From: fortunati.luigi@gmail.com   
      
   Richard Livingston mercoledì 07/09/2022 alle ore 05:12:12 ha scritto:   
   >> 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.   
      
   I absolutely agree with what you wrote.   
      
   But how does all this affect the direction of motion that a body   
   decides to take when it is left free to follow its geodesic?   
      
   I simplify with my animation   
   https://www.geogebra.org/m/zdevssyz   
   where there is an elevator that (initially) cannot follow its geodesic   
   since it is constrained at point A.   
      
   If we eliminate the constraint (Start) the elevator starts moving   
   towards B to follow its geodesic which I have highlighted with the blue   
   dotted line.   
      
   This blue line has no limits and has no privileged directions, it is   
   not a vector that goes in an obligatory direction.   
      
   One direction is as good as the other.   
      
   But the elevator always goes in one direction only, the one that goes   
   down, towards point B.   
      
   And it never goes up.   
      
   What is it that forces the elevator to always get started downwards and   
   never upwards?   
      
   What has the down direction more than the up direction?   
      
   In other words, are geodesics open lines or are they vectors with one   
   direction privileged over the other?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)