On Thursday, March 8, 2018 at 4:35:35 PM UTC-5, Brent wrote: > > > > On 3/8/2018 9:48 AM, [email protected] <javascript:> wrote: > > > > On Thursday, March 8, 2018 at 12:36:07 PM UTC-5, Brent wrote: >> >> >> >> On 3/8/2018 4:24 AM, [email protected] 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. >
*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 * > > 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 * > > Brent > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > To post to this group, send email to [email protected] > <javascript:>. > Visit this group at https://groups.google.com/group/everything-list. > For more options, visit https://groups.google.com/d/optout. > > > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
