Space Pulled into Gravity Well: What Happens?

• B
• pforeman
In summary, although the surface of the earth is in the way, space still flows towards the center of the earth. This flow of space is what causes gravity, and the strength of the field builds up as space flows past the surface of the earth.
pforeman
TL;DR Summary
If gravity is the result of space flowing into a massive object (9.8 m/ sec on earth ) then does it accumulate there.
Many explanations out there for gravity. One that I saw a few months ago explained that our gravity hete on earth is space moving toward the center of the earth, and we should be falling to the center of earth at a rate of 9.8 m/ s/s. But the surface of the earth is in the way...
What happens to the " fabric of space" after it gets there?
Even if space is a "field", wouldn't the strength of the field start to build up?
Thanks,
Paul

pforeman said:
One that I saw a few months ago
Can you give a specific reference?

vanhees71
pforeman said:
One that I saw a few months ago explained that our gravity hete on earth is space moving toward the center of the earth, and we should be falling to the center of earth at a rate of 9.8 m/ s/s.
As Peter says, a reference would help. Keeping track of where you learned something is almost as important as actually learning it - sources (for anything) vary widely in their reliability and the skill with which they present their information.

Your description sounds like the "river" model, which is an attempt to visualise how gravity works in GR by having space flowing downwards and "naturally" carrying things with it. It only works as a local description, and (IMO) a fairly poor one. The point about local models is that asking global questions (like "what happens to all that space after it flowed past us") isn't possible because the model has no description of it. It's like taking the highschool level "gravity is ##mg## and always in the same direction" and wondering why people on the other side of the world don't fall off. The only answer is to use a more general model, and in more general models of relativistic gravity space doesn't flow anywhere.

Wes Tausend, Demystifier, russ_watters and 1 other person
Ibix said:
Keeping track of where you learned something is almost as important as actually learning it

I'm doomed.

Demystifier and vanhees71
Not sure, maybe PBS SPACETIME.
But I have seen multiple explanations of gravity that has space moving into massive objects, usually black holes. This movement of space into a massive object is usually given to be the explanation of how gravity works.

PeroK
pforeman said:
Not sure, maybe PBS SPACETIME.
But I have seen multiple explanations of gravity that has space moving into massive objects, usually black holes. This movement of space into a massive object is usually given to be the explanation of how gravity works.
It's a continuing problem with popular science that they tend to present interpretations that have particularly vivid visualisations as "what REALLY happens" or "the TRUTH about" whatever. Then people find that they don't really make sense outside certain bounds and come here...

I can assure you that there's no flowing space in the full mathematics of GR. There are better visualisations of how GR models gravity than the river model, but they all have limitations because you can draw honest representations of curved 4d pseudo-Riemannian manifolds but not one-to-one accurate ones. The animation below is by forum member @A.T., based off work by Epstein, I believe.

russ_watters
pforeman said:
This movement of space into a massive object is usually given to be the explanation of how gravity works.
I've never seen this. Gravity is one aspect of the curvature of spacetime. In particular, that massive particles follow timelike geodesics in spacetime. And, that spacetime curvature itself depends on the mass.

vanhees71
PeterDonis said:
Can you give a specific reference?

I would imagine the OP is refering to "The river model of general relativity". Google finds about three popularizations on the web from this search term. -I believe the idea originated with Hamilton et al, see for instance https://arxiv.org/abs/gr-qc/0411060.

A quote from the abstract.
This paper presents an under-appreciated way to conceptualize stationary black holes, which we call the river model. The river model is mathematically sound, yet simple enough that the basic picture can be understood by non-experts.

A case could be made for having the Original Poster confirm or deny the origins before discussing this in depth. This is both to avoid discussing something because it's not relevant to the OP, and also because it is good policy to encourage posters to think about their sources. This is important not just for this particular issue, but in general to encourage critical thinking about the mass of information on the world-wide web.

That said, I'll give my own opinions on the "River model".

Personally, I don't really like the "river model" much, but it does have some validity. I see it as a bit of a dead-end, it doesn't really lead anywhere, and I don't think it can make the grade as a complete explanation of general relativity. It does have a peer-reviewed source (though I recall reading comments somewhere by the author himself that getting it through peer review was difficult). It can apparently be interpreted in a mathematically valid way, which is a plus over many popularizations. But I don't really like it much still, in spite of some of the positive features.

Wes Tausend
pforeman said:
If gravity is the result of space flowing into a massive object (9.8 m/ sec on earth ) ...
It isn't.Matter tells spacetime how to curve; spacetime tells matter how to move.

Neither matter nor spacetime tells spacetime how to move.

Because spacetime does not "move".

vanhees71
pforeman said:
Not sure, maybe PBS SPACETIME.
That's not a valid reference.

pforeman said:
But I have seen multiple explanations of gravity that has space moving into massive objects, usually black holes. This movement of space into a massive object is usually given to be the explanation of how gravity works.
As @pervect says, there is a description called the "river model:" of black holes that is something like this. However, it's just a description. It is not an explanation, because, as you note in your OP, it does not provide answers to what seem like obvious questions. The standard GR picture of spacetime curvature, by contrast, does.

vanhees71
DaveC426913 said:
It isn't.Matter tells spacetime how to curve; spacetime tells matter how to move.

Neither matter nor spacetime tells spacetime how to move.

Because spacetime does not "move".

how would things be different if spacetime did move?

gamerlibrarian said:
how would things be different if spacetime did move?
We would be talking about a different theory than GR, with a different mathematical basis and a different notion of what spacetime is. Someone would have to propose such a theory before anyone could say what it predicts.

Since we have no valid reference as a basis for discussion, this thread is closed.

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