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The visualization of gravity as shown by this video is pretty good at explaining how massive objects bend space, and such bending causes objects around it to fall towards it (a.k.a: gravity).

However, here's what I do not understand. The reason why the experiment in the video works in the first place, is because real gravity -the one in that room- is acting on the marvels and causing those marvels to bend the sheet. In the real world, there must be a force that causes massive objects to bend space in the first place. What is that force?

To put my question in a simple set of equations:

Experiment: Marvel + Sheet + Real Gravity = Sheet Bending (Fake Gravity).

Real World: Planet + Space + (?) = Space Bending (Real Gravity).

There are other questions about this on StackExchange, but they don't seem to address the missing variable in the equations above.

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    Possible duplicate of http://physics.stackexchange.com/q/51198/; see links therein. See also http://physics.stackexchange.com/q/3324/. – HDE 226868 Jul 28 '15 at 18:36
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    You should just plain ignore that video. – paparazzo Jul 28 '15 at 18:37
  • The above comment by @Frisbee is totally correct. You have to keep following the math, not your physical intuition because you, me, everybody on Earth can only think in 3 D space, and the real world is 4 D spacetime. You will much better off in the long run trying to learn the math if you want to really understand advanced physics. –  Jul 28 '15 at 18:48
  • @AcidJazz Well, can you give an approximate explanation of the variable missing on the second equation of the set? Is it a force? Does it even have a name? Basically, what causes mass to bend spacetime in the first place? –  Jul 28 '15 at 18:53
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    "The visualization of gravity as shown by this video is pretty good at explaining how massive objects bend space" If that the the usual #@&(#%&(#% rubber sheet, then I say: no, it's not; it's rubbish. And the question that you've developed here is only one of several reasons for its being junk. – dmckee --- ex-moderator kitten Jul 28 '15 at 19:04

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The analogy between how a massive object bends a rubber sheet and how sources of mass-energy-momentum bend spacetime is a poor one. It's essentially comparing the mechanical deformation of a two dimensional sheet in space to the curvature of four dimensional (3+1) spacetime, which have a mathematical similarity, but not a physical one. In Newton's theory of universal gravitation, gravity is a force which acts pairwise between massive objects. In General Relativity, it is no longer a force, but rather a geometric affect of energetic objects on spacetime. That is, things with energy are the source of curvature in four-dimensional spacetime (3 spatial, 1 temporal). Because gravity is no longer a force, objects in curved spacetime that experience no net force still travel in what are called geodesics (paths of the shortest distance on a curved surface).

Ultima
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    But that still leaves the question open as to what is the cause of massive objects curving spacetime in the first place. –  Jul 28 '15 at 18:56
  • I think you are missing the point. Mass curves spacetime. Cause: mass. Effect: curvature of spacetime. There is nothing more fundamental at work here. The mathematics itself is, in some sense, (curvature of spacetime) = (energy content). – Ultima Jul 28 '15 at 19:09
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    Has anyone or can anyone actually answer this question? What Bends space time, how does mass do it? And I never seen answer when I see this question asked? – Bill Alsept May 09 '17 at 16:22
  • @BillAlsept I think you're looking for an answer like "space is empty so wherever mass exists space cannot exist, thus shrinking the actual (empty) distance between objects, which cause the localized curvature we observe" although I have no idea if that's really the case. – Michael Nov 21 '18 at 02:59