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A way some people explain (or try to explain) how gravity works is using space-time curvature: an object with high mass distorts the surrounding space-time plane like a bowling ball distorts a sheet of plastic and the surrounding objects are always trying to roll down to that other object.

However, this explanation to me seems flawed, because in order to work, it needs to be influenced by the very thing it tries to explain. The reason a bowling ball distorts a sheet is because Earth's gravity is affecting it. However, the space-time curvature which this distortion is an analogy to is trying to explain gravity. The implication of this space-time curvature is that something is pulling down on the space-time plane, but this pulling itself requires gravity from a perpendicular source, it seems to me.

I'm having difficulty wrapping my head around this. The method we use to explain gravity itself uses gravity to work, sort of like some kind of recursive loop. it's comparable to "to get fuel, we need to drive, and to drive, we need fuel", and I don't know enough about gravity to figure this out myself.

Am I understanding this analogy correctly, or are there deeper things at play?

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marked as duplicate by John Rennie, Alfred Centauri, Kyle Kanos, Brandon Enright, Qmechanic Jun 3 '14 at 16:12

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    $\begingroup$ xkcd.com/895 But seriously, the rubber sheet thing is just an analogy, and you're correct in saying that is somewhat recursive. Einstein's equations don't say why spacetime is curved when there is mass; they just say that it is, and as far as we know (or as far as I know) there's no reason why it must be so. $\endgroup$ – Javier Jun 3 '14 at 14:41
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    $\begingroup$ I'm unclear about your question. Are you asking a specific question about the analogy or are you asking for a better general explanation of how gravity curves spacetime? $\endgroup$ – Jim Jun 3 '14 at 14:42
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    $\begingroup$ @Jim Javier Badia kinda explains my question. In essence, to understand the analogy you need to understand the thing that it's an analogy for, and it's that which I'm struggling to cope with. Shouldn't an analogy be self-contained? $\endgroup$ – Nzall Jun 3 '14 at 14:48
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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/65363/2451 , physics.stackexchange.com/a/13839/2451 , physics.stackexchange.com/q/7781/2451 and links therein. $\endgroup$ – Qmechanic Jun 3 '14 at 14:49
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    $\begingroup$ @NateKerkhofs No an analogy does not have to be self-contained. An analogy is only meant to describe a particular aspect of an idea. Very few analogies do not break down at some point. In this case, the bowling ball on a sheet is meant to show how gravity affects spacetime, it has nothing to do with why that happens $\endgroup$ – Jim Jun 3 '14 at 14:53
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I think the curvature of space due to presence of massive mass is just an analogy. The space is NOT a membrane and mass doesnt push it downward.

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