From: ram@zedat.fu-berlin.de   
      
   helbig@asclothestro.multivax.de (Phillip Helbig (undress to reply)) writes:   
   >Minor nitpick (the summary is otherwise excellent): in practice, the   
   >distance between the two test bodies in an elevator will INCREASE   
   >because gravity is slightly stronger lower down.  That is an example of   
   >a tidal force.   
      
     By the way, recently there were news[1] about a quantum   
     gravity gradiometer that can detect tiny changes (like, by   
     one nano-g) in gravitation (ten times more accurate than   
     previous comparable devices). Such a device is also   
     described in [2], where one can read:   
      
   |Our gradiometer is based on two atom interferometers   
   |(gravimeters) apart in the vertical direction to measure the   
   |differential acceleration between two clouds of cold rubidium   
   |atoms in free fall.   
   from [2].   
      
     So, that would be an application and direct illustration   
     of that tidal force! In the case of [1], the two sensors   
     really seem to be 1 meter apart, just as in my thought   
     experiment.   
      
   |This places the measurement positions of the sensor at   
   |approximately 0.5 m from the road surface for the lower   
   |sensor, and 1.5 m for the upper sensor.   
   from [1]   
      
     Such devices allow, for example, to detect structures hidden   
     in the ground, such as tunnels, by their gravitational force.   
      
     1 Michael Holynski, Quantum sensing for gravity cartography,   
     Nature (2022). DOI: 10.1038/s41586-021-04315-3.   
      
     2 Wei Lyu et al, "Development of a compact high-resolution   
     absolute gravity gradiometer based on atom interferometers"   
      
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