On 3/8/2018 4:31 PM, agrayson2...@gmail.com wrote:
On Thursday, March 8, 2018 at 4:35:35 PM UTC-5, Brent wrote:
On 3/8/2018 9:48 AM, agrays...@gmail.com <javascript:> wrote:
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:
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:
*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.
*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
*
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.
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.
*
*
*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
*
Right. It's any mass-energy.
Brent
Brent
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