From: fortunati.luigi@gmail.com   
      
   Luigi Fortunati lunedì 31/10/2022 alle ore 18:46:21 ha scritto:   
   >>> If we let all the elevators in the world move freely (unhindered), they   
   would head towards the center of the Earth where they would all meet.   
   >>>   
   >>> If we rested a body anywhere else, it would invariably accelerate towards   
   the center of the Earth, and only if we rested it in the center of the Earth   
   would it remain stationary in its place.   
   >>>   
   >>> So there is no doubt that the center of the Earth is not the same as all   
   the other points but it is very special, because it is a center of gravity   
   (and the other points don't).   
   >>>   
   >>> Also for General Relativity, is the center of the Earth different from all   
   other points?   
   >>   
   >> Not different from all other points, but different from all other points on   
   Earth because, as you mentioned, it is the center of gravity.  But there is   
   nothing special about the location per se, as one can change the center of   
   gravity by, say, digging    
   a hole and piling the dirt into a hill nearby.   
   >   
   > It is not true that "there is nothing special about the location per se" it   
   is the location itself that is special.   
   >   
   > And nothing changes if you move the privileged point by making a hole and a   
   small hill beyond: the privilege passes from one point to another but does not   
   disappear.   
   >   
   > And what is this privilege?   
   >   
   > If you are on Earth and you throw a stone anywhere, the stone does not come   
   back on its own.   
   >   
   > But if you are in the center of the Earth and you throw a stone in any   
   direction, it will come back * by itself *, accelerating towards you.   
   >   
   > That's what's special about the center of gravity location, that's its   
   privilege.   
   >   
   > [[Mod. note -- I think Phillip Helbig's point is that if you change the   
   > Earth's mass distribution a bit (e.g., by digging a hole and piling the   
   > dirt into a hill nearby), that the center of gravity will change to   
   > a different point.  If you are in the original (pre-digging) center   
   > of the Earth-with-hole-and-pile-of-dirt and throw a stone, the stone   
   > does *not* come back to you.   
   > -- jt]]   
      
   He doesn't come back to me because I'm no longer in the center of   
   gravity.   
      
   The stone comes back to me (obviously) only if I am in the center of   
   gravity.   
      
   It is the center of gravity that has these characteristics, not me.   
      
   What I wanted to point out is that the center of gravity (wherever it   
   is) is * unique *, that is, it is the only point where gravity   
   converges.   
      
   And no other point has the same characteristics.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)