Hi John, Alan was claiming that motion of a free particle along a geodesic was an unjustified assumption in relativity. 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. But people have given answers to this, not completely satisfactory perhaps.
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]> 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]. > 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. -- ---------------------------------------------------------------------------- Dr Russell Standish Phone 0425 253119 (mobile) Principal, High Performance Coders Visiting Senior Research Fellow [email protected] 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.
