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Why do objects rest on space thus causing gravity?

  1. Mar 14, 2015 #1
    So gravity is supposedly caused by objects such as planets, stars, etc. resting on space which curves space. The more massive the object, the larger the bulge it creates, the stronger the gravity.

    My question is, why do these objects even rest on space in the first place? It's as if there's a downward pulling force in space itself. There must be something other than gravity pulling objects down because otherwise, orbits won't exist.

    I apologize in advance if I just made a really idiotic thread. I'm not a pro at physics, but I'm hoping to learn.
     
  2. jcsd
  3. Mar 14, 2015 #2

    DaveC426913

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    You're talking about the "rubber sheet" analogy of gravity. It is only an analogy. An object resting on a rubber sheet just happens to mimic - in two dimensions - a similar geometry to masses in spacetime in 3 dimensions.

    Objects do not "rest on space". There is no "downward pulling".
     
  4. Mar 14, 2015 #3
    Then how do orbits work?
     
  5. Mar 14, 2015 #4

    DaveC426913

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    Well, that's very hard to describe without using the equations. This is why we use analogies like the rubber sheet.
     
  6. Mar 15, 2015 #5
    Is there at least a basic principle or concept?
     
  7. Mar 15, 2015 #6

    russ_watters

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    Well, yes -- it's the rubber sheet analogy. It works fine, as long as you don't read past it.
     
  8. Mar 15, 2015 #7
    The analogy though uses the gravity on Earth as some sort of downward pull. If there's no downward pull in space and space is 2D, then how do satellites orbit? Isn't an orbit caused when a satellite is caught in the bulge of space? And that satellite's momentum keeps it in orbit rather than it plummeting to the center of the bulge?
     
  9. Mar 15, 2015 #8

    Drakkith

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    The bending of the rubber sheet is an analogy for the bending of space. The geometry of space is actually changing because of the presence of mass and energy. So, like a ball rolling across the curved surface of the rubber sheet, an object's path through space is altered because of this change in geometry.
     
  10. Mar 15, 2015 #9

    A.T.

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    It makes perfect sense... if you don't think about it.
     
  11. Mar 15, 2015 #10

    A.T.

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    No. The spatial distortion, represented by the rubber sheet, has only relatively tiny effects for orbiting bodies. For example, the precession of Mercury's orbit is partialy due to that spatial distortion. The orbiting itself (as opposed to moving straight) is mostly due to the distortion of the time dimension, with respect to space:



     
  12. Mar 15, 2015 #11

    russ_watters

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    I've never heard it described that way.
    The curvature of space isn't "downward", it's 3-dimensional, not 2-dimensional. The satellite feels a pull toward the earth.
     
  13. Mar 16, 2015 #12

    A.T.

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    Right, but the relevant curvature here is that of space-time, which is 4-dimensional.

    The satellite doesn't "feel a pull", it measures zero proper acceleration.
     
  14. Mar 16, 2015 #13

    DaveC426913

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    This is a common problem I have seen people have with the rubber sheet analogy. They see that a heavy mass is pulled downward onto a rubber sheet by gravity, and they ask why the analogy is circular: it uses gravity to explain gravity.

    They are taking the analogy literally. They should really just be seeing the geometry, not the causes of the geometry in the analogy.
     
  15. Mar 16, 2015 #14

    A.T.

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    Yes, and when you do that, then it becomes obvious that the whole analogy is bad, because spatial geometry alone cannot explain the"attraction" that causes orbits. If you care only about the geometry, and not about up & down, then you can flip the surface upside down, with a hill instead of the dent. But that would create "repulsion" of the rolling balls.
     
  16. Mar 18, 2015 #15
    This has really bugged me about gravity=space curvature.
    I get that two objects moving with respect to each other would be diverted from their normal paths by mass-induced curved space. I get that totally, it makes complete intuitive sense.
    But to consider two objects at complete rest to each other, why would curved space (or even curved spacetime) start them moving towards each other?
    If they're not initially moving, then no curvature would seem to start the ball moving so to speak.
    (In the sheet analogy, obviously you've got actual downward pulling gravity that gets the ball moving, but that's just the poor analogy at work)

    I just don't get it for actual 4-dimensional gravity.
     
  17. Mar 18, 2015 #16

    Drakkith

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    Because they arent at rest. They are moving through spacetime.
     
  18. Mar 18, 2015 #17

    A.T.

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    Curved space won't. Curved space-time will.
    They advance in space-time, even when at rest in space.
    For a vertical fall, you just need one spatial dimension and time, as shown in the videos in post #10. But this doesn't work for whole orbits, as they require two spatial dimensions and time, so 3 dimensions which are then distorted and would need to be embedded in higher than 3D flat space for visualization.
     
  19. Mar 18, 2015 #18
    The video was helpful.
    I guess the difficult part is accepting the idea of "traveling through time" as if it's just something the apple is moving through.
    I think of time as just how rapidly or slowly things move with respect to other things (like clocks, photons, etc.)
    I.e. without any movement in the universe I couldn't conceive of time, so it seems like there would have to be movement in the first place for time to have any meaning.
    But for space-time, time exists separately and conversely causes movement (by bending space)

    Although the equations work I'm told :-)
     
  20. Mar 18, 2015 #19

    A.T.

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    Things are aging without moving.
    The whole point of space-time is that space and time are not separate.
     
  21. Mar 18, 2015 #20
    No, I don't think you'd age if all your molecules, quarks etc. stopped moving.

    Yeah space-time is one entity. What i mean is "time is something separate in the sense of being a dimension", instead of just how one measures the regularity of movment of objects in space.
     
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