On Friday, March 9, 2018 at 12:51:29 PM UTC-5, Lawrence Crowell wrote: > > On Friday, March 9, 2018 at 9:44:30 AM UTC-6, [email protected] wrote: >> >> >> >> On Friday, March 9, 2018 at 12:10:13 AM UTC-5, Brent wrote: >>> >>> >>> >>> On 3/8/2018 8:40 PM, [email protected] wrote: >>> >>> >>> >>> On Thursday, March 8, 2018 at 7:59:50 PM UTC-5, Brent wrote: >>>> >>>> >>>> >>>> On 3/8/2018 4:31 PM, [email protected] 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 * >>> >>> >>> They have more relationship than they did when Maxwell discovered EM. >>> It was purely a field on a fixed background. So you've been troubled since >>> 1862. >>> >>> Under GR the EM field is a source of gravity and hence warps spacetime; >>> and warped spacetime deflects EM waves. >>> >>> Brent >>> >> >> *Good perspective on the situation. OTOH, for Newton movement is caused >> by an attractive force, whereas for Einstein it's caused by the advancement >> of time. So, IMO, the mystery of movement in a gravity field persists. AG* >> >> > > I am not exactly sure why you are stuck on the idea that the advance of > time causes motion. >
*Because in the absence of a force causing non-geodesic motion, the increase in time results in a change in spatial position since in the equations of motion, spatial coordinates are not independent variables; they depend on time IIUC. AG* > In one sense you can say that by being at rest in a frame one is moving at > T = ct distance along a fourth dimension. But that is not really a cause > for motion in spatial directions. > > LC > -- 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.
