Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How does gravity get away from a source so it can pull back to the source?

  1. Mar 5, 2009 #1
    If gravity is generated inside the sun by the mass of the sun, how does it initially get away from the sun out to the edges of our solar system(or the edge of the universe) so that it can pull everything back to the sun? I've heard the analogy about how it's like a funnel filled with sand and the sand is all flowing towards the hole in the bottom of the funnel but the problem I find with that is that the sand(representing gravity in this analogy) has already been put into the funnel. In reality, the sand(gravity) is generated inside the hole in the funnel(by the source) and would have to first get away from the hole so that it could flow back towards the hole. The analogy makes no sense without having some means of getting the sand into the funnel in the first place. A fountain in the center of the funnel could send the sand out so it could flow back to the center of the funnel but gravity is a one way force(always pulling back to the source) so that's not really an option. I have the same problem with the analogy of a stretched out rubber sheet with a bowling ball placed on it. Sure, it would make a depression in the sheet but in this particular analogy the sheet represents gravity and once again we have the problem of how the sheet exists where it is to begin with given that it's created inside the bowling ball. Maybe I don't have a good enough imagination to be able to understand physics.
  2. jcsd
  3. Mar 5, 2009 #2


    Staff: Mentor

    I have never heard that odd sand analogy.

    Anyway, if there are two distinct objects then there must be some geometrical relationship between them (otherwise they would not be distinct objects). That geometrical relationship is called spacetime. If there is a lot of stress-energy then that geometrical relationship will be detectably different from a flat Minkowski geometry. This curved geometry is what is called gravity in modern physics (GR).
  4. Mar 5, 2009 #3
  5. Mar 5, 2009 #4


    User Avatar

    Lay off the weed man.
  6. Mar 5, 2009 #5


    User Avatar
    Gold Member

    The sand analogy is incomprehensible as given by you (probably your misrepresentation as you also mess up the following analogy).

    Incorrect. Gravity is not a one way street, even in the Newtonian view.

    No, the sheet represents spacetime and serves to "explain" gravity according to GR.
  7. Mar 6, 2009 #6


    User Avatar
    Science Advisor

    No, the sheet represents curved space only (2 space dimensions), which is why it cannot explain how mass attraction works according to GR. You need the time dimension for this:
    A curved sheet with a depression can also be used to visualize potential in Newtonian gravity. But this must not be confused with GR:
  8. Mar 6, 2009 #7


    User Avatar
    Gold Member

    I guess the conventional sheet analogy (gravitational well) only serves to represent 2D, but the analogies I have seen propose it in 3D and seemed to serve as a basic explanation for gravity in GR. I will concede only conceptual knowledge of GR, so can't comment more than that.

    I can see how the gravitational well like the one linked is a poor analogy.
  9. Mar 6, 2009 #8


    User Avatar
    Science Advisor

    In both cases the pictures are 3D, but they contain a curved 2D manifold which is embedded in flat 3D space. The crucial point is, which of the 4 spacetime dimensions are represented by the curved 2D manifold. To explain mass attraction, you need to include the time dimension, so you have only one space dimension:

    The problem of the rubber sheet is, that it represents two space dimensions, and no time dimension.
  10. Mar 6, 2009 #9
    No, no, you're fine. It's everyone else who has the problem. Just think about screwing a helical screw into a piece of wood. The helix turns and comes backwards towards you and then pushes against the wood. It is the reaction force of this which then pushes the screw forward. Think about it for a bit. It does make sense. This is a way of visualising how a gravity particle can cause a force of attraction despite it moving away from the object.
  11. Mar 6, 2009 #10
    Gravity is not something that travels from the sun to our planet, somehow takes hold of our planet and then travels back to the sun...

    Gravity is just a force between two masses. This force exists due to curvature of space-time as explained by general relativity.
  12. Mar 7, 2009 #11
    You don't quite have the correct mental picture of the screw analogy above. The particle can exert a force of attraction due it's helical spin, although the particle itself continues on it's original direction of travel.
    The standard model will be tested once and for all in the forthcoming LHC experiment.
  13. Mar 10, 2009 #12
    Is there anybody else who believes that a simple particle picture of reality is entirely feasible? (i.e. that in the near future, General Relativity and the fabric of space-time will be proved to be the wrong picture)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook