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
> > >
> > > 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
> 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*
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