On Friday, March 9, 2018 at 11:23:42 PM UTC-5, Brent wrote: > > > > On 3/9/2018 6:10 PM, John Clark wrote: > > On Fri, Mar 9, 2018 at 6:22 PM, Russell Standish <li...@hpcoders.com.au > <javascript:>> wrote: > > Hi Russell > > *> Alan was claiming that motion of a free particle along a geodesic was >> an unjustified assumption in relativity.* > > But its not unjustified or an assumption if that's the way we observe > things move, and it is. > > I simply asked if geodesic motion is assumed by GR. I never alleged it was unjustified. And it could be an assumption if that's the way it is. AG

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> *> If he were asking why is momentum conserved, then one could answer it >> along the lines of Vic Stenger's symmetries, utilising Noether's theorem.* > > > Yes but Noether's theorem couldn't be invoked and the conservation of > momentum produced from it unless there was a symmetry, in this case > the fact that the laws of physics are the same at all points in space. > Someone could then ask why that is, and at this time the best answer we > could give is that’s just what we observe. As far as I can see it is not a > logical necessity, physics could have been different from one place to > another but we see that is not the case. > > > If that were the case then we would look for some other variable(s) that > would account for the difference in order to arrive at a more comprehensive > theory that, with the new variable(s), made the theory the same both > places. The idea of physics as a fundamental theory is that it should be > the same at all times and places. So if it's not, we either look for a > better theory or (temporarily) give up and call the variations > "geography". As my friend Vic Stenger put it, physics assumes POVI, Point > Of View Invariance. > > Brent > > And Einstein said mass/energy tells spacetime how to curve and spacetime > tells mass/energy how to move, but if this were not true it would > contradict observations but not produce any logical self contradictions > that I know about. In physics if you keep asking recursive "why is that?” > questions eventually you'll come to a brute fact. Put it another way, I > don't think the laws of physics could be derived from pure logic alone > regardless of how intelligent you are, that's why we need observation. > > John K Clark > > > -- > 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 everything-li...@googlegroups.com <javascript:>. > To post to this group, send email to everyth...@googlegroups.com > <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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.