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trees and plants
Hello there.My question is:can Newtonian gravity be generalised to include not only bodies with mass but energy also?Thank you.Can my thread be moved to classical physics?
You mean kinetic energy?Sorry for my thread and question perhaps it should be deleted.Ibix said:Um... bodies in Newtonian physics do typically have energy, and Newtonian physics works as is. What generalisation do you think is needed?
You already know that it does, from how you phrased your question here.universe function said:Does gravity pull electromagnetic radiation like light?
Where did you read this? It's possible to kinda sort of put light into a model of Newtonian gravity, but the obvious way to do it gives the wrong answer for the deflection angle and some approaches say it won't be deflected at all. Fundamentally, Newtonian gravity is a low-speed weak field approximation to general relativity and is invalid for fast moving objects, of which light is an excellent example.universe function said:I somewhere read that Newtonian gravity also predicts the bending of light from gravity.
universe function said:Can my thread be moved to classical physics?
universe function said:Does gravity pull electromagnetic radiation like light?
What about other kinds of electromagnetic radiation like gamma rays, radio waves, microwaves or other types of radiation like gravitational radiation, beta radiation or sounds?Are they pulled by the gravity of a body like the sun or a star?Are there other physical objects like radiation without mass that are energies?Are they pulled by gravity?Ibix said:You already know that it does, from how you phrased your question here.
Where did you read this? It's possible to kinda sort of put light into a model of Newtonian gravity, but the obvious way to do it gives the wrong answer for the deflection angle and some approaches say it won't be deflected at all. Fundamentally, Newtonian gravity is a low-speed weak field approximation to general relativity and is invalid for fast moving objects, of which light is an excellent example.
universe function said:Is spacetime only a mathematical construct or is it something physical?
universe function said:Because it is in four dimensions we can not know because we do not perceive it in real life?
universe function said:Some people I think say that general relativity imply that the current universe we perceive is a shadow of the real is this wrong?
universe function said:Is curvature of spacetime a consequence of the unification of space and time
universe function said:without time passing we would not know how spacetime is curved?
The universe we perceive is a 3d subspace of the 4d whole, following GR's notions of spacetime.universe function said:Some people I think say that general relativity imply that the current universe we perceive is a shadow of the real is this wrong?
Well, you couldn't have curvature of spacetime without spacetime. But you can have flat spacetime.universe function said:Is curvature of spacetime a consequence of the unification of space and time
Ibix said:The universe we perceive is a 3d subspace of the 4d whole
Can Newtonian gravity be generalised to include not only bodies with mass but energy also?
Well, the past light cone is a 3d surface, isn't it? ##S^2\times R##? It's not spacelike, indeed, but that's why I said "subspace" and avoided words like "slice", which I'd agree fit a spacelike "now" better.PeterDonis said:I don't think that's the same as perceiving it.
Ibix said:Well, the past light cone is a 3d surface, isn't it? ##S^2\times R##? It's not spacelike, indeed, but that's why I said "subspace" and avoided words like "slice", which I'd agree fit a spacelike "now" better.
I think general relativity is the answer as the next generalisation to Newtonian gravity.I try to learn general relativity,trying to learn about light cones, world lines, solutions of the einstein field equations, but I think the introduction has pretty much taken a big part of my interest and although I want to read about de Sitter spacetime or other spacetimes I find it difficult to not consider these topics on general relativity as of less interest compared with the introduction of general relativity.Vanadium 50 said:What is this thread about? It seems to be meandering all over the place.
Has a simple answer: "No, because then it would be something other than Newtonian gravity." If you then want to change this to something kinda-sorta-like-Newtonian gravity, you need to explain to us what you mean by kinda-sorta-like-Newtonian gravity and why GR doesn't "count".
If you instead want to know what GR is, that is probably too big a bite for one thread.
Ah, yes. Had forgotten that bit.PeterDonis said:the apex point of the cone, which is the event "here and now"
Well, you could add that point - it makes sense to do so for a directly experienced notion of now. Then, I believe the topology does become R3.PeterDonis said:##S^2\times R## is not a "3d surface" in the usual sense of that term--that would be ##R^3##. It is a topological 3-manifold, but it is "missing a point" (the apex point of the cone, which is the event "here and now").
PAllen said:you could add that point - it makes sense to do so for a directly experienced notion of now.
PAllen said:Then, I believe the topology does become R3.
universe function said:I think general relativity is the answer as the next generalisation to Newtonian gravity.I try to learn general relativity,trying to learn about light cones, world lines, solutions of the einstein field equations, but I think the introduction has pretty much taken a big part of my interest and although I want to read about de Sitter spacetime or other spacetimes I find it difficult to not consider these topics on general relativity as of less interest compared with the introduction of general relativity.
haushofer said:the Newtonian limit can be considered as a c-->oo limit of GR. On dimensional grounds this explains why time derivatives, going as 1/c, and energy terms, going as 1/c^2, are contracted away.
haushofer said:can Newtonian gravity be extended such that the gravitational potential becomes time-dependent (i.e. the partial derivative w.r.t. t becomes non-zero)?
PeterDonis said:Not all time derivatives. Velocities have to be small compared to ##c##, so terms in velocities that are in GR but not in Newtonian gravity go away. But not all time derivatives are like that. See below.
This already happens in Newtonian gravity for non-static systems, i.e., systems where mass is in motion. The GR terms in ##v / c## induced by such motions vanish in the Newtonian approximation; but the time derivatives of the direct Newtonian potential terms due to the moving masses don't.
haushofer said:I meant that it's impossible (AFAIK) to extend the Poisson equation with a time derivative of the potential.
Newtonian gravity is a classical theory that describes the force of gravity as a mutual attraction between two objects with mass. On the other hand, Einstein's theory of general relativity is a modern theory that describes gravity as a curvature of spacetime caused by the presence of mass and energy.
Yes, Newtonian gravity can be generalized to include energy by using the concept of gravitational potential energy. This potential energy arises from the gravitational force between two objects and is dependent on their masses and separation distance.
In Newtonian gravity, mass and energy are considered to be separate entities. Mass is a measure of an object's resistance to acceleration, while energy is the ability to do work. However, in Einstein's theory of general relativity, mass and energy are two forms of the same thing and can be converted into each other through the famous equation E=mc^2.
The inclusion of energy in Newtonian gravity does not significantly affect its predictions in most situations. However, in extreme cases such as near the speed of light or near massive objects like black holes, the predictions of Newtonian gravity may deviate significantly from those of general relativity.
Yes, Newtonian gravity is still considered a valid theory in many situations and is widely used in fields such as engineering and astronomy. However, it has been superseded by Einstein's theory of general relativity in cases where high precision and accuracy are required, such as in the study of gravitational waves or the orbits of planets around the sun.