From: helbig@asclothestro.multivax.de   
      
   In article ,   
   Carl K  writes:   
      
   > I used a new perfect (but simple) Newtonian physics engine to reverse a   
   > billiards break. The simulation surprised me. I expected it to re-form a   
   > stationary triangle of balls and spit out the cue ball. It did not.   
   >   
   > This means that classical physics is non-deterministic and not practically   
   > reversible.   
      
   It does not mean that it is non-deterministic.  Make a movie of the   
   billiard break and run it backwards: perfectly valid physics.  Classical   
   physics is reversible.   
      
   That it is not reversible in practice is a different matter, but that   
   was known before the simulation.   
      
   > The simulator is perfect in that it keeps times, positions, and velocities   
   > as expressions--3*sqrt(7)/7--rather than using numeric approximations. Its   
   > worlds are 2D with circles, infinite walls, and elastic collisions.   
      
   But even 3*sqrt(7)/7 or whatever, when it comes down to actually   
   calculating something, will be used a finite approximation.   
      
   Taking a broader view, questions such as whether classical physics is   
   deterministic and/or reversible aren't determined by simulations.  There   
   are some complicated processes which simulations can shed light on, but   
   with regard to fundamental issues, one must know the fundamental issues   
   when programming the simulation.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)