Re: Newbie question about gravity

On May 20, 9:59 pm, "Bill Hobba" <rubb...@xxxxxxxx> wrote:
"Martin Hogbin" <goatREMOVETHIS...@xxxxxxxxxx> wrote in message

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"z" <genesinpie...@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.

It's kind of ironic. The same concept can be said of real
accelerations. In other words, when your time path is curved by
acceleration, your motion through time causes you to move through
space. The treatment of gravity in GTR seems to resemble more of a
"acceleration by virtue of inertia" theory rather than purely an
inertial theory of gravity.

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.

I would appreciate a link to your google article. For clarification,
I take it that "space curvature" is what we commonly consider as tidal
accelerations, a differential of acceleration over a spatial
interval. But the time curvature, is the curvature which determines
the geodesics of point masses. Time curvature definitely causes
relative accelerations of local freely-falling frames which swamp any
tidal effects, meaning that, their coordinate systems are relatively
accelerated (dv/dt) wrt to one another even though they are moving
inertially. Spacetime Physics only addresses the space curvature
(tidal property of curvature) for the purpose of defining an adequate
domain for applying STR. So I would definitely appreciate a link.

I suspect time curvature is dimensionally similar to the space
curvature, acceleration/distance, only distance is in time as opposed
to three space? In any event, thanks for any help.

.

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