From: richalivingston@gmail.com   
      
   On Wednesday, September 7, 2022 at 1:22:46 AM UTC-5, Luigi Fortunati wrote:   
   > A geodesic passes from A and also from B.   
   >   
   > Is the direction from A to B fully equivalent to the direction from B   
   > to A?   
   >   
   > Or can it happen that one of the two directions prevails over the   
   > other?   
      
   In general relativity, the physical (spatial) distance from A to B is always   
   the same as from B to A.  This is not true of the time for light travel   
   however.  If A is higher in a gravitational field than B, the round trip   
   light travel time is longer for A to B to A (from the point of view of A)   
   than it is for B to A to B (from the point of view of B).  For any single   
   observer the round trip times are the same either way.  That may seem   
   paradoxical, but it is actually true and consistent.   
      
   Rich L.   
      
      
   [[Mod. note -- Referring to Richard's opening sentence, I take it that   
   he is referring to the metric distance integrated along the geodesic   
   between points A and B.  This is also known as the "geodesic distance".   
   -- jt]]   
      
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