From: tjroberts137@sbcglobal.net   
      
   On 9/1/21 2:27 PM, J. J. Lodder wrote:   
   > [...]   
      
   Here's a description of a laboratory experiment to measure any variation   
   in the vacuum speed of light during a year, at the part per billion   
   level. Please explain why you think that it could not detect such   
   variations.   
      
   The basic idea is to construct a very stable vacuum optical cavity of   
   length L, and measure any variations in the frequency of its free   
   spectral range (= c/(2L)). The precise value of L does not matter,   
   as this is looking at variations.   
      
   Construct a temperature-controlled cell a meter or so on a side (c.f.   
   Kennedy-Thorndike), and inside it construct a vacuum optical cavity   
   whose length is ~ 0.5 meters, determined by material with essentially   
   zero coefficient of thermal expansion (e.g. invar). The free spectral   
   range of such a cavity is c/(~1 meter), which is ~ 300 MHz. Use   
   Pound-Drever-Hall laser locking to lock two high-quality lasers to   
   adjacent fringes and count their heterodyne frequency, using at least   
   four Cs-133 atomic clocks to generate the timebase [#]. By counting for   
   1000.000000000000 seconds and averaging multiple counts this should   
   easily have a resolution of ~0.1 Hz (out of ~300 MHz). Make measurements   
   repeatedly over at least a year.   
      
        [#] Don't use GPS, as they will steer its clocks to offset   
        any variation in c.   
      
   This should detect variations in c over one year, at the part per   
   billion level. In principle it could do better....   
      
   Tom Roberts   
      
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