From: helbig@asclothestro.multivax.de   
      
   In article ,   
   Nicolaas Vroom  writes:   
      
   > [[Mod. note -- Please limit your text to fit within 80 columns,   
   > preferably around 70, so that readers don't have to scroll horizontally   
   > to read each line.  I have rewrapped the lines in this article.  -- jt]]   
   >   
   > Op vrijdag 17 september 2021 om 06:16:55 UTC+2 schreef Phillip Helbig   
   > (undress to reply):   
   >   
   > > c to be constant and that we believe that it is constant. I don't think   
   > > that likely; my point is merely that we cannot prevent the speed of   
   > > light from changing simply by defining it to be constant. Rather, it is   
   > > defined to be constant as a practical matter because we have evidence   
   > > that it is.   
   > If that is true you should explain us what that evidence is.   
   > I prefer to write: because we have no evidence that it is not.   
      
   OK, but that is a linguistic/philosophical issue which detracts from the   
   main argument.   
      
   > My point is mainly that if 'you' define c to be physical constant,   
   > that means that photons always have the same speed, 'you' should   
   > also define how this speed is calculated.   
   > What Rømer did (very cleverly) was to calculate the average   
   > speed of light between Jupiter and the Earth. This calculation is   
   > a mathematical description of the mechanical Sun, Earth, Jupiter   
   > system.   
      
   Right.  But his speed agrees with other measurements.   
      
   > Is his result valid for the whole of the universe?   
      
   That is indeed an assumption, but we have no evidence that it is not   
   true.  People have investigated cosmological models with a variable   
   speed of light.   
      
   > We use the speed of light to measure hugh distances.   
   > Is it realy correct to claim that this speed, along that whole path, is   
   > everywhere the same?   
      
   The speed of light in a vacuum is the same (and is the same no matter   
   who measures it).  There is no evidence that that is not true.   
      
   > What about the influence if 'space' is a vacuum?   
      
   If it is not a vacuum, the speed is less.   
      
   > What about the influence of matter?   
      
   General relativity describes that.  It is a matter of taste whether one   
   interprets some phenomena as slowing the speed of light or increasing   
   the distance.   
      
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