On Tuesday, April 16, 2019 at 5:41:35 PM UTC-6, Brent wrote: > > > > On 4/16/2019 7:56 AM, [email protected] <javascript:> 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: >>> >>> >>> >>> On Thursday, April 11, 2019 at 10:56:08 PM UTC-6, Brent wrote: >>>> >>>> >>>> >>>> On 4/11/2019 9:33 PM, [email protected] wrote: >>>> >>>> >>>> >>>> On Thursday, April 11, 2019 at 7:12:17 PM UTC-6, Brent wrote: >>>>> >>>>> >>>>> >>>>> On 4/11/2019 4:53 PM, [email protected] wrote: >>>>> >>>>> >>>>> >>>>> On Thursday, April 11, 2019 at 4:37:39 PM UTC-6, Brent wrote: >>>>>> >>>>>> >>>>>> >>>>>> On 4/11/2019 1:58 PM, [email protected] wrote: >>>>>> >>>>>> >>>>>>>> >>>>>>> He might have been referring to a transformation to a tangent space >>>>>>> where the metric tensor is diagonalized and its derivative at that >>>>>>> point in >>>>>>> spacetime is zero. Does this make any sense? >>>>>>> >>>>>>> >>>>>>> Sort of. >>>>>>> >>>>>> >>>>>> >>>>>> Yeah, that's what he's doing. He's assuming a given coordinate system >>>>>> and some arbitrary point in a non-empty spacetime. So spacetime has a >>>>>> non >>>>>> zero curvature and the derivative of the metric tensor is generally >>>>>> non-zero at that arbitrary point, however small we assume the region >>>>>> around >>>>>> that point. But applying the EEP, we can transform to the tangent space >>>>>> at >>>>>> that point to diagonalize the metric tensor and have its derivative as >>>>>> zero >>>>>> at that point. Does THIS make sense? AG >>>>>> >>>>>> >>>>>> Yep. That's pretty much the defining characteristic of a Riemannian >>>>>> space. >>>>>> >>>>>> Brent >>>>>> >>>>> >>>>> But isn't it weird that changing labels on spacetime points by >>>>> transforming coordinates has the result of putting the test particle in >>>>> local free fall, when it wasn't prior to the transformation? AG >>>>> >>>>> It doesn't put it in free-fall. If the particle has EM forces on it, >>>>> it will deviate from the geodesic in the tangent space coordinates. The >>>>> transformation is just adapting the coordinates to the local free-fall >>>>> which removes gravity as a force...but not other forces. >>>>> >>>>> Brent >>>>> >>>> >>>> In both cases, with and without non-gravitational forces acting on test >>>> particle, I assume the trajectory appears identical to an external >>>> observer, before and after coordinate transformation to the tangent plane >>>> at some point; all that's changed are the labels of spacetime points. If >>>> this is true, it's still hard to see why changing labels can remove the >>>> gravitational forces. And what does this buy us? AG >>>> >>>> >>>> You're looking at it the wrong way around. There never were any >>>> gravitational forces, just your choice of coordinate system made >>>> fictitious >>>> forces appear; just like when you use a merry-go-round as your reference >>>> frame you get coriolis forces. >>>> >>> >>> 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 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.
