From: tjroberts137@sbcglobal.net   
      
   On 7/7/21 3:31 AM, J. J. Lodder wrote:   
   > Tom Roberts  wrote:   
   >> Certainly the (vacuum) speed of light COULD vary, it's just that in the   
   >> world we inhabit, with current technology, it is observed to not vary   
   >> significantly (when measured using standard clocks and rulers at rest in   
   >> some locally inertial frame).   
   >   
   > There is no such thing as a god-given 'standard clock'   
   > or 'standard ruler'.   
      
   Of course. Such standards are determined by humans. Organizations such   
   as ISO have been created to agree upon such standards and publish them.   
      
   It OUGHT to be obvious that a standard clock measures its elapsed proper   
   time using standard seconds, and a standard ruler measures distance   
   using some standard of length, such as meters.   
      
   >> But it certainly is possible that in the   
   >> future we will develop technology with greatly improved resolution and   
   >> discover that it actually does vary in the world we inhabit.   
   >   
   > A meaningless statement.   
   > If variation is found we will have to discover (or decide!)   
   > what it is that varies.   
   > (speed?, rulers?, clocks?, all three?, some 'fundamental' 'constant'?)   
      
   My statement is not meaningless: if the speed of light is measured to   
   vary, then it is certainly varying -- DUH!   
      
   Whether something else is also varying is a different question; to date   
   no significant variation has been found in any of the things you   
   mention. Such measurements have excellent accuracy, 9 or more   
   significant digits.   
      
   > This is not a matter that can be settled   
   > by means of naive empiricism,   
   > by just 'measuring' the 'speed of light',   
      
   How else could one detect a variation in the speed of light????   
      
   Tom Roberts   
      
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