XPost: sci.electronics.design   
   From: ram@zedat.fu-berlin.de   
      
   DJ Delorie  wrote or quoted:   
   >So... a photon moving at speed of light has some   
   >energy-mass-whatever-equivalent, which is sufficient to warp spacetime.   
      
     Everything has a "stress-energy tensor", including photons.   
     (There is a Wikipedia page "Stress-energy tensor")   
      
   >That change may cause other photons' paths to "bend"   
      
     That tensor changes the curvature of space-time, which influences   
     the paths of everything moving in space-time, including photons.   
      
   >                                                     as if gravity were   
   >a force, regardless of time effects, because it's not a force, it's a   
   >shape.   
      
     Whether gravity is a force depends on the model. In general   
     relativity, it's not a force, but in other fields it's sometimes   
     a force. In high-school physics, it certainly is a force.   
      
   >I wonder, though, if there were some non-spacetime actual "force"   
   >between photons (I realize there isn't, but imagine one for a moment).   
      
     Yes, there is photon-photon scattering in QED (virtual charged particle   
     loops), but it is a very weak effect that rarely matters in practice.   
      
     Both effects (a photon's stress-energy tensor and photon-photon   
     scattering) are very weak and therefore not observable in practice.   
      
   >If said force obeyed the usual F=ma, *now* would time dilation come into   
   >play wrt photons?  Two photons travelling next to each other, from their   
   >point of view (or any other), how would that force evolve itself over   
   >"time" ?   
      
     Time dilation does not matter here because everything can be   
     described from one single frame of reference (the lab frame)   
     while it cannot be described from the "point of view of a photon".   
      
   >[*] except if photons move at the speed of light, and "gravity" does   
   >    too, do photons create gravitational shock waves as they travel?  Or   
   >    does the distortion just travel happily along with the photon?  Or   
   >    just behind it?  How could we find out?   
      
     The photon is a quantum of an electromagnetic field; that field   
     has a stress-energy tensor; that stress-energy tensor modifies   
     the curvature of space-time. But we do not yet have a "theory   
     of everything" that describes quanta (such as photons) /and/   
     gravitation. Quantum electrodynamics describes quantum effects   
     in electrodynamics and general relativity describes gravitation.   
      
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