On Friday, March 9, 2018 at 6:22:41 PM UTC-5, Russell Standish wrote: > > Hi John, > > Alan was claiming that motion of a free particle along a geodesic was > an unjustified assumption in relativity.
I never made that claim. AG > But it follows from > conservation of the mass-energy tensor, which is the equivalent of > conservation of momentum (ie Newton's first law) in 4D Riemannian > spacetime. 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. But its really a background question to GR. If the > question is why do we experience motion at all (given a block > spacetime), then that is really outside of GR. That was my question, more or less. Why outside GR? AG But people have given > answers to this, not completely satisfactory perhaps. > What do they say? AG > > Cheers > > On Fri, Mar 09, 2018 at 01:34:37PM -0500, John Clark wrote: > > On Wed, Mar 7, 2018 at 8:39 AM, <[email protected] <javascript:>> > wrote: > > > > *> Suppose a test particle is restrained spatially, say in the Sun's > > > gravitational field. When released, it starts to move (toward the > Sun). How > > > does GR explain this motion? By the advance of time? AG* > > > > Gravity isn't really a force, its a curvature of spacetime. If not acted > on > > by a force an object will always follow a straight path through > spacetime, > > and if spacetime is curved, as it will be in the proximity of matter > then, > > the shortest distance between 2 events in spacetime is a geodesic. That > > means if I tie a wristwatch to an apple and let go the the apple will > take > > the path that maximizes the time, as seen by the wristwatch, between > > my dropping it and the apple hitting the ground; this is because the > > spacetime distance formula in Minkowski Space is s^2= X^2 -T^2 so the > > larger the time is the smaller the spacetime distance is. > > > > Think about it this way, gravitational time dilation causes the watch to > > run faster 3 feet above the ground than it does when its 3 feet lower, > so > > the shortest path through the 2 events (my letting go and the apple > hitting > > the ground) would involve a path that spent more time more distant from > the > > ground than closer to it. And that means acceleration. > > > > > > 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 [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. > > -- > > ---------------------------------------------------------------------------- > > Dr Russell Standish Phone 0425 253119 (mobile) > Principal, High Performance Coders > Visiting Senior Research Fellow [email protected] > <javascript:> > Economics, Kingston University http://www.hpcoders.com.au > ---------------------------------------------------------------------------- > > -- 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.
