From: jappel@linux01.gwdg.de   
      
   Hi Phil,   
      
   Phil Hobbs wrote:   
      
   > Back in the '70s, iirc, people were doing things like dropping the   
   > corner cube mirror in a vacuum column and taking the path difference to   
   > be 1/2 g*t**2, but it turned out that surface stiction and ambient   
   > vibration made the initial velocity and rotation rate too uncertain.   
      
   Amazingly, and as far as I know, such corner-cube dropping machines are   
   still the sate-of-the art machines for absolute measuring of the local   
   gravitation: You pour a little concrete foundation, set up your gravimeter   
   and let it drop its cube a few thousand times...   
      
   The next best more modern technology is based on free-falling atoms: Atoms   
   are prepared in a superposition of internal quantum states and then they are   
   shot along different parabola trajectories depending on their internal   
   states. When their trajectories cross a second time, the superposition is   
   undone and the phase of the quantum interference and the area between the   
   paths gives the gravitation potential.   
      
   But possibly something has changed there in the last 3 years...   
      
   Cheers,   
   	Jürgen   
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