From: dr.j.thornburg@gmail-pink.com   
      
   Luigi Fortunati  wrote:   
   > An even more general principal is that "free fall" means "fall without   
   > obstacles".   
   >   
   > No one can deny that, in the most remote space far from all gravity,   
   > a metal elevator falling towards an electromagnet is in "free fall".   
      
   This is mistaken: In this context a magnetic field counts as an "obstacle",   
   and a magnetic material interacting with a magnetic field is *not* in   
   free fall.   
      
      
      
   > And a metal robot in a metal elevator resting on an electromagnet   
   > (without being able to look outside) cannot know if there is a motor   
   > that is accelerating the elevator or if there is an electromagnet that   
   > is attracting it towards the floor (principle of equivalence).   
      
   A metal elevator will partially, but not fully, screen the magnetic field,   
   so there will still be a nonzero magnetic field inside the elevator.   
   This means that the robot can easily tell the difference: just see if,   
   inside the elevator, there's a differential acceleration between test   
   masses inside made of iron (ferromagnetic) vs oxygen (diamagnetic) vs   
   helium (non-magnetic).   
      
   --   
   -- "Jonathan Thornburg [remove -color to reply]"    
      currently on the west coast of Canada   
      "what I still don't understand to this day is why a suicide bomber is   
       cowardly and deceitful, and the bomber pilot who throws bombs at innocent   
       people from a height of five kilometers is courageous and brave."   
                                              -- Volker Pispers (German comedian)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)