From: nospam@de-ster.demon.nl   
      
   Phillip Helbig (undress to reply)    
   wrote:   
      
   > 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.   
      
   It is incredible how much misunderstanding there is   
   on such a simple subject.   
   To clear things up:   
   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.   
   Now this can be done in many different ways.   
   For example, we can define the second on basis of a seconds pendulum,   
   or on basis of the vibrations of a quartz crystal,   
   or on basis of some atomic hyperfine transition, or....   
   With some thought you can figure out   
   how those definitions scale, when fundamental constants vary.   
   (different of course, the pendulum has a G in it,   
    the other second definitions don't)   
   Likewise for length units, like platinum bars, atomic wavelengths,   
   stabilised lasers, the AU, etc.)   
      
   Now if some, or all of the fundamental constants vary,   
   so does the measured c, depending on how we set up   
   the definitions of the length and time units.   
   (with all kinds of possible answers)   
   This 'measured' c is not something to do with nature,   
   it depends on our -human- measurement definitions.   
      
   Now it should be obvious to anyone with any sense   
   of how physics should be done   
   that the units of time and length   
   should be chosen in mutually compatible ways.   
   (so differing by a factor of c)   
   So, with the right definitions of units c cannot possibly be variable.   
      
   Or, saying the same in different words:   
   c is not really a fundamental constant of nature   
   in any way that makes -physical- sense.   
   It is merely a conversion factor between units.   
   You could just as well ask how 5280 feet/mile   
   is going to change with the age of the universe,   
      
   Jan   
      
      
      
      
   >  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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