Undergrad The Shape of Space: How Does It Move?

[ddjj77]

TL;DR

Mass causes space to change shape, but does space move when it changes shape?

Mass causes space to change shape, and that's what we see as gravity. But does space move when it changes shape? If yes, relative to what?

 

[Dale]

Spacetime does not have a velocity. So, no, it does not move.

 

[Ibix]

Mass causes space to change shape, and that's what we see as gravity.

That's a popsci description, and isn't really very helpful for understanding.

But does space move when it changes shape?

No. Relativity talks about spacetime. What you call "all of space at this time" is a slice of spacetime. What you call "all of space at a slightly later time" is a different slice of spacetime. It may well have a different "shape" from the earlier slice, but that's because it's a different part of spacetime, not because one piece changed shape.

 

[Vanadium 50]

What would it even mean for space to move? Move where? How could you tell?

 

[ddjj77]

Spacetime does not have a velocity. So, no, it does not move.

It's baffling how something can change shape without moving.

 

[Ibix]

It's baffling how something can change shape without moving.

It doesn't change shape, anymore than a film of a guy running actually moves. You are just looking at different parts of a static whole.

 

[Dale]

It's baffling how something can change shape without moving.

That is because you are used to more or less rigid objects made out of a fixed amount of matter which change shape by having one part of the matter move relative to another. Spacetime isn’t like that at all.

 

[PeterDonis]

Mass causes space to change shape

No, mass (more precisely, stress-energy) causes spacetime to curve. This is not a "change" in shape; it's just that the mass determines the shape. But the shape of spacetime is a single 4-dimensional geometry; it's not something that "changes".