XPost: sci.physics, sci.physics.relativity   
   From: nomail@nomail.net   
      
   "George Kinley"  wrote in message   
   news:nmNFd.34881$g4.647017@news2.nokia.com...   
      
   > Hi,   
   > If our galaxy is rotating around other group of galaxy under influenece   
   > of combined gravity of galaxies, which mean that combined gravity of   
   > all the galaxy should be more then gravity of individual black hole   
   > present in the centre of every galaxy, so how does light travel under   
   > such emmence gravity   
      
   Why doesn't the matter in a galaxy collapse into its central   
   black-hole ?   
      
   Orbital angular momentum.   
      
   For a hyperbolic trajectory (escape trajectory), a body of   
   mass, m, a distance, R, from a black-hole of mass, M,   
   must possess orbital angular momentum, L, exceeding   
      
   L  >  L_escape  =  sqrt[ (G M m) R ]   
      
   That's from Newtonian gravity. For GTR, there's an   
   extra term that's responsible for orbital  precession   
   of massive bodies and for null geodesics for light.   
      
   Except for the case of a head-on collision ( L= 0 ),   
   light possesses orbital angular momentum WRT the   
   central black-hole.  Light follows a null geodesic that's   
   due to the extra term described above.   
      
   [Old Man]   
      
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