From: fortunati.luigi@gmail.com   
      
   Jonathan Thornburg [remove -color to reply] il 21/12/2023 09:19:38 ha   
   scritto:   
   > In a moderator's note a few days ago, I wrote:   
   >> Starting with Newtonian mechanics for simplicity,   
   >> "gravity" could plausibly mean any of several things:   
   >> * gravitational potential energy (which is a scalar in Newtonian mechanics).   
   >> * the Newtonian "little g" (which is a 3-vector at any given position   
   >>   and time   
   >> * the *difference* in the Newtonian "little g" between nearby objects   
   >>   at a given time; this difference is what you can measure about the   
   >>   gravitational field if you're in a freely falling elevator.  This   
   >>   difference is a 3-vector which depends on the separation between the   
   >>   nearby objects,   
   >>     difference = M * separation   
   >>   where M is a 3x3 matrix and "*" denotes matrix multiplication.  This   
   >>   3x3 matrix M (which is really a rank 2 tensor) provides a complete   
   >>   description of the local gravitational field at a given position and   
   >>   time.   
   >   
   > In article , Luigi Fortunati replied:   
   >> Does all this exclude that, in classical mechanics, gravity is a   
   >> fundamental force and, therefore, is necessarily a vector?   
   >   
   > To respond to this, I want to elaborate on my 3rd bullet point quoted   
   > above.  (This is from the perspective of Newtonian mechanics and   
   > gravitation.)   
   >   
   > Suppose we're in a freely-falling elevator.  What can we measure   
   > about the local gravitational field?   
      
   I will gladly answer you if you specify which gravitational field you   
   are talking to me about.   
      
   In the free falling elevator there are 3 masses and 3 gravitational   
   fields.   
      
   There are (1) the mass of the elevator, (2) that of the body inside it   
   and (3) that of the Earth outside it.   
      
   The 3 gravitational fields are:   
      
   - that between the mass of the elevator and the mass of the body inside   
   it   
      
   - that between the mass of the Earth and the mass of the elevator   
      
   - that between the mass of the Earth and the mass of the body inside   
   the elevator.   
      
   Which of the 3 gravitational fields are you talking about?   
      
   Luigi Fortunati   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)