XPost: sci.physics.relativity   
   From: PointedEars@web.de   
      
   Stefan Ram wrote:   
   > 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.   
      
   Nonsense.   
      
   >   Whether gravity is a force depends on the model.   
      
   No, _gravitation_ is always just a phenomenon.  People who say "gravity is a   
   force" are oversimplifying twice.   
      
   > 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.   
      
   No, it _can be_ described as the consequence of a force.  It can be better   
   described as a consequence of the curvature of spacetime.  There is probably   
   (because we alredady know that there are circumstances where GR stops   
   working) an even better description, perhaps as an interaction with a   
   graviton field.   
      
   >> 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   
      
   A photon does not have a stress-energy tensor (what would be the stress?),   
   and a stress-energy tensor (field) is not an effect, but a quantity in   
   theoretical physics.   
      
   > and therefore not observable in practice.   
      
   Nonsense, photon--photon scattering has been observed:   
      
      
      
   >> [*] 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;   
      
   It is *the* quantum of *the* (quantum-)electromagnetic field.  That is, it   
   is an elementary excitation state of that field.   
      
   > that field has a stress-energy tensor;   
      
   Yes, but a photon has not.   
      
   > that stress-energy tensor modifies the curvature of space-time.   
      
   No, it _corresponds to_ the curvature of _spacetime_.   
      
   > But we do not yet have a "theory of everything" that describes quanta   
   > (such as photons) /and/ gravitation.   
      
   Correct.   
      
   > Quantum electrodynamics describes quantum effects in electrodynamics   
      
   In a sense.  More precisely, is a quantum field theory of electrodynamics.   
      
   > and general relativity describes gravitation.   
      
   Not only.   
      
   Why are you continuing this mindless amok-crosspost?   
      
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