# Re: Does GR tell us why anything moves?

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On Saturday, March 10, 2018 at 10:54:50 PM UTC-5, Brent wrote:
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> On 3/10/2018 7:32 PM, agrays...@gmail.com <javascript:> wrote:
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> On Saturday, March 10, 2018 at 10:09:45 PM UTC-5, Brent wrote:
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>> On 3/10/2018 5:04 PM, agrays...@gmail.com wrote:
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>> On Sunday, March 4, 2018 at 11:35:24 PM UTC-5, agrays...@gmail.com
>> wrote:
>>>
>>> I don't think so. I think its equations of motion mix time and space, so
>>> if time increases, spatial position must change. That is, the assumption
>>> that time increases, produces changes in spatial position. CMIIAW. AG
>>>
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>> Here's a related question; if you assume a 4d spacetime manifold, how
>> does one imagine the shortest path between two points, aka a geodesic path,
>> using the Lorentzian metric? AG
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>> In the Lorentz metric a geodesic is the longest path between two
>> time-like events.
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>> Brent
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> Right; we've discussed this in the past. But if that's true, why choose
> motion along a geodesic in spacetime for a model of gravity
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> He didn't "choose" it, it's implicit in the motion being force-free.  As
> Russell pointed out it's implied by conservation of energy-momentum.
>
> Brent
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Where exactly is it argued that conservation of energy-momentum implies
motion along a geodesic in the absence of external forces? TIA, AG

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> if it's the LONGEST path between two time-like events. Wasn't Einstein
> motivated by the fact that a geodesic along a sphere using the Euclidean
> metric for path length, is the SHORTEST distance between two points on a
> sphere? AG
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