in many ways. For one, you're explaining gravity with gravity(What makes the bowling ball push on the trampoline? What makes the tennins ball move down the indentation?). Another problem is that you're supposed to represent the curvature of spacetime, and there is no time dimension in the picture.

    We've had many threads about it, here's the most recent one:
    https://www.physicsforums.com/showthread.php?t=760793
     
  9. Jul 15, 2014 #8
    Its true that it isn't a sound analogy, but It does show the same outcome, does it not?
     
  10. Jul 15, 2014 #9

    Bandersnatch

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    No, it doesn't. You can't get anything similar to the precession of Mercury's orbit out of it, for example.
    It's a sound visualisation of gravitational potential well, and how bodies move according to newtonian gravity. But once you invoke spacetime, you're talking about General Relativity, and the analogy is no longer valid.
     
  11. Jul 15, 2014 #10
    I see... well, I'll keep that in mind. Thanks
     
  12. Jul 15, 2014 #11

    Chronos

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