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Posted by Daniel Johnson on October 22, 2002 03:55:38 UTC

"Besides. There is no gravity in General Relativity. There is only space dynamics and curvature do to the presence of mass."
Yes, of course. Sometimes, however, when succeeding theories explain the same observed phenomena, it is simpler in a short discussion to use the same vocabulary as one moves from theory to theory.

"Moreover, the accelerations on the fringe of galaxies are not low. They are easily measured. They are known."
Well, no, yes, and yes. The accelerations are easily calculated and therefore easily known. But they are very low--the accelerations of stars relative to galactic cores, and the accelerations of galaxies relative to superclusters, are quite small. Acceleration equals velocity squared divided by the radius. Near the centers of galaxies the velocities are high and radii are small, giving high acceleration. At the galactic fringes, the opposite holds--vastly lower velocities and vastly larger radii. We cannot reproduce such low accelerations for Earth-based or even Solar-system-based experiments. Hence such very low accelerations are an experimental area where Relativity and Newtonian mechanics alike cannot be easily disproven or verified by testing. Hence the need for dark matter remains a matter of faith in Newtonian and Relativistic mechanics in areas where evidence for their accuracy is lacking.
And yes, both Newton and Einstein provided us with magnificent, useful equations that I truly love.

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