XPost: sci.physics   
   From: PointedEars@web.de   
      
   Stefan Ram wrote:   
   > john larkin  wrote or quoted:   
   >> The gamma rays had no gravitational effect on the rest of the mass in   
   >> the universe, up until the instant that they collided to form a   
   >> particle pair. Then a gravitational object magically appeared.   
   >   
   >   As I argued before, that pair of photons /does/ have mass.   
      
   Then you are a hopeless case because a photon has mass _zero_ (in the   
   Standard Model of particle physics).   
      
   That is how it not only can, but why (according to special relativity) it   
   MUST, propagate at c.  We can see that as follows:   
      
   The energy dispersion relation for a photon is E = ℏ ω(K), and one finds   
   ω(K) = k c, so E = ℏ k c = ℎ/(2π) (2π/λ) c = (ℎ/λ) c.  But   
      
     p = ℎ/λ,   
      
   so for a photon   
      
     E = p c.   
      
   The energy-momentum relation for a free particle in Minkowski space is   
      
     E^2 = m^2 c^4 + p^2 c^2  <==>  m = 1/c √(E^2 - p^2 c^2)   
      
   so   
      
     E = p c  ==>  m = 0. ∎   
      
   >   But even if it would /not/ have mass.   
      
   It does not.   
      
   > The source of gravity   
      
   _gravitation_   
      
   > is   
   >   not mass, but energy-momentum (the energy-momentum tensor T)   
   >   (in general relativity).   
      
   Correct.   
      
   >   A photon passing by the sun is attracted to it.   
      
   No, it follows the curvature of spacetime.   
      
   > To conserve   
   >   momentum, the sun must also be attracted by the photon!   
      
   /Ex falso quodlibet./   
      
   F'up2 .relativity   
      
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   PointedEars   
      
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