From: tjroberts137@sbcglobal.net   
      
   On 7/5/21 1:45 PM, Phillip Helbig (undress to reply) wrote:   
   > In article ,   
   > Nicolaas Vroom  writes:   
   >> tindividual photons also have energy, and as a consequence   
   >> individual photons also have a mass?   
   >   
   > E = mc^2 so in that sense photons have mass...   
      
   That is not mass in any sense, that is ENERGY.   
      
   >> This implies when a star emits light it also emits mass.   
      
   No, the light it emits carries no mass.   
      
   > ...and as a result the mass of a star decreases when it emits light.   
      
   Yes, even though the light itself carries no mass. As the star emits   
   light with total energy E, the star's mass decreases by m, with E=mc^2.   
   This is what that equation actually means, not your misinterpretation above.   
      
   > The question is whether the rest mass is zero.   
      
   "Rest mass" is an archaic term. Today "mass" refers to the norm of an   
   object's 4-momentum. In the object's rest frame that corresponds to its   
   energy [#]. The "c" in E=mc^2 is just a units conversion factor;   
   physicists often use units with c=1 and omit "c" from equations. So, for   
   instance, the PDG now lists particle masses in Ev, not the older Ev/c^2.   
      
   	[#] This of course does not apply to light, which has no   
   	rest frame. "Rest mass" could never apply to light; mass   
   	does, and is zero.   
      
   Tom Roberts   
      
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