From: helbig@asclothestro.multivax.de   
      
   In article ,   
   ram@zedat.fu-berlin.de (Stefan Ram) writes:   
      
   > "J.B. Wood"  writes:   
   > >Hello, and so what we conclude is curved space-time is indistinguishable   
   > >from gravity.  They're both manifestations of the same phenomenon (some   
   > >as yet-to-be-discovered property of the cosmos?) Kind of like talking   
   > >about "matter/mass" and "energy" separately when we know E = MC^2.   
   >   
   >   There /is/ a difference between energy and mass because   
   >   E=mc^2 is only a simplification. In fact, E^2 is (mc^2)^2   
   >   /plus/ (pc)^2. In other words, kinetic energy is not mass.   
      
   That depends on terminology.  In  E = mc^2, m is the "rest mass".  If   
   something is not at rest, one can think of the total mass-energy in   
   terms of a relativistic increase in mass, so still E = mc^2 but m is   
   larger, or (somewhat more modern) the sum of the rest mass and the terms   
   involving momentum.   
      
   >   (However, some kind of movement can become part of the mass:   
   >   a heated gas with total momentum 0 has more mass than the   
   >   same gas cooled down and still with total momentum 0.)   
      
   Right.  This refers to ALL kinds of movement and, indeed, to all kinds   
   of energy, for example the electric field in a nucleus contributes to   
   its rest mass.   
      
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