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Originally Posted by yusufcam
thank you,
i suppose but to more accurately reflect the original question, would be to ask how an object curves space time.
i would guess that its isn't by displacement in a body (space time) but through the force that contracted the body pulling spacetime in towards it with it. like an weighted object dropped on a sheet and pulling in the sheet around it.
now to say that gravity is the curvature of space time is good for working out how orbits work, but i am not so sure explains how gas clouds collapses/gravitate into stars. Then i suspect you are looking at gravity as a force.
the emergent idea makes sense here, that gravity works differently at different levels.
a simple view, but i am wondering if it can be explained at these levels.
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No one knows how a mass curves space time because General Relativity is only a semi phenomenological theory where the emphasis is to explain the effect rather than the cause. It explains the concept of the Newtonian potential of gravity as a distortion of space time without explaining how space time curves. Newtonian gravity is purely a phenomenological theory as it offers absolutely no reason for the cause behind gravity.
The use of General Relativity for working out orbits has only a very limited application.
Firstly there needs to an enormous difference in mass between the central body and the orbiting body. Mathematically the orbiting body is known as a test mass, a mass which is not large enough not to create its own space time curvature.
Secondly the orbiting body needs to be sufficiently close to the central mass and/or the mass/density of the central body is great enough for space time curvature to be significant.
With the exception of Mercury all other planets are described by Newtonian physics where planets orbit in 3 dimensional flat space and gravity is a force.
Similarly the formation of stars is also based on the Newtonian version of gravity.
While it appears that Newtonian physics and General Relativity are two totally different theories of gravity there is a subtle connection between the two. Newtonian gravity is a first order approximation to General Relativity.
If you took the mathematics behind General Relativity and applied "normal conditions" (low gravity conditions and velocities well below the speed of light) the equations break down to Newton's equations.
Steven