From: fortunati.luigi@gmail.com   
      
   Richard Livingston luned=EC 12/09/2022 alle ore 07:36:11 ha scritto:   
   >> The time coordinate is part of the geodesic.   
   >>   
   >> If the time coordinate has a preferred direction, the geodesic also has   
   >> a preferred direction.   
   > ...   
   >   
   > This is somewhat over simplified, but you can think of the g_00 metric   
   > component as similar to a potential, in that it multiplies the particle   
   > energy rather than adds/subtracts like a true potential.  Never the less, the   
   > "potential" decreases as you get closer to the gravitating mass, and thus   
   > objects feel a "force" in that direction.   
      
   Why is "force" in quotes?   
      
   If you feel it, it is a force without quotes.   
      
      
   [[Mod. note --   
   In the context of general relativity (GR) you don't feel gravity as   
   a force.   
      
   In GR the apparent downward "force" which I feel right now (sitting in   
   a chair in a room near the Earth's surface) is understood not as a   
   gravitational "force", but rather as follows:   
   (a) An inertial reference frame (IRF) at my location is by   
       definition in free-fall, and therefore must be accelerating   
       downwards at 9.8 m/s^2 with respect to the Earth's surface.   
   (b) Therefore, my chair (which is stationary with respect to the Earth's   
       surface) is accelerating *upwards* at 9.8 m/s^2 with respect to   
       an IRF at my location.   
   (c) Because my chair is accelerating upwards at 9.8 m/s^2 with   
       respect to an IRF, my chair must push up on my body in order   
       to accelerate my body upwards at 9.8 m/s^2 with respect to an IRF.   
   (d) More generally, because a coordinate system fixed to the Earth's   
       surface is accelerating upwards at 9.8 m/s^2 with respect to an   
       IRF, every object feels an apparent "force" pointing downwards.   
   -- jt]]   
      
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