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. 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.
