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Wrong analogy

  1. Sep 10, 2004 #1
    I read recently about an analogy to how general relativity tries to picture gravity.
    Its the one about a rubber sheet and a heavy sphere placed on it deforming it to cause a depression.Another ball in the region will tend to move towards this depression .But doesn't this assume the existence of a force which does this(gravity),which is what we are trying to picture in the first place?So isn't this analogy wrong?
    Also according to relativity, gravity is a deformation of spacetime and is not really a force like others.But quantum theory pictures it the other way as a force due to interaction through graviton.How do you explain this contradiction
  2. jcsd
  3. Sep 10, 2004 #2


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    The resolution of that contradiction is the biggest goal in physics, at this point ni history.

    The important thing is that the presence of a mass causes a deformation of the fabric. In the analogy, gravity is the erason why mass distorts the medium. In the real universe, nobody knows why or by what mechanism mass causes space to warp. All we now is that the model matches observation.
  4. Sep 13, 2004 #3
    yep ,this is a big problem in physics today, the two theories contradict eachother. I believe that Einstein was right in his General theory of relativity, since the graviton is such a farfeched idea (well, it sounds farfetched to me)
  5. Sep 13, 2004 #4


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    Consider the history of gravitation:

    First Aristotle said objects fall because they have a natural or final place towards which all objects tend to move.
    But nobody knew what caused them to move.

    Then Newton said there was a force, gravitation, which caused objects to move according to his laws of motion.
    But nobody knew what caused this force, or how it could act a great distance across a vacuum.

    Then Einstein said that there was no force, but mass locally causes space-time to warp. Objects travel along straight lines, geodesics, along the curved surface of space-time. Mass tells space-time how to curve, curved space-time tells mass how to move.
    But nobody knew how mass curved space-time.

    Then …….. (fill in the blanks yourself!)

    Last edited: Sep 13, 2004
  6. Sep 13, 2004 #5
    Cheez! Thanks Garth. You leave me a way out and a little room to wiggle? :smile:
    Last edited: Sep 13, 2004
  7. Sep 13, 2004 #6
    Yes, the analogy is rather dubious. It's trying to claim that it is the curvature which makes the other ball move towards the depression, when we know that it's because its falling down. If you turned it upside down then the curvature is the same but your intuition is different.
    The problem is that in GR masses curve space-time rather than just space, so even removing a dimension for visualization you still have three dimensions, and then need more dimensions in order to visualise the curvature.
  8. Sep 13, 2004 #7
    I have a new analogy that you should consider and I am calling it the New Toy Model for developing this intuition.

    It also brings you up to speed with the brane scenarios. Some people are better equiped, to envision Three sphere considerations :smile:

    I would like to personally thank Chronos for this linksite in bubble views. I hade been working onthis for a while, and that help was icing on the cake.

    So thanks to Chronos.
    Last edited: Sep 13, 2004
  9. Sep 13, 2004 #8
    I came up with a way to explain it to some friends of mine with no science background at all. As with all of these analogies, it is highly imperfect, but they did seem to get "something" from it. It's completely visual, so it might be hard to explain. I would like input from anyone who can think of refinements (or just tell me it is too far wrong to bother with). It's supposed to be a tool for explaining to non-scientists the "general idea".

    First, I made a "track", like a train track, that curved all over the place in some random manner. I then took a ball and a little toy figurine and put it on the ball. The ball was the earth and figurine was my friend.

    I said that he and Earth are always moving together on this track, though he never feels it, the same way we don't "feel" the Earth turning. I said not to worry about where this track was, or what it was, just that we were always moving forward, and the shape of the track (ie bends) determined which "direction" we were moving at any one time.

    I said as long he stayed on the Earth, this was fine. However, in "reality" the table where the track lay was filled with a horrible maze of overlapping side tracks. I then made a half-circle branch at one point that left the main track and then rejoined it a little later.

    I said if he were to jump up, he would switch tracks, onto this little half circle branch. He and the earth would still be moving forward the whole time, however he was on a slightly different track now. So I moved the ball and the figurine "forward" on the tracks, and low and behold he ended up right back on the Earth farther down the track.

    I said it just so happened that whenver he jumped there would always happen to be one of these side tracks there, and this was "gravity", it wasn't pulling him down, it was just a change in the shape of the track.

    Judging by how freaked out he got, he did seem to get something from this (actually, "they", I explained it this way to a group of non-scientists).

    It struck me a very simple way to get the very basics of the idea across, without getting into complications.

    One guy asked me how airplanes stay "up" then, to which the best I could come up with was that they "continuously switch tracks" to not end up meeting the Earth again.

    Anyway, what do you folks think of this as a simple analogy?
  10. Sep 13, 2004 #9


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    The point about the "rubber sheet analogy" is that in the analogy it is not gravity that makes the second ball move down. You do have the wrong (but common) analogy. MTW give a better one at the beginning of their book 'Gravitation' of ants crawling over an apple. Their paths converge as they go over the area near the stalk. They are actually walking along straight lines over a curved surface. As they look down at their feet they are locally putting one foot in front of the other and moving in a straight line, however, because of the curvature of the surface they are walking upon, globally their paths are curved and converge. An observer not knowing the surface was dimpled might think a force was drawing them together and call it 'gravity'.
    - Garth
  11. Sep 13, 2004 #10


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    I wouldn't call it "wrong". "Incomplete" perhaps. That analogy is a helpful tool for people new to the idea to visualize what's going on. But yes, beyond that, the analogy is an inaccurate model.

    As noted, relativity and quantum mechanics have yet to mesh. However, each is a highly successful explanation of how the universe works. If you're dealing with the macro-universe then use Relativity. If you're exploring the subatomic universe, then use QM. The ultimate answer to "what is gravity" remains a mystery.
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