From: nospam@de-ster.demon.nl   
      
   Tom Roberts  wrote:   
      
   > 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.   
      
   Thank you for this perfect illustration of my point.   
   Supposing there would be an effect, what would we conclude? [1]   
      
   Your naive assumption is that blocks of metal must have a constant   
   length. They feel real solid, don't they?   
   However, if we assume that fundamental constants   
   (such as alpha for example) might be variable   
   there is no reason to believe in constancy of the length of   
   metal rods.   
      
   Given what we know about spacetime, and about the physics of metals,   
   my guess is that the second interpretation will be the preferred one,   
      
   Jan   
      
   [1] I ignore the practical point that the limited accuracy   
   of your setup (a mere 10^-9) will not yield a meaningful result anyway.   
   We already know that things are far more stable than that.   
   So the practical conclusion will be some kind of experimental error.   
      
      
      
   > 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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