From: helbig@asclothestro.multivax.de   
      
   In article , Jos   
   Bergervoet  writes:=20   
      
   > > First you must measure the speed of light. Or better, you have to des=   
   cribe   
   > > a general accepted way, how the speed of light is measured.   
   >=20   
   > But we have that! Observing the propagation of light using length and   
   > time units based on the propagation of light. The outcome is fixed.   
   >=20   
   > > If you have such a recipe, you can measure and decide if the speed of=   
    light   
   > > is everywhere the same and if this speeed is the same in -time- at a   
   > > specific location   
   >=20   
   > The only way to change it is to abandon the accepted definition (which   
   > always keeps the speed fixed). And perhaps this could happen, if   
   > for instance all speeds in physics suddenly became 10% higher, except   
   > light. Then most physicists would be open to the idea that actually we   
   > should change this definition.   
      
   Indeed.  It could happen.  It has happened.  The metre used to be=20   
   defined as 1/10,000,000 of the quadrant of the meridian through Paris. =20   
   The second used to be defined as a certain fraction of a year.  These=20   
   definitions were changed.  Why?  In part because other definitions can=20   
   be reproduced with greater accuracy, but also because they can change. =20   
   Of course, it would have been silly to say that since the definition of=20   
   the metre is fixed, the size of the Earth, or the length of the year,=20   
   could not change, even in principle.   
      
   > > The same type of problems exists between: what is mass and how is thi=   
   s   
   > > directictly measured or calculated based on different measurements.   
      
   Like some other units, the kilogramme has recently been redefined.  Why?=20   
   In part because the standard kilogramme in Paris was losing mass.  So,=20   
   it was possible to detect it, even though it was the standard.   
      
   The Universe does not care how we define our units.  Certain quantities=20   
   either vary or they don't.  We cannot prevent them from varying by using=20   
   them to define a unit.  When we are REASONABLY SURE that something does=20   
   not vary (at least not detectably over interesting timescales), as a=20   
   purely PRACTICAL matter one can define units in terms of constants of=20   
   nature such as the speed of light.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)