On Tuesday, January 10, 2017 at 5:46:13 PM UTC-8, Nagaraju Palagani wrote:   
   > Because of gravity, if we drop something, it falls down, instead   
   > of up. Well everybody knows that! But we do not know the mechanism   
   > that governs gravity?   
      
   No. We only have a model that tells us how paths of free fall are coupled   
   to energy-momentum(*). General relativity is a non-quantum theory so this   
   sort of thing is perhaps not that surprising:   
   this is a situation similar to where Lagrangian mechanics was with its   
   least action principle: no mechanism underlying the odd property of nature   
   choosing extremal paths for particle motions. Then came quantum mechanics   
   in the Feynman formulation showing how those extremal paths arise from   
   certain wave reinforcement and cancellation.   
      
   (*)Strictly speaking, GR doesn't even say that gravity (spacetime curvature)   
   is _caused_ by energy-momentum (mass, etc.), it only says that the two are   
   always correlated, with the correlation specified by the Einstein equation.   
   So there is a possibility that both are actually caused by some yet unknown   
   _third_ agency ("correlation does not imply causation" and all that).   
      
   --   
   Jan   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)