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 * > > 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 everything-li...@googlegroups.com <javascript:>. > To post to this group, send email to everyth...@googlegroups.com > <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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
