Gravity as geometry vs gravity traveling at c

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Discussion Overview

The discussion revolves around the relationship between gravity as a manifestation of spacetime geometry and the concept that gravity travels at the speed of light. Participants explore the implications of these ideas within the framework of general relativity and the nature of gravitational fields.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that changes in gravity travel at the speed of light, while gravity itself is described as a field and a manifestation of spacetime geometry.
  • There is a question about why the spacetime distortion view is preferred for gravity, as opposed to other forces which are viewed as fields.
  • Some participants suggest that gravity being spacetime geometry implies it is not merely a choice but a fundamental aspect of how gravity operates.
  • One participant expresses uncertainty about interpreting gravity as a field, questioning how it can be considered a field if it changes spacetime.
  • Another participant proposes that gravity is not a field but rather the inherent geometry of spacetime, emphasizing that it is pervasive and does not travel, while only changes in gravity travel.
  • There is a discussion about whether gravity is constantly changing, particularly in relation to the movement of celestial bodies like the Sun.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of gravity, its relationship to spacetime, and whether it can be classified as a field. The discussion remains unresolved with no consensus reached on these points.

Contextual Notes

Participants highlight limitations in their understanding of the concepts discussed, particularly regarding the definitions and implications of gravity as a field versus spacetime geometry.

Canis Lupus
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Hi,

I am seeking to understand better how this well accepted idea:

"...according to general relativity, gravity is a manifestation of the geometry of spacetime."
(https://en.wikipedia.org/wiki/Loop_quantum_gravity)

is compatible with the equally well accepted idea that gravity travels at the speed of light.

Any help would be appreciated.
 
Last edited:
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Canis Lupus said:
Hi,

I am seeking to understand better how this well accepted idea:

"...according to general relativity, gravity is a manifestation of the geometry of spacetime."
(https://en.wikipedia.org/wiki/Loop_quantum_gravity)

is compatible with the idea that gravity travels at the speed of light.

Any help would be appreciated.
CHANGES in gravity travel at the speed of light. Gravity itself is a field and is a manifestation of spacetime or, it is, I think, more appropriate to say that gravity IS spacetime geometry.
 
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phinds said:
CHANGES in gravity travel at the speed of light. Gravity itself is a field and is a manifestation of spacetime or, it is, I think, more appropriate to say that gravity IS spacetime geometry.

Thanks for the reply.
 
A follow-up question: how come for gravity the space-time distortion view is preferred (to my understanding, a hypothetical graviton field would be equivalent), whereas the other forces are preferred to be viewed as fields?
 
rumborak said:
A follow-up question: how come for gravity the space-time distortion view is preferred (to my understanding, a hypothetical graviton field would be equivalent), whereas the other forces are preferred to be viewed as fields?
Gravity IS spacetime geometry so it's not really a choice
 
I may be mistaken, but I thought the graviton point of view does away with space-time distortion.
 
rumborak said:
A follow-up question: how come for gravity the space-time distortion view is preferred (to my understanding, a hypothetical graviton field would be equivalent), whereas the other forces are preferred to be viewed as fields?
Is there a field theory that changes time?
 
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phinds said:
Gravity itself is a field
I'm not sure that I understand this if I take it literally. When I interpret a field as a force vector at every position and time, I have to wonder which position and time. If gravity changes space-time, can it really be considered a field? -- or do I need to give up the distortion part to accept it as a field? In that case, I guess the force is also changing motion in time -- which I could accept. Does that make it more of a tensor than a force in the F=ma sense, since the changing motion in time must not depend on its mass?
EDIT: A field is not necessarily a force field where acceleration would need to depend on mass.
 
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phinds said:
Gravity IS spacetime geometry so it's not really a choice

The gravity field and spacetime are one and the same? Is this a new understanding?
I thought that space-time was a coordinate system; a mathematical construct only. A gravity field is more than that as it has the capability to exert a force if a mass is present. Gravitational field intensity has the dimensions of [Force]/[Mass], Same as acceleration. Is this wrong?
 
  • #10
FactChecker said:
I'm not sure that I understand this if I take it literally. When I interpret a field as a force vector at every position and time, I have to wonder which position and time. If gravity changes space-time, can it really be considered a field? -- or do I need to give up the distortion part to accept it as a field? In that case, I guess the force is also changing motion in time -- which I could accept. Does that make it more of a tensor than a force in the F=ma sense, since the changing motion in time must not depend on its mass?
I think you're right; it's not a field (that was a poor choice of description --- I did better w/ the last sentence in that post), it's just the geometry inherent in spacetime. What I meant was that it is pervasive and does not travel. It's the changes in gravity that travel.
 
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  • #11
phinds said:
I think you're right; it's not a field (that was a poor choice of description --- I did better w/ the last sentence in that post), it's just the geometry inherent in spacetime. What I meant was that it is pervasive and does not travel. It's the changes in gravity that travel.

Does this mean it is constantly changing?
 
  • #12
Canis Lupus said:
Does this mean it is constantly changing?
calling @PeterDonis
 
  • #13
Canis Lupus said:
Does this mean it is constantly changing?
When the Sun moves through space, the associated change of space-time due to gravity moves with it. The change travels out from the Sun at the speed of light.
 
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  • #14
FactChecker said:
When the Sun moves through space, the associated change of space-time due to gravity moves with it. The change travels out from the Sun at the speed of light.

Thanks. That gives me a better idea.
 
  • #15
phinds said:
calling @PeterDonis

Sorry, I don't understand this post.
 
  • #16
Canis Lupus said:
Sorry, I don't understand this post.
I'm calling on one of our experts so that you can get a better answer than what I could provide.
 
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  • #17
phinds said:
I'm calling on one of our experts so that you can get a better answer than what I could provide.

thanks
 

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