From: richalivingston@gmail.com   
      
   On Thursday, May 5, 2022 at 2:26:53 AM UTC-5, Luigi Fortunati wrote:   
   > Luigi Fortunati marted=C3=AC 03/05/2022 alle ore 10:17:57 ha scritto:   
   > > [Moderator's note: Even if gravity is not localized, the concept of an   
   inertial frame can be. -P.H.]   
   > Ok, so let's ask ourselves who is at rest and who is not in the "local"   
   > reference.   
   >   
   > Let us ask ourselves: if the man in the elevator stopped at the floor   
   > drops the ball he is holding, is it the ball that falls towards the   
   > floor (Newton) or is it the floor that falls towards the ball   
   > (Einstein)?   
   >   
   > It is entirely reasonable to imagine that there may be a force capable   
   > of accelerating the ball downwards but it takes a lot of faith to be   
   > able to accept that there may be a force capable of accelerating the   
   > entire Earth towards the ball.   
      
   I think this will be my last post on this issue:   
      
   -"Who is at rest?" is the wrong question.  The relevant question is "who   
   is in an inertial frame?"  If you are in an inertial frame you can let go   
   of an object and it will float where you left it.  If you let go of an   
   object and it accelerates away, then you are not in an inertial frame.   
      
   -In the paradigm of General Relativity there are no "forces" due to   
   gravity, only curvature of space-time.  The result is that a reference   
   frame that is at a fixed position away from the center of mass of   
   a large massive object is no longer an inertial frame.  That is, if you   
   release an object that is initially stationary in that frame it will start   
   to accelerate away from you.  In this paradigm the released object   
   has no forces on it, it is merely following its normal world line through   
   space-time.  You, on the other hand, feel a force on your feet that is   
   accelerating you upwards relative to the inertial frame that is   
   accelerating downwards wrt you.   
      
   -You can choose to ignore this point of view and say the released   
   object is experiencing a force downwards, but if that is the case   
   why does someone in free fall feel "weightless"?  If you were inside   
   an elevator far from any mass you would feel weightless.  If you   
   were in an elevator in free fall near a large mass you would again   
   feel weightless.  In one case you would say there is no force, in the   
   other you would say there is.  What difference does it make?   
      
   -If you don't want to think in terms of the curvature of space-time and   
   the effect that has on an object's world line, you will not progress   
   much in understanding General Relativity.   
      
   Rich L.   
      
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