From: nospam@de-ster.demon.nl   
      
   Tom Roberts  wrote:   
      
   > 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).   
   >   
   > > This should make clear that a change cannot be observed using the   
   > > local metric, but not everyone will agree that this means it cannot   
   > > 'really' change.   
   >   
   > The constancy of the vacuum speed of light applies only locally, so   
   > everyone who understands the issues will agree for a massless field.   
   > But of course that's the rub -- we don't really know whether the photon   
   > field is truly massless.   
      
   Why this insistence on photons being or not being 'truly' massless?   
   It is nothing but a red herring.   
   All troubles that might arise from a non-zero photon mass   
   are easily killed in advance   
   by adding 'in the limit of infinite frequence'   
   to the definition of the speed of light.   
      
   Since the photon mass cannot be measured,   
   even the longest radio waves that we can make   
   still have an 'infinite' frequency.   
      
   > > 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.   
   >   
   > All this only applies to massless fields, and we don't really know   
   > whether the photon field is truly massless. Of course we never will....   
      
   Indeed, and of course not.   
   A photon mass corresponding to a wavelength of the size of the universe   
   cannot be measured in principle.   
   Our poor photon has only a few decades left to have mass in,   
      
   Jan   
      
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