# What is the fabric of space time really like?

## 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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Dale
Mentor
Neither analogy is very good. I think that it is probably best to learn about spacetime directly rather than by poor analogies to other things which do not reflect most of the properties of spacetime.

Neither analogy is very good. I think that it is probably best to learn about spacetime directly rather than by poor analogies to other things which do not reflect most of the properties of spacetime.
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.

phinds
Gold Member
2019 Award
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.
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.

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.
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.

Nugatory
Mentor
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.

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

timmdeeg
Gold Member
Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?
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.
In my opinion any static picture might be misleading as the function of time being part of "space-time" is missing.
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:

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.
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.

Chronos
Gold Member
The problem with spacetime diagrams is it is very difficult to depict time distortion. That is the missing key to understanding gravity. I see A.T. beat me to the punch.

Dale
Mentor
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.
The mathematical language used to describe spacetime is called Riemannian geometry.

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.

timmdeeg
Gold Member
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.
That's right. In case my wording wasn't correct, kindly let me know.

Cyber Space said:
Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?
Every time I see someone use the phrase fabric of spacetime it makes me ill. As stated above, there's no such "thing" as the "fabric" of spacetime. The term "fabric" is a term used as an analogy to allow one to visualize something which is curving.

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.

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

But in every theory of (quantum) gravity there is a "mathematical entity" like a 4-manifold, a Hilbert space of spin networks etc.
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.

tom.stoer