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My understanding is that general relativity concludes that gravity isn't real because it does not exist in all frames of reference. Also that mass and energy warp spacetime into a curved geometry. Does that then mean that objects in orbit are actually traveling in straight lines in curved spacetime?

Qmechanic
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Wjdavis5
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1 Answers1

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Your first line is incorrect, gravity is real, just jump up and you will confirm that. Frames of reference are used to find how gravity affects our measurements of spacetime.

Your second line is correct.

Objects, from photons to planets, moving in general relativity, obey the geodesic equation, so a curved line can be a "straight line", that is, if you define a straight line as the shortest distance between two points.

  • I guess I may be misunderstanding. But what I understood is that Einstein concludes that there is no force of gravity, and instead it is a warping of space time by large mass. He concluded that the force of gravity didn't exist because it disappears while in free fall. For a moment in the jump you mention you feel weightless. Therefore the force does not exist in all frames of reference and if something does not exist in all frames of reference it cannot be objectively real. – Wjdavis5 Jun 09 '15 at 16:27
  • Einstein said , and you could Google this using "equivalence Principle",not so much that gravity did not exist, as that a person accelerating in a spaceship could not tell if they were in a gravity field or an accelerting frame of reference. –  Jun 09 '15 at 16:33
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    @Wjdavis5: this is a semantic nicety. Because motion is explained by the geometry some like to claim that gravity isn't a force. This is a bit silly because the mass times the norm of the four-acceleration is non-zero in loads of situations, and to me $ma$ certainly seems equal to $F$. – John Rennie Jun 09 '15 at 16:34
  • @johnrennie. I was about to say I think I just got stuck on the semantics of it. Thank you all! – Wjdavis5 Jun 09 '15 at 16:36
  • Would it be fair to say then that gravity is jot a force exerted on objects, but rather a force acting on spacetime and objects then react to spacetime? – Wjdavis5 Jun 09 '15 at 16:40
  • Best way to say it, for me anyway, is kinda like you said in your second line, mass-energy produces gravity, which curves spacetime and objects moving in spacetime follow curved space according to the geodesic equation. –  Jun 09 '15 at 16:45
  • @Wjdavis5, According to G.R., the force that you feel when you stand on the Earth's surface is not gravity. Rather, what you feel is the force that the solid Earth exerts on you to stop you from falling into its center. – Solomon Slow Oct 09 '15 at 20:56
  • Way late. No force between 2 masses, [[[A(the least massive), and B exists at all]]];Forget that. Mass curves spacetime. Since no force between A and B actually exists, Newton's laws of motion apply, here: it's path is the exact same as there is no force. However this path happens to be curved and bent towards the larger mass. Therefore, it appears that the smaller mass falls in the larger mass. Geodesic equation, if time is a dimension, then the objects will accelerate. It therefore only appears that an inverse-square force between 2 masses is pulling the smaller mass to the larger mass. – Damon Blevins Nov 08 '15 at 07:52