On Wednesday, March 7, 2018 at 2:17:15 AM UTC-5, Russell Standish wrote: > > On Tue, Mar 06, 2018 at 09:18:46PM -0800, agrays...@gmail.com > <javascript:> wrote: > > > > > > They follow from the principle of conservation of momentum, also > > > sometimes known as Newton's first law. > > > > > > > Can you elaborate on that? Is there always motion even if time > > doesn't exist? Motion in space or spacetime AG > > Clearly, with motion in flat space, conservation of momentum > implies that motion will be along a straight line. If the direction of > movement changed, that is automatically a change of momentum. > > In curved space, the corresponding curve must be a geodesic - there > are no such things as straight lines in curved space. IIRC, the > equivalent expression of conservation of momentum is that the > covariant derivative of the mass-energy tensor must vanish. There is a > discussion on page 386 of Misner, Thorne & Wheeler's epic book > gravitation... > > You can also come to the same conclusion using an extremum principle > such as Laplace's principle of least action, but for sheer intuition, > the above explanation works best for me.

*Thanks for your time and effort, but I don't think you understand my* *question. 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* -- 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.