From: tjroberts137@sbcglobal.net   
      
   On 9/30/22 4:29 AM, Luigi Fortunati wrote:   
   > Richard Livingston gioved=C4=97 29/09/2022 alle ore 17:44:00 ha scritto=   
   :   
   >> The same question can be asked of each atom in each of the bodies:=20   
   >> Why do they not accelerate toward the center of mass of the body?=20   
   >> The answer is electromagnetic forces, the same ones that keep your=20   
   >> feet from sinking into the earth.  These EM forces cause the atoms   
   >> to accelerate w/in the curved space-time generated by all the mass   
   >> in the objects. and the result is the common motion of the combined   
   >> mass.   
   >=20   
   > They don't accelerate (I think) because there's another atom below=20   
   > blocking it.   
      
   You both are using too imprecise terminology, with a PUN on "accelerate"   
   here -- WHAT TYPE of acceleration do you mean?   
      
   Luigi is clearly thinking of coordinate acceleration relative to   
   coordinates at rest with respect to the body in question.   
      
   But Richard is thinking of each atom's proper acceleration. relative to   
   the atom's instantaneously co-moving locally inertial frame. Such   
   locally inertial frames, of course, are always falling due to the local   
   gravity, and here the EM forces make the atoms accelerate relative to   
   such (falling) frames.   
      
   In GR, proper acceleration is physical, while coordinate acceleration   
   can be merely an artifact of the coordinates used (as happens here for   
   Luigi's choice [#]). Gravitation never generates proper acceleration,   
   but real forces, such as EM, always do.   
      
       [#] Luigi chose coordinates relative to which the   
       accelerations of atoms are zero, even though their proper   
       accelerations are nonzero.   
      
   Tom Roberts   
      
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