From: nospam@de-ster.demon.nl   
      
   Eric Flesch  wrote:   
      
   > On 01 Sep 2021, nospam@de-ster.demon.nl (J. J. Lodder) wrote:   
   > >The assertion was that you can at least in principle use laboratory   
   > >measurements of the speed of light to see if it varies.   
   > >To see that you can't you need to have at least a vague idea   
   > >of how such measurements are done.   ...   
   >   
   > I just wanted to thank the OP for his excellent precis.  It has   
   > bothered me for a long time that with defining our length scale in   
   > reference to c-dependent physical outputs, that we've given up an   
   > absolute length scale as a basis of measurement.  That is, we've   
   > assumed c to be ever unchanging WRT a physical rod.  If that   
   > assumption is wrong, we've disabled our ability to find out.  We have   
   > put blinkers on ourselves.  It can't be right to do that.   
      
   It is not just that we have given up on having an absolute lenght unit,   
   we have understood that we never had one to begin with,   
   if we look at things to sufficient accuracy.   
      
   We have not assumed c to be ever unchanging wrt to a physical rod,   
   we have defined distances (to much greater accuracy and reproducibility)   
   by giving the speed of light a defined value.   
   This has not diminished our ability to measure things in any way,   
   it only means that we have agreed   
   to incorporate observed changes somewhere else.   
   (if there are any)   
      
   The effect of this is to replace the physically meaningless question   
   of whether the -dimensioned- quantity c can vary   
   with the physically observable and meaningful question   
   of whether the dimensionless ratios of differently defined length units   
   are changing wrt to each other.   
      
   Jan   
      
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