From: jos.bergervoet@xs4all.nl   
      
   On 21/06/29 8:41 AM, J. J. Lodder wrote:   
   > Phillip Helbig (undress to reply)    
   >> In article , Thomas Koenig   
   >>> Nicolaas Vroom  schrieb:   
   >>>   
   >>>> Ofcourse you could claim that the speed of light is constant.   
       ...   
      ...   
   > It is incredible how much misunderstanding there is   
   > on such a simple subject.   
      
   I think there are several reasons for it. See below..   
      
   > The speed of light cannot 'really' be variable.   
   > Why?   
   > In order for the speed of light to be measurable at all   
   > we need to define both a length and a time unit.   
   >     ...   
      
   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.   
      
   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.   
      
   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.   
      
   Obviously, by specifying the "locally observed speed of light"   
   this problem is avoided, but if you do not want to talk about   
   different points in space or in time, then it becomes a bit   
   too obvious that 'change' cannot be observed, it looks a lot   
   like not wanting to observe it, then..   
      
   Finally, it's also conceivable that the photon mass at some   
   point in the future will become nonzero, theoretically (another   
   theory says that this has already happened).   
      
   All these things explain the ongoing discussion, I think..   
      
   --   
   Jos   
      
   --- SoupGate-Win32 v1.05   
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