From: PointedEars@web.de   
      
   Paul B. Andersen wrote:   
   > You are standing on the floor in a small room   
   > without windows with an accelerometer fixed to the wall.   
   > The accelerometer shows that it is accelerating   
   > in the direction which to you is upwards.   
   >   
   > If the accelerometer shows that the acceleration   
   > is constant 1g, then there is no way you can   
   > decide whether you are stationary at the ground,   
   > or if you are accelerating at 1g in a spaceship.   
   >   
   > If the accelerometer shows that the acceleration   
   > is changing with time, then you know that you are   
   > in a spaceship.   
      
   Notice that this thought experiment which became known as the "elevator   
   thought experiment" depends on the assumption that the gravitational field   
   is uniform.  In practice that is not so, so one *can* tell whether one is   
   in a constantly accelerating reference frame or at relative rest in the   
   vicinity of a celestial body, say Terra:   
      
   As the gravitational field is not actually uniform but   
   spherically-symmetric, objects that are falling due to gravitation and would   
   be falling in parallel all the way, are actually also moving closer together   
   as they are falling.  This would not happen in a constantly accelerating   
   reference frame.   
      
   The Einstein Equivalence Principle only states that *locally* gravitation is   
   indistinguishable from constant acceleration; that is, if the considered   
   volume is small enough that the tidal forces described above can be neglected.   
      
   --   
   PointedEars   
      
   Twitter: @PointedEars2   
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