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Possible Duplicate:
Does the curvature of spacetime theory assume gravity?

Since I read Cosmos long ago, I see the same analogy about the balls rolling on a rubber sheet used to explain how gravity works. But a ball rolls on a surface because gravity is pulling it down. In space it will follow a straight line and go over any hole on the surface.

So, in the analogy we all know, where the curvature of the rubber sheet is gravity, what is pulling the ball down? If space is curved, what keeps an object attached to that "surface" of space?

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    $\begingroup$ Possible duplicates: physics.stackexchange.com/a/13839/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Jan 14, 2013 at 16:25
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    $\begingroup$ That's just one of the reasons it's a rotten analogy. Pay no attention to the stupid rubber sheet metaphor. Nor to the man behind the curtain. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jan 14, 2013 at 16:34
  • $\begingroup$ Someone has been reading xkcd. $\endgroup$
    – John Alexiou
    Commented Jan 14, 2013 at 16:43
  • $\begingroup$ I think you have to take it as in real life things naturally go in a straight line, but in the analogy of the rubber sheet the natural path is to follow the rubber sheet. The same way if you look at a straight line through a non uniform piece of glass the line doesn't look straight but it still is. $\endgroup$
    – Jonathan.
    Commented Jan 14, 2013 at 16:45
  • $\begingroup$ I think it's useful to have this question asked exactly once because it's a very common misunderstanding, and it is the kind of conceptual issue we deal with. And as far as I can tell, it hasn't been asked here before. $\endgroup$
    – David Z
    Commented Jan 14, 2013 at 18:22

2 Answers 2

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This is one of the problems with analogies (when not treated as analogies). Obviously, spacetime is not a sheet and masses are not rolling on it. You encounter these kinds of problems whenever you take an analogy too far or out of its context.

For example, you get in the same mess when you try to describe (attractive) electromagnetic forces as rubberbands between particles. Because what's going on inside the rubberbands? That's electromagnetic forces at work! So, as with your question, this analogy gets you into trouble if you ask about the nature of the elements in the analogy.

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The problem is that once you go so far and take "actual" gravity into account, the analogy does not work anymore. The only use of the analogy is to show that something is curved in the presence of massive bodies and that this curvature affects the path of other objects.

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    $\begingroup$ But that still leaves the question open as to what force causes objects' path to be affected by this curvature of space. This, I still do not understand. $\endgroup$
    – user73879
    Commented Jul 28, 2015 at 18:50
  • $\begingroup$ Very good explanation. First explanation of this analogy which makes sense to me. $\endgroup$
    – Vojtěch
    Commented Aug 28, 2016 at 19:02

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