# Re: Does GR tell us why anything moves?

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On 3/8/2018 8:40 PM, agrayson2...@gmail.com wrote:
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On Thursday, March 8, 2018 at 7:59:50 PM UTC-5, Brent wrote:

On 3/8/2018 4:31 PM, agrays...@gmail.com <javascript:> wrote:
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On Thursday, March 8, 2018 at 4:35:35 PM UTC-5, Brent wrote:

On 3/8/2018 9:48 AM, agrays...@gmail.com wrote:
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On Thursday, March 8, 2018 at 12:36:07 PM UTC-5, Brent wrote:

On 3/8/2018 4:24 AM, agrays...@gmail.com wrote:
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On Wednesday, March 7, 2018 at 11:04:09 PM UTC-5, Brent
wrote:

On 3/7/2018 5:39 AM, agrays...@gmail.com wrote:
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```                *Thanks for your time and effort, but I don't
think you understand my*
*question. Suppose a test particle is restrained
spatially, say in *
*the Sun's gravitational field. When released, it
starts to move (toward *
*the Sun). How does GR explain this motion? By the
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force causing the particle to follow a non-geodesic
path through space-time.  When you released it, it
then followed the "straightest path possible", i.e.
a geodesic.

Brent

So time is the "culprit". What has this resumption of
spatial motion (along a geodesic in spacetime) have to
do with conservation of momentum, if at all ? TIA, AG
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It's not a "resumption" of motion; it's just tilting the
direction of motion from being along your coordinate
time line (which you think of as 'not moving') to being
along the geodesic (which you think of as 'falling').
The 4-momentum of the system, including whatever device
you were using to keep the particle from falling is
conserved.

Brent

I said I was reading Epstein. I have it with me while
traveling. If 4 momentum is conserved, isn't that the same
as saying motion on a geodesic is postulated?
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No. Motion on a geodesic is force-free motion.  If you have
rocket, for example, you can travel on a non-geodesic, but
4-momentum is still conserved considering your rocket and its
exhaust.

*OK, but what I meant was this; when the force causing a
non-geodesic motion is discontinued, geodesic motion is restored.
Is this baked into the field equations and thus can be understood
as the result of the postulates of GR? AG
*
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I wouldn't say "baked in".  You have to represent a particle as
concentrated mass point in the equations and then they tell you
that, absent other forces, it follows a geodesic.

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```        Incidentally, if one accepts GR as a "valid" model of
gravity, doesn't that preclude any coupling between gravity
and EM? AG
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```        Photons couple the same as other mass-energy, they travel on
geodesics absent some other interaction.

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*OK, but what I meant by "coupling" would be if EM had a role in
producing the gravitational phenomenon other than its mass-energy
contribution. As I understand GR, it is solely the mass-energy of
anything that produces the geometry of spacetime, and thus
gravity, nothing specifically electromagnetic. AG
*
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Right.  It's any mass-energy.

Brent

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*This I find troubling. We have two fundamental physical phenomenon, gravity and EM, and they seem to have no intrinsic relationship between each other. AG
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They have more relationship than they did when Maxwell discovered EM.  It was purely a field on a fixed background.  So you've been troubled since 1862.
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Under GR the EM field is a source of gravity and hence warps spacetime; and warped spacetime deflects EM waves.
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Brent

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