XPost: sci.electronics.design   
   From: PointedEars@web.de   
      
   [Supersedes to fix premature submission of incomplete text]   
      
   Stefan Ram wrote:   
   >   [...] please see the section "Electromagnetic stress-energy tensor" on   
   >   the Wikipedia page "Stress-energy tensor". (I think this also is   
   >   equation 4.2.27 in "General Relativity" (1984) - Robert M. Wald.)   
      
   [The book can probably be recommended (it was in the list of recommended   
   literature in our GR lecture notes; I have also heard a talk by Wald at my   
   university, the University of Bern, recently, which he gave after receiving   
   the Einstein Medal for his life's work there); although it is not suited for   
   beginners (read Hartle's "Gravity" and Carroll's "Spacetime" first); but ...]   
      
   As if you would have any clue what you are talking about.   
      
   >   A single photon does not have mass,   
      
   So, finally you saw the light? ;-)   
      
   > while a pair or gas of photons can have mass.   
      
   It _does_ have mass.  That has to do with the fact that the mass of a   
   *system* is equivalent to its rest energy (that is what E_0 = m c^2 actually   
   means), and so the *kinetic* energy of the photons (which is equal to their   
   total energy as their mass is zero) contribute to that as they are (MUST be)   
   in motion in the rest frame of that system.  [It does not make sense to   
   attribute to seemingly constrained photons a potential energy because they   
   are quantum objects and are not actually constrained: the probability to   
   find them outside the enclosure is just smaller than inside.]   
      
   See also:   
      
   PBS SpaceTime: The Real Meaning of E=m c^2   
      
      
   >   Usually, the space-time curvature generated by realistic light is   
   >   small and therefore rarely has an observable effect.   
      
   "Realistic"?   Otherwise this is correct.   
      
   > However, in the early universe it once dominated over matter.   
      
   Misleading.  The energy density of electromagnetic radiation was larger than   
   the energy density of matter then.  See also:   
      
      
      
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   PointedEars   
      
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