From: helbig@asclothestro.multivax.de   
      
   In article , Thomas Koenig   
    writes:   
      
   > Nicolaas Vroom  schrieb:   
   >   
   > > Ofcourse you could claim that the speed of light is constant.   
   >   
   > The way that the SI units are defined now, the speed of light   
   > in vacuum is indeed constant.  If you measure anything else than   
   > 299792458 m/s, recalibrate your measurement devices.   
      
   The speed of light is now a defined quantity, that is true.  However,   
   that is merely a practical matter.  If the speed of light really were   
   variable, that could still be detected just as easily as before the   
   redefinition.  Suppose that the speed of light did drop by a measurable   
   amount.  People would not immediately redefine the length of everything   
   because of that.   
      
   Many other SI units were recently redefined, so they are "exact" in that   
   sense.  The same caveats apply.   
      
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