From: relativity@paulba.no   
      
   Den 21.01.2026 01:12, skrev Thomas 'PointedEars' Lahn:   
   > 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:   
      
   I am well aware of this assumptions in Einstein's   
   elevator thought experiment.   
      
   That's why my thought experiment is different and   
   doesn't rely on that assumption.   
      
   The accelerometer is fixed to the wall, so it is   
   measuring the acceleration at that point.   
   If you had held the accelerometer in your hand you   
   could - in principle if the accelerometer was very   
   sensitive - measure the acceleration at different   
   positions. If the acceleration is constant at each   
   position, but different at the different positions,   
   then you would know that you are at a constant   
   height above the ground.   
      
   >   
   > 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.   
      
   Yes, of course.   
      
   >   
   > 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.   
   >   
      
   Right. You cannot get it more local than at a point.   
      
   --   
   Paul   
      
   https://paulba.no/   
      
   --- SoupGate-Win32 v1.05   
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