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## Main Question or Discussion Point

Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?

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Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?

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Dale

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- #3

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i was just using those as example because it is most seen and worded as space time being one dimensional and the water one was just an example of it being surrounding and 3d that surrounds the matter.

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As Dale said, the analogies are seriously flawed. Space-time isn't a THING at all, it is a framework in which matter and energy exist. It's just like length. It isn't a thing, but you can measure the relative positions of things in it.i was just using those as example because it is most seen and worded as space time being one dimensional and the water one was just an example of it being surrounding and 3d that surrounds the matter.

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what i was trying to refer to was the concept of the gravity of matter bending space time and how it would be, since most diagrams show it as only bending space/time on one side.As Dale said, the analogies are seriously flawed. Space-time isn't a THING at all, it is a framework in which matter and energy exist. It's just like length. It isn't a thing, but you can measure the relative positions of things in it.

- #6

Nugatory

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Those diagrams are seriously misleading.what i was trying to refer to was the concept of the gravity of matter bending space time and how it would be, since most diagrams show it as only bending space/time on one side.

Here's a recent thread you may find helpful: https://www.physicsforums.com/showthread.php?t=760793 and pay particular attention to the video in #3 of that thread.

- #7

Drakkith

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Actually it's a mix of both. The "bending" of spacetime is like the trampoline, but this happens in three dimensions like the water surrounding a submerged object. The best picture I've seen is a 3d grid of lines that bend when passing by a massive object.Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?

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timmdeeg

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In my opinion any static picture might be misleading as the function of time being part of "space-time" is missing. I therefore prefer to identify the action of gravity with the motion of neighbouring inertial bodies. If their geodesics are bent towards or away from each other (due to relative acceleration) then the space-time in this region is curved, otherwise it is flat.

In this sense you can use the trampoline picture by watching how the distance between some balls which are rolling towards the center develops over time.

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A.T.

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If time is a dimension of the diagram, then the diagram itself can be static and yet show the development over time. That is the whole point of a space-time diagram. But it still can be helpful to animate a space-time diagram, to show the progress of time:In my opinion any static picture might be misleading as the function of time being part of "space-time" is missing.

https://www.youtube.com/watch?v=jlTVIMOix3I

https://www.youtube.com/watch?v=DdC0QN6f3G4

The trampoline is not a space-time diagram, just a space diagram. But the worldlines of free fallers are geodesics in space-time, not space.If their geodesics are bent towards or away from each other (due to relative acceleration) then the space-time in this region is curved, otherwise it is flat. In this sense you can use the trampoline picture by watching how the distance between some balls which are rolling towards the center develops over time.

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Chronos

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- #11

Dale

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The mathematical language used to describe spacetime is called Riemannian geometry.i was just using those as example because it is most seen and worded as space time being one dimensional and the water one was just an example of it being surrounding and 3d that surrounds the matter.

Riemannian geometry is used to describe geometry on spaces (called manifolds) that may be curved, such as the 2D surface of a sphere. It describes the curved manifold geometry entirely from quantities that can be measured within the manifold, e.g. on the 2D surface of a sphere without reference to the 3rd dimension.

In Riemannian geometry spacetime is a 4D curved manifold with a "signature" that leads to 3 dimensions of space and one dimension of time.

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timmdeeg

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That's right. In case my wording wasn't correct, kindly let me know.The trampoline is not a space-time diagram, just a space diagram. But the worldlines of free fallers are geodesics in space-time, not space.

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Every time I see someone use the phraseCyber Space said:

It makes people suspicious when they hear physicists say that gravity is curving the "fabric" of spacetime when there's no "thing" to be curved. At least it was that way for me.

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tom.stoer

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But in every theory of (quantum) gravity there is a "mathematical entity" like a 4-manifold, a Hilbert space of spin networks etc.Every time I see someone use the phrasefabric of spacetimeit makes me ill. As stated above, there's no such "thing" as the "fabric" of spacetime.

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Yes. That's quite right. Spacetime is very much a mathematical construct, a manifold. It's just not something which is made of paper and fig leaves or any other kind of matter exotic or otherwise.But in every theory of (quantum) gravity there is a "mathematical entity" like a 4-manifold, a Hilbert space of spin networks etc.

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tom.stoer

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I fully agreeIt's just not something which is made of paper and fig leaves or any other kind of matter exotic or otherwise.

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