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 
 

> *> 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
>
>  
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