From: tjoberts137@sbcglobal.net   
      
   On 1/1/24 2:18 AM, Luigi Fortunati wrote:   
   > [...] But is it really like that?   
      
   We humans have no hope of knowing "what is really happening". The best   
   we can do is formulate models of what is happening, apply them to   
   various physical situations, and improve them -- this is called science.   
   Guessing "what is really happening" is the province of theology,   
   astrology, and other arcane wastes of time.   
      
   > [... confused writing]   
      
   If you want to understand General Relativity, you must STUDY it. Posting   
   random questions in a newsgroup is outrageously inadequate. Get a good   
   book and STUDY. You have never posted a question that could not be   
   answered by yourself if you had STUDIED.   
      
   Note in particular you must distinguish between various types of   
   acceleration: proper acceleration, coordinate acceleration in different   
   coordinate systems, and 4-acceleration. By just using the term   
   "acceleration", and failing to distinguish among the different types,   
   the PUNS destroy any argument you make.   
      
        Hint: observer A at the center of the earth would have zero   
        proper acceleration, as the earth is in freefall [@]. Observer   
        B on the surface has a proper acceleration of 9.8m/s^2 upward,   
        due to the force exerted on them by the surface. If either   
        observer extends their coordinates to the other [#], each would   
        say the other observer has zero acceleration in their   
        coordinates.   
      
   	[@] This is really an approximation in GR, valid only   
   	for "small" objects. Compared to the sun, the earth is   
   	indeed small, and this is an excellent approximation.   
      
   	[#] extending such coordinates is problematical,   
   	so just assume it can be done without distortions.   
      
   Note that "all physics is local" [Einstein and others], so these various   
   descriptions of accelerations of distant objects never appear in the   
   laws of physics. They are useful only for personal amusement.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)