Light travels at the speed of light, and we know how it behaves on galactic scales--gravitational lensing is well known now, and is consistent with Einstein's general relativity. Light is affected by gravity much as ordinary matter is. However, we do not know with certainty that the observed behavior of ordinary matter on galactic scales is consistent with general relativity. The observed motions of galaxies, and of stars on the fringes of galaxies, cannot be explained by general relativity IF only ordinary matter exists. Hence, if general relativity is correct, there must exist vast amounts of unseen "dark" matter to give additional gravity to account for the motions in question, and that dark matter cannot all be ordinary matter that is too cold to radiate--even cold ordinary matter absorbs light and hence is still "visible" by obscuring things behind it. The dark matter must be of some sort completely undetectable (so far) except for its gravitational effects. It is not protons, neutrons, electrons, et cetera.
General relativity replaced Newton's gravity. At the velocities encountered in the Solar system, and far from very massive bodies, Einstein's equations simplify to approach Newton's. (The Sun is just massive enough that the orbit of Mercury departs measurably from Newtonian mechanics over several decades, and this was one of the historical clues that set Einstein to thinking.) So another way of phrasing my question is, which is simpler:
1)To believe in dark matter of a completely unknown type, and to believe that it is more plentiful that all presently known matter, in order to "save" general relativity, or
2)To believe that Einstein's theory, like Newton's, is merely a very good approximation that doesn't work under some circumstances (such as at accelerations that exist on the fringes of galaxies, but that are so low that they cannot be made to exist anywhere within the Solar system)?
--Daniel

• What about supersymmetric particles? - harvey1 - October 21, 2002 - 03:58 UTC
• Newton's theory needs to be saved as well - yaniru - October 21, 2002 - 11:24 UTC
  • vacuum energy - emperor__alikar - October 22, 2002 - 01:40 UTC
    • Vacuum energy - yanniru - October 22, 2002 - 19:15 UTC
  • More dark matter - danieldgj - October 22, 2002 - 03:55 UTC
    • Velocities are measured - yaniru - October 22, 2002 - 11:11 UTC
      • velocities - danieldgj - October 22, 2002 - 12:57 UTC
        • Do you have an alternative theory. - yanniru - October 22, 2002 - 19:20 UTC
          • Occam's razor - danieldgj - October 22, 2002 - 23:12 UTC
            • Gravitational colapse - emperor__alikar - October 22, 2002 - 23:56 UTC
              • Time limits - danieldgj - October 23, 2002 - 22:22 UTC
                • What if they dicover one day... - emperor__alikar - October 23, 2002 - 22:39 UTC
                  • The Sun fits - danieldgj - October 25, 2002 - 01:23 UTC
                    • Well, not really.... - kc2gwx - October 25, 2002 - 04:02 UTC
            • I don't think Occam's razor applies here - harvey1 - October 23, 2002 - 13:51 UTC
              • What about nuetrinos - yaniru - October 23, 2002 - 13:56 UTC
                • Neutrinos are plentiful. - danieldgj - October 23, 2002 - 22:35 UTC
              • Either way, Einstein is approximate - danieldgj - October 23, 2002 - 22:33 UTC
                • I agree with your objection to that assumption - harvey1 - October 23, 2002 - 23:02 UTC
                  • How does supersymmetry dispense with dark matter? - yaniru - October 24, 2002 - 15:11 UTC
                    • That's right - harvey1 - October 24, 2002 - 16:31 UTC
                      • Agreed - yaniru - October 24, 2002 - 17:21 UTC
                        • Yes and no. - danieldgj - October 25, 2002 - 01:27 UTC
                          • How do you know that - yaniru - October 25, 2002 - 02:37 UTC