4

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?

Nzall
  • 388
  • 3
    http://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. – Javier Jun 03 '14 at 14:41
  • 1
    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? – Jim Jun 03 '14 at 14:42
  • 1
    @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? – Nzall Jun 03 '14 at 14:48
  • 1
    Possible duplicates: http://physics.stackexchange.com/q/65363/2451 , http://physics.stackexchange.com/a/13839/2451 , http://physics.stackexchange.com/q/7781/2451 and links therein. – Qmechanic Jun 03 '14 at 14:49
  • 1
    possible duplicate of Bowling ball on a rubber sheet analogy - what pulls the ball down – John Rennie Jun 03 '14 at 14:52
  • 3
    @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 – Jim Jun 03 '14 at 14:53
  • @NateKerkhofs: You are absolutely right, and there is yet another thing. If we have a large curvature around a large mass, and we have a smaller object "rolling down" towards the large mass, then ... why is it rolling down at all? Rolling down (a hill) requires real force (gravitation) underneath (this hill). If there is no force and only the curvature, an object put on such a slope will not move - it will stay right where you put it despite the curvature. Curvature by itself does not make things move. – bright magus Jun 03 '14 at 15:05
  • 1
    And even more importantly than all of this, it's not space that's curved. It's spacetime that is curved. If space is perfectly flat, it's still possible to observe curved paths. For instance, with the metric $ds^{2} = -(1-\phi(r))dt^{2} + \delta_{ij}dx^{i}dx^{j}$. – Zo the Relativist Jun 03 '14 at 15:19

1 Answers1

2

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.

Normie
  • 1,456