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|WikiProject Physics / Relativity||(Rated Start-class, High-importance)|
Gravity in QM ?
- To the extent that it is described by a field theory. I would have added the section on it myself, if I knew anything about it...Someguy1221 20:00, 3 November 2007 (UTC)
Is there something we don't know about Gravity
RE: Article in Astronomy Today - RE: Flybys not following predictions
Gravity begins at the center of the Earth and as you continue out it becomes progressively less and less. If a flyby goes into a position of less gravity, just by doing so, it should accelerate the speed of the flyby. What do you think. I can explain gravity further. email@example.com Galaxy2010 (talk) 19:00, 25 January 2010 (UTC)
- Questions about gravity should be directed to the science reference desk, at WP:RD/s. This page is for discussion of the article itself, and not its subject. Someguy1221 (talk) 19:12, 25 January 2010 (UTC)
If gravitational fied is around,then why does the flame of a candle is always pointing in the upward direction?
Force is NOT perceived subjectively.
This is pseudo-relativity at it's absolute worst:
"In a field model, rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is what is perceived subjectively as a "force". In fact there is no force in such a model, rather matter is simply responding to the curvature of spacetime itself."
Force is not a subjective perception. The definition and units of force are defined by international agreement. In general relativity, gravitational force is measured with a spring scale.
The tired cliché "gravity is not a force in general relativity" is not even wrong. The correct concept is "gravity is a fictitious force in general relativity". —Preceding unsigned comment added by NOrbeck (talk • contribs) 14:33, 23 August 2010 (UTC)
- Please sign your talk page messages with four tildes (~~~~)? Thanks.
- We have sources saying that gravity is "no force", and that it is a "fictitious force" in general relativity, so it seems to depend on the author. By the way, I would not say that "gravitational force is measured with a spring scale". I'd say that "spacetime curvature is measured with a spring scale".
Anyway, the quoted statement is rather poor. I propose we say something like:
- "In a field model, rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is what is perceived and measured as a "force". In fact one can state that is no gravitational force in such a model (ref 1), or that gravity is a fictitious force (ref 2), and that matter is simply responding to the curvature of spacetime itself."
- DVdm (talk) 15:45, 23 August 2010 (UTC)
Spacetime curvature is equivalent to the gravitational gradient (tidal acceleration); it is measured with a gravity gradiometer. Spacetime curvature is not measured with a spring scale, a spring scale measures the gravitational (pseudo) force. Uniform gravity does not imply curved spacetime, and zero gravity (e.g. at the center of the earth) does not imply flat spacetime. The topic at hand, the gravitational field, is measured with a gravimeter.
In classical mechanics, a gravitational field isn't a force field, but rather a specific force field. The force at a point in a gravitational field depends on the quantity of mass placed there, so gravitational force cannot be abstracted as a field.
In both theories, the concept of force is irrelevant when describing a gravitational field. Also, in metrical theories, the field quantity is usually the metric tensor. In that context, the derivative of the metric field is the equivalent of the gravitational field in Newtonian mechanics.
It is misleading at best to claim that "there is no gravitational force" in metric theories of gravity. I personally have been yelled at by engineers who feel that it is absurd to suggest that the quantity measured by a torsion balance (as in the Cavendish experiment) is not a force, and I agree 95%. One could just as easily argue that there is no gravitational acceleration in GR, or that there is no tidal forces in GR, or that there is no tidal acceleration in GR. The last statement is the most correct, since tidal acceleration in GR is caused by the curvature of spacetime, not a direct physical interaction.
Existing: "In a field model, rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is what is perceived and measured as a "force". In such a model one states that matter moves in certain ways in response to the curvature of spacetime, and that there is either no gravitational force, or that gravity is a fictitious force."
Suggested: "In some field models, rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is measured as an acceleration. In such models, one states that matter experiences accelerations due to inertial motion through curved spacetime."
For a related discussion see: Two-body_problem_in_general_relativity#Force_exists_in_GR. NOrbeck (talk) 09:06, 29 August 2010 (UTC)
- I'd rather keep the existing formulation, the idea being not to formulate what we think is the best formuation, but just to show what is stated in the literature. In this regard I think we have 3 points of view (or better, formulations) from pretty solid sources. I do agree that the opening "In a field model" can be replaced with "In some field models". DVdm (talk) 11:09, 29 August 2010 (UTC)
- In all formulations of all viable theories the concept of force is irrelevant to this article. Readers must be spared from trite philosphical digressions. NOrbeck (talk) 11:44, 31 August 2010 (UTC)
- added the gravitational field equations for both classical and GR,
- added some referances, hence removed the tag. Before I started there were still referances, so the tag wasn't even nessersary
- The expansion became larger than I expected: some content ended up re-ordered, but I added 5 additional referances, and clarified all classical forms of the gravitational field equation, including the field due to a number of descrete external masses. I have a stomach-wrenching feeling the edits will be reverted just for that...-- F = q(E + v × B) 11:55, 18 December 2011 (UTC)
Definition of the gravitational field
If I remember correctly from my physics clases, the gravitational field is not just defined as Gravitational Force over a test mass but with a limit. Also, it is not any Gravitational Force -as it might be interpreted from the article-. I think it should be written as
- m is the test mass
- F is the Gravitational Force between the mass that generates the field and the test mass JuancitoxTw (talk) 13:49, 18 July 2012 (UTC)
Generally accepted fundamental hypothesis
I think the citation from Jesse L. Greenstein in this article is very confusing and taken out of context. I learned about it from a question on Quora: Are gravitational waves the new (a)ether?, which links to this Wikipedia article. The analogy between gravitational field and ether is very far fetched and reflects the author's subjective opinion. I propose to remove the whole section.