On Tue, Mar 06, 2018 at 09:18:46PM -0800, agrayson2...@gmail.com 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.


Dr Russell Standish                    Phone 0425 253119 (mobile)
Principal, High Performance Coders
Visiting Senior Research Fellow        hpco...@hpcoders.com.au
Economics, Kingston University         http://www.hpcoders.com.au

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