From: helbig@asclothestro.multivax.de   
      
   In article ,   
   Nicolaas Vroom  writes:   
      
   > Op woensdag 30 juni 2021 om 21:37:22 UTC+2 schreef Tom Roberts:   
   > > On 6/29/21 1:49 PM, Jos Bergervoet wrote:   
   > > > Indeed we can agree that basically this is determined by the metric   
   > > > of space. Any massless field will have a propagation speed defined   
   > > > by the metric, but any measurement of speed also has to use that   
   > > > metric. So the result is fixed.   
   > > You mean the metric of spacetime (not space). And this applies only to a   
   > > massless field -- there is no fundamental reason for the photon to be   
   > > massless, it's just that its mass is observed to be consistent with zero   
   > > and an extremely tiny upper limit (< 10^-18 eV).   
   >   
   > How is this mass observed? Or should I write upper limit?   
      
   The upper limit comes from the observed accuracy of the inverse-square   
   law.  Also, if photons had rest mass, then photons of different energies   
   would travel at different speeds.  That effect is used to set limits on   
   neutrino masses.   
      
   > Is it not true,   
   > that when it is possible to measure the energy of a light pulse,   
   > that then individual photons also have energy,   
   > and as a consequence individual photons also have a mass?   
      
   E = mc^2 so in that sense photons have mass...   
      
   > This implies when a star emits light it also emits mass.   
      
   ...and as a result the mass of a star decreases when it emits light.   
      
   The question is whether the rest mass is zero.   
      
   > > > We know that seen from another point in space, the speed of light   
   > > > can be different if space-time is curved (as it usually is..) You   
   > > > may then claim that it is only an 'apparent' slowing down if e.g.   
   > > > light falls into a black hole, but then we only change the discussion   
   > > > to the meaning of 'apparent' and 'really'. We can't maintain that it   
   > > > is unobservable, in that case.   
   > > That's just an argument over the meanings of words. Moreover it's an   
   > > argument that never comes up because the constancy of the vacuum speed   
   > > of light applies only locally.   
   >   
   > Does that implies that globally, considering a light pulse (explosion)   
   > emitted over a long distance, that its speed is not constant?   
      
   Look up "Shapiro delay".   
      
   > Is this text from Wikipedia true?:   
   > "Photons are massless,[a] so they always move at the speed of light in   
   vacuum,   
   > 299792458 m/s (or about 186,282 mi/s).   
   > [a] The photon's invariant mass (also called "rest mass" for massive   
   particles)   
   > is believed to be exactly zero. This is the notion of particle mass generally   
   > used by modern physicists. The photon does have a nonzero relativistic mass,   
   > depending on its energy, but this varies according to the frame of   
   reference."   
      
   Yes.   
      
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