From: fortunati.luigi@gmail.com   
      
   Jonathan Thornburg [remove -color to reply] martedì 20/12/2022 alle ore   
   20:35:04 ha scritto:   
   > Geodesic motion is also known as "free-fall".   
      
   This thing you say is certainly true for the Moon which is in free fall   
   towards the Earth (or, rather, towards the Earth-Moon common center of   
   mass).   
      
   It is also true for airplanes and it is true for men who move on the   
   earth's surface always going straight.   
      
   What is the first characteristic of geodetic motion? It is that of   
   moving along a curve without ever passing through its centre: the Moon   
   never passes through the Earth-Moon center of mass, man and the plane   
   never pass through the center of the Earth.   
      
   The second characteristic is that the geodetic motion does not converge   
   towards a single point: planes and men go in any direction and there is   
   no one that is privileged over the others.   
      
   The third characteristic is that if the body moving along a geodesic   
   encounters an obstacle and stops, when we remove the obstacle, it does   
   not start moving again along the same geodesic as before.   
      
   In fact, if the earth were a perfect sphere of ice, a skater who goes   
   straight forward, and is stopped, when we free him to move, remains   
   stationary in place of him.   
      
   These are the three characteristics of geodetic motion.   
      
   But the famous free fall of Einstein's elevator is not a geodesic motion   
   at all!   
      
   In fact, it lacks even one of the characteristics of geodetic motion.   
      
   First, it doesn't go around the center of the Earth, it just goes right   
   through it.   
      
   Secondly, it *always* converges towards a single point: if there are a   
   thousand different elevators, in points very distant from each other,   
   all the thousand converge towards a single point: the center of the   
   Earth (which a thousand airplanes and a thousand people, who follow   
   their geodesics, do not).   
      
   Thirdly, the elevator that is stopped by a constraint, as soon as we   
   remove it, automatically and inevitably resumes the same geodesic motion   
   it had before.   
      
   Thus, the "free fall" of the Moon is a geodesic motion, the flight of   
   airplanes and the movement of men on the earth are geodesic motions but,   
   not having any of the characteristics of geodesic motion, the "free   
   fall" of the elevator is not a geodesic motion.   
      
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    * Origin: you cannot sedate... all the things you hate (1:229/2)