Re: Newbie question about gravity


"Martin Hogbin" <goatREMOVETHIS123@xxxxxxxxxx> wrote in message
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"z" <genesinpieces@xxxxxxxxx> wrote in message
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With my superficial grasp of spacetime physics I have come to a basic
understanding of such complex and occasionally bizarre gravitational
phenomena as orbital pathways, light bending, and black holes, but
there remains one simple every day fact of gravity that does not make
sense to me within this framework: why do things fall down?

If I drop a baseball, what gives it its downward motion? I understand
how the curvature of spacetime affects the trajectories of objects
that are already moving, but if an object is initially at rest, how
does the curvature of space time make any difference?


Remember, it is spaceTIME that is curved. You are always
travelling (if that is the right word) throught time. When spacetime
is curved by matter or energy, your 'motion' through time
causes you to move through space.

Just to expand a bit on what Martin said, when gravity is weak the curvature
of space is negligible - it is the curvature of time that results in motion.
I posted the exact mathematical detail a while ago (a variant of one found
in Landau - Classical Theory of Fields) - if you are interested I can dig
it up from google - but the derivation can be found in any good book on
relativity.

Thanks
Bill



--
Martin Hogbin





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