From: roland.franzius@uos.de   
      
   Am 12.12.2020 um 19:13 schrieb Bruce Scott:   
   > On 2020-11-13, Luigi Fortunati  wrote:   
   >> The bridge and the train have the same length at rest.   
   >>   
   >> The bridge collapses only if the entire weight of the train rests   
   >> on it.   
   > [...]   
   >   
   >> [Moderator's note: This is essentially the same puzzle as the   
   >> ladder paradox, which even has its own Wikipedia entry.  In fact,   
   >> it is closer to the "man falling into grate" version originally   
   >> discussed by the late, great Wolfgang Rindler. -P.H.]   
   >   
   > The version we got in class (way back when) was the train entering   
   > the barn with the doors opening/closing just in time.  The answer,   
   > of course, is relativity of simultaneity.   
      
   With respect to gravity things are different. Objects at speed of light   
   follow "straight lines" near worldlines of light on local light cones.   
      
   If light rays traverse the bridge without beeing bend down to touch the   
   opposite wall, any massive train will reach the other side, too, in the   
   limit v->c. Since bridges are constructed using light rays, no extra   
   engeneering art is necessary.   
      
   For slow trains, the engineer should form the bridge as a ballistic   
   parabola in order to save steel.   
      
   The question of the bridge collapse is a question of energy-momentum   
   transfer in its rest system.   
      
   The train at the speed of light is releivistically compressed to a point   
   and acts like a point mass on a ballistic hyperbole at its perigaeum. It   
   does not transfer energy-momentum to the bridge. It will leave the earth   
   surface tangentially and will disappear somwhere behind Uranus.   
      
   --   
      
   Roland Franzius   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)