From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >Stefan Ram venerd=EC 04/03/2022 alle ore 10:12:00 ha scritto:   
   >>From the point of view of physics, however, gravity is   
   >>always there;   
   >On this, of course, I absolutely agree: the force of gravity in the   
   >free-falling elevator does not disappear.   
   >But doesn't Einstein say quite the opposite?   
      
     In Newton's worldview, there is an attracting force F, with   
      
   F = G m_1 m_2 / r^2                     and   
   G = 6.67430(15) x 10^(-11) N m^2 kg^(-2),   
      
     along the (imagined) connecting line between two systems of   
     masses m_1 and m_2, respectively, and with distance r. This   
     does not change when one of the masses is placed within an   
     elevator, free falling or not. That force F does not depend   
     on the speed or acceleration of either of those two systems.   
      
     In the theory of general relativity (GR), there is never any   
     "force of gravity" at all. Bodies always move on what is the   
     generalization of a straight line in curved space-time, when   
     no force (a concept which does not include gravity now) is   
     acting on them. This theory is much more complicated and not   
     necessary here.   
      
     One should not mix concepts from different theories,   
     but needs to describe everything using the same theory.   
     I decided to use Newton's worldview. In Newton's worldview,   
     the force of gravity between two systems with masses never   
     disappears, it is always "G m_1 m_2 / r^2".   
      
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