On Thursday, March 8, 2018 at 7:59:50 PM UTC-5, Brent wrote: > > > > On 3/8/2018 4:31 PM, [email protected] <javascript:> wrote: > > > > On Thursday, March 8, 2018 at 4:35:35 PM UTC-5, Brent wrote: >> >> >> >> 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] 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 * > > > 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 >
*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 * > > >> * 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]. >> 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. >> >> >> -- > 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.
