From: hobbs@electrooptical.net   
      
   On 5/8/2014 5:19 PM, Jürgen Appel wrote:   
   > 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   
   >   
      
   Interesting, thanks.   
      
   Cheers   
      
   Phil Hobbs   
      
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC   
   Optics, Electro-optics, Photonics, Analog Electronics   
      
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