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I understand why a released object moves as it does once it has started moving, and why all objects fall at the same speed, removing other effects. But I don't see why the curvature of space would make it start to move in the first place, thank you.

user141183
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  • Can you give some details on your first statement: what makes you think that a released object moves as it does once it has started moving? – devCharaf May 22 '20 at 18:30
  • Have you looked up geodesic deviation? If not, I would start there. – Alfred Centauri May 22 '20 at 18:43
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    Hint: it's the curvature of spacetime, not just space. – PM 2Ring May 22 '20 at 18:53
  • It’s moving through spacetime even before it’s released. – G. Smith May 22 '20 at 19:28
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    @G.Smith, that whole "moving through spacetime" thing can get some feathers here ruffled. I was thinking of this but heck, when did Ben Crowell leave PSE? – Alfred Centauri May 22 '20 at 21:01
  • Looked up geodesic deviation, it's about objects already in motion.: "geodesic deviation describes the tendency of objects to approach or recede from one another while moving under the influence of a spatially varying gravitational field. If two objects are set in motion along two initially parallel trajectories, the presence of a tidal gravitational force will cause the trajectories to bend towards or away from each other..." I'm wondering why the shape of the space can make an object stationary in relation to the mass it is near start moving. – user141183 May 22 '20 at 21:30
  • I was asked about why an object already in motion moves as it does, that is explained in GR via geodesic paths through the field. – user141183 May 22 '20 at 21:38
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    @David To be in (inertial) motion or to be static is the same, even in classical mechanics. Two objects carefully placed to freely float in the space station can be initially stationary for the crew and moving for an approaching ship. – Claudio Saspinski May 23 '20 at 03:01
  • I think the crucial misunderstanding is in the word "released". It implies a force in Newtonian mechanics, that keeps the object motionless, and boundary conditions in relativistic.. – anna v May 23 '20 at 05:42

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In the weak field limit (which applies near a planet) curvature of space is so small that its effect can be ignored completely; gravity is explained by looking only at time. We know that clocks on GPS satellites do not keep time with identical clocks on Earth, and we also know that energy is the time component of a 4-vector. It follows that energy is changed when an object moves up or down. We measure the energy change directly, as light is redshifted when it goes upwards, and we can calculate that the change is equivalent to potential energy in Newtonian gravity. The apparent force of gravity is minus the gradient of potential energy, so that the apparent change in the rate of clocks with height is directly responsible for the apparent force of gravity.

Charles Francis
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  • It has been proved via the Rietdijk-Putnam argument that Minkowski spacetime means no motion through time. It is out of the question to explain this via 'moving through spacetime'. And gravitational time dilation is merely a pattern in the block of block time, it cannot be used to explain why an object starts moving. – user141183 May 22 '20 at 21:46
  • @David, this website is about mainstream physics, not the misunderstandings of individuals. – Charles Francis May 23 '20 at 06:45
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    @David, the answer concerns general relativity, not Minkowski spacetime – Charles Francis May 23 '20 at 07:11
  • Because general relativity uses Minkowski spacetime, and depends on it, there's a need to explain things in a way that is consistent with Minkowski spacetime. I'm still unsure about the question, but thanks for your help. – user141183 May 23 '20 at 12:37
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    General relativity uses local Minkowski spacetime. The explanation is fully consistent. You cite theories which are not – Charles Francis May 23 '20 at 12:56
  • The Rietdijk-Putnam argument shows that depending on Minkowski spacetime, as GR does, motion through time does not exist in any form, except as a perception-based illusion. Local or not makes no difference. Whether an event is past or future is frame dependent, so motion along the time axis cannot have enough existence to cause an object to start falling. There must be a better explanation from GR that trying to use motion through time. – user141183 May 23 '20 at 16:20
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    Let us continue this discussion in chat. – Charles Francis May 23 '20 at 16:38