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
> 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.
> 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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