From: nicolaas.vroom@pandora.be   
      
   Op woensdag 7 juli 2021 om 19:20:49 UTC+2 schreef Phillip Helbig:   
   > In article <1pbx48j.1xmowyy1fnnxmfN%nos...@de-ster.demon.nl>,   
   > nos...@de-ster.demon.nl (J. J. Lodder) writes:   
   > > Thomas Koenig  wrote:   
   > > > 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.   
   > >   
   > > Nonsense.   
   > > In the SI as it stands it is impossible in principle   
   > > to measure the speed of light,   
   > Because, for practical reasons, the metre is now defined as the   
   > distance light travels in a certain time. That is our definition,   
   Etc,   
      
   You can do that, but now you create a new issue:   
     How is this CERTAIN TIME defined and more important measured in   
     detail in practice.   
   That is a very important issue because we can all measure the same   
   time, but when we compare all the distances measured,   
   (which should be identical) they are not.   
   That means at the most 1 person measures the distance of 299792458   
   meters correct assuming we all measure 1 second.   
      
   It is the same as the above ambiguous advice:   
   	"YOU should recalibrate your measurement device."   
   But if my measurement also is different from all of the others   
   how much should I adapt my time measurement device?   
      
   The above raised issue about CERTAIN TIME becomes even more important   
   if you want to measure the speed of a cosmic ray (etc).   
      
   Nicolaas Vroom   
      
   [[Mod. note -- An old nautical saying is "never go to sea with two   
   chronometers; always take one or three".  In this context, that means   
   that people doing precision timing & clock development often use an   
   an ensemble of co-located clocks (typically 5-10 are used), all of similar   
   construction and method-of-operation, so that they can inter-compare the   
   clocks.  Since all the clocks in the ensemble are co-located, they should   
   all record the same elapsed-time readings; more accurately, any differences   
   in their elapsed-time readings can be ascribed to clock drifts (errors).   
   Inter-comparing the clocks can thus give a statistical estimate of the   
   clocks' accuracy (quantified by "Allen variance" -- see the Wikipedia   
   article of that name if you want details).  If any clock is an outlier   
   in the ensemble, it's flagged as not-working-properly (a.k.a "broken").   
      
   For example, if I have 6 co-located clocks A,B,C,D,E,F which were   
   initially all synchronized, and after one mean solar day they read   
     A: 86400.00000124   
     B: 86400.00000382   
     C: 86400.00000226   
     D: 86400.00000275   
     E: 86400.00229071   
     F: 86400.00000390   
   seconds, then I can reasonably say that   
   (a) clock E is a clear outlier and is not working properly   
   (b) on average, clocks A,B,C,D,F run fast by about 3 +/- 1 microseconds/day   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)