On Tuesday, April 16, 2019 at 9:21:32 PM UTC-6, Brent wrote: > > > > On 4/16/2019 6:25 PM, [email protected] <javascript:> wrote: > > > > On Tuesday, April 16, 2019 at 5:41:35 PM UTC-6, Brent wrote: >> >> >> >> On 4/16/2019 7:56 AM, [email protected] wrote: >> >> >> >> On Monday, April 15, 2019 at 9:26:59 PM UTC-6, Brent wrote: >>> >>> >>> >>> On 4/15/2019 7:14 PM, [email protected] wrote: >>> >>> >>> >>> On Friday, April 12, 2019 at 5:48:23 AM UTC-6, [email protected] >>> wrote: >>>> >>>> ... >>>> If gravity is a fictitious force produced by the choice of coordinate >>>> system, in its absence (due to a change in coordinate system)* how >>>> does GR explain motion?* Test particles move on geodesics in the >>>> absence of non-gravitational forces, but * why do they move at all?* AG >>>> >>> >>> Maybe GR assumes motion but doesn't explain it. AG >>> >>> >>> The sciences do not try to explain, they hardly even try to interpret, >>> they mainly make models. By a model is meant a mathematical construct >>> which, with the addition of certain verbal interpretations, describes >>> observed phenomena. The justification of such a mathematical construct is >>> solely and precisely that it is expected to work. >>> --—John von Neumann >>> >> >> *This is straight out of the "shut up and calculate" school, and I don't >> completely buy it. E.g., the Principle of Relativity and Least Action >> Principle give strong indications of not only how the universe works, but >> why. That is, they're somewhat explanatory in nature. AG* >> >> >> Fine, then take them as explanations. But to ask that they be explained >> is to misunderstand their status. It's possible that they could be >> explained; but only by finding a more fundamental theory that includes them >> as consequences or special cases. Whatever theory is fundamental cannot >> have an explanation in the sense you want because then it would not be >> fundamental. >> >> Brent >> > > *I don't think I asked them to be explained, and I don't think* > * I misunderstand their status. In the examples I gave, the principles are > pretty fundamental and nonetheless seem to explain something substantive > about the universe even though they're not part of a deeper theory. AG * > > > You wrote, "...how does GR explain motion? Test particles move on > geodesics in the absence of non-gravitational forces, but why do they move > at all?" > > GR hypothesizes that force-free motion of test particles is along > geodesics. In 4-space they "move" because there is a time coordinate and a > particle is by definition something that persists in time (in contrast to > an "event"). > > Brent >
*Yes, I asked for an explanation of motion in the context of GR, but my response to Von Neumann was NOT meant in that context; namely, that physics sometimes DOES give explanations in what we could consider fundamental theories. But particles can hypothetically persist at a fixed time and still be particles. I don't think GR says anything about WHY test particles move, other than to postulate HOW they move; along geodesics. By distinction, our other force theories do IMO explain WHY particles move. AG * > -- > 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.
