From: tjroberts137@sbcglobal.net   
      
   On 7/5/19 5:48 PM, Nicolaas Vroom wrote:   
   > [...] The first step is   
   > to agree that length contraction is something physical.   
      
   Be careful, as the phrase "something physical" means different things to   
   different people. Your sense (demonstrable via experiment) is OK, but   
   many/most people would consider it to mean "a physical change in the   
   object's length", which is wrong.   
      
   > The experiment I have in mind is the following take a very long   
   > locomotive.  You can also take a train with wagons. In that case the   
   > wagons should be connected in such away that the whole train becomes a   
   > rigid object. Put the train on a very long straight track and put two   
   > markers near the track, one in front and one in the back such that the   
   > distance between the two markers is just slightly smaller that the   
   > distance of the train. The observer (video camera) should be situated a   
   > large distance from the center of the train. When that is the case the   
   > observer should see only one marker. When you move the train slightly   
   > the observer should see the other marker but never both markers. From   
   > the observers point of view the markers are behind the train or track.   
   > Now you bring the train completely at the far left end of the track. The   
   > experiment starts when the train starts to move towards the right at a   
   > high speed with the video camera on. The central question to answer is:   
   > when the train is in between the two markers will you observe both   
   > markers? Yes or no. When the answer is no, there is no length   
   > contraction involved. When the answer is yes, there is length   
   > contraction involved. In this experiment there are no clocks involved,   
   > which removes the issue of clock synchronisation.   
      
   While there are no clocks, you are assuming that the observations of the   
   two ends are simultaneous in the frame of the rails and video camera.   
   While that might seem obvious, is _IS_ an assumption you are making, and   
   is an essential part of the measurement. Moreover, you are ignoring   
   optical diffraction....   
      
   Put in some numbers: assume the train is 100 meters long, moving at the   
   outrageous speed of mach 2 (686 m/s). The predicted "length contraction"   
   is 3E-10 meters. The ends of the train cannot be made flat to that   
   accuracy, and the experiment must fail (even ignoring diffraction).   
      
   Imagine a 1 meter projectile fired from a rail gun at the outrageous   
   speed of mach 20 (6860 m/s). The predicted "length contraction" is also   
   3E-10 meters, with the same result.   
      
   Tom Roberts   
      
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