From: helbig@asclothestro.multivax.de   
      
   In article , "Richard D.   
   Saam"  writes:   
      
   > "In the simple newtonian model, gravity propagates instantaneously:   
   > the force exerted by a massive object points directly toward   
   > that object's present position.   
   > For example, even though the Sun   
   > is 500 light seconds from the Earth,   
   > newtonian gravity describes a force on Earth   
   > directed towards the Sun's position "now,"   
   > not its position 500 seconds ago.   
   > Putting a "light travel delay" (technically called "retardation")   
   > into newtonian gravity would make orbits unstable,   
   > leading to predictions   
   > that clearly contradict Solar System observations."   
   >   
   > How does this concept translate in to universe observations?   
   > It would appear that gravity travels instantaneously   
   > among all "now" time objects (galaxies etc).   
   >   
   > Is that correct?   
   >   
   > Richard D Saam   
   >   
   > [[Mod. note -- If you're assuming Newtonian gravity, then gravity travels   
   > instantaneously among all "now" time objects.  If you're assuming general   
   > relativity then it doesn't (and the calculations are much more complicated).   
   >   
   > Roughly speaking, the difference between a general-relativity and a   
   > Newtonian-gravity will be determined by the ratio between the typical   
   > velocities of the masses vs their typical separations.  I am not certain,   
   > but I think this ratio (difference) is a lot smaller for typical cosmological   
   > galaxy-clustering simulations than it would be for solar-system simulations,   
   > i.e., Newtonian gravity is a relatively better approximation for   
   > cosmological galaxy-clustering simulations than it is for solar-system   
   > simulations.   
   > -- jt]]   
      
   The Newtonian concept is wrong.  Gravity propagates at a finite speed,   
   the same as the speed of light.  It turns out, though, that the standard   
   Newtonian approximation is better than including this finite speed in   
   the Newtonian picture.  The fully relativistic treatment is the most   
   correct, of course, but not worth the trouble in many cases.   
   Cosmological simulations use the standard Newtonian approximation in   
   most cases.  This is a lucky break for simulators.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)