On 3/8/2018 9:48 AM, [email protected] wrote:
On Thursday, March 8, 2018 at 12:36:07 PM UTC-5, Brent wrote:
On 3/8/2018 4:24 AM, [email protected] <javascript:> wrote:
On Wednesday, March 7, 2018 at 11:04:09 PM UTC-5, Brent wrote:
On 3/7/2018 5:39 AM, [email protected] wrote:
*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 advance
of time? AG*
Time was advancing all along. Your restraint was a 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
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.
Didn't you say you had read Epstein?
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?
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.
Incidentally, if one accepts GR as a "valid" model of gravity, doesn't
that preclude any coupling between gravity and EM? AG
Photons couple the same as other mass-energy, they travel on geodesics
absent some other interaction.
Brent
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