Re: sources of gravity
From: Jimi (abc_at_def.com)
Date: 12/01/04
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Date: Wed, 1 Dec 2004 17:06:47 +0000 (UTC)
On Tue, 30 Nov 2004 18:46:53 +0000 (UTC),
alistair@goforit64.fsnet.co.uk (alistair) wrote:
>Jimi wrote in message:
>>There is no reason to assume that background energy does not curve
>>spacetime, but the fact that it exists uniformly throughout the
>>universe means that any net curvature is cancelled out.
>
>Space-time should curve up to a point in space because gravity would
>be so strong with 10^120 Joules / m^3 throughout the universe.In a
>previous post I asked (using a Newtonian line of thinking): can a
>large sphere of mass stop a smaller one inside it (the smaller sphere
>representing the space around us which is filled with vacuum energy)
>from collapsing to a point under its own gravity.If the answer is
>yes,the question would then arise:what stops the large sphere from
>collapsing under its own gravity? So even using a Newtonian
>argument,it is difficult to see why the universe wouldn't become very
>small indeed if vacuum energy exists at the density quantum mechanics
>says it should.A short range repulsive force
>could prevent vacuum particles from getting close together but the
>evidence from general relativity applied to black holes is that this
>force would have to exist at 10^-15 metres (once neutrons get closer
>together than this no force of any size can stop a singularity from
>forming as a neutron star collapses).
>No such force has been found.The existence of the casimir force shows
>that vacuum particles behave as theory predicts they will (the
>Casimir force is dominated by virtual photons - so we can only be sure
>virtual photons are behaving as expected).If vacuum particles are
>behaving "normally", then we are left to conclude that there must be
>something about space itself that stops
>the universe from collapsing to a point because of high vacuum energy
>density.
>If space were made from strings (which do not compress below about
>10^-34 metres) the problem would be solved.And the same is true for
>any such physical entity with a small but non-zero size limit.
That is a good point, an energy density of that magnitude would equate
to a large gravitational potential and should spontaneously create
black holes everywhere (or just one giant one). This is not the case
for one very good reason - gravity is not a force, but the result of
curved spacetime only. The energy with which all particles are
ultimately made of is always travelling at C (although trapped in very
tiny spaces) and continue to follow curved paths. This is why a mass
at rest will accelerate downward. Another way to think of this is to
imagine parallel mirrors (ideal of course) with a photon trapped
between them. This can be a rough model of the high frequency
electromagnetic energy which matter is composed of. In the absence of
a net curvature, the photon will simply bounce back and forth or in
other words, remain stationary. If however the mirrors are near a
large body the photon path will curve "downward" as it moves and after
being reflected it will continue to curve "downward" and "accelerate"
down the mirrors at the same rate that a stationary mass would. So
you see, energy density is irrelevent, only relative density and
ultimately spacetime curvature dictates gravitational potential.
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