From: nicolaas.vroom@pandora.be   
      
   On Tuesday, 9 July 2019 21:01:39 UTC+2, Tom Roberts  wrote:   
   > 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.   
      
   Let us perform an even simpler experiment. (in stead of rod you can also   
   read train) Take one reference rod of for example 100m. Position the   
   observer a certain distance away from the centre of the rod,   
   perpendicular on the direction of the rod. To make this simpler you can   
   also place two lights at both end of the reference rod. Take a second   
   rod of 50m. Place this rod between the observer and the reference rod   
   just as close as possible against the reference rod. What that means the   
   observer can see both ends of the reference rod. Now you perform the   
   experiment the same as before: You place the second rod completely   
   towards the left and you move the rod with a constant speed completely   
   towards the right. The question is: when second rod is straight in front   
   of the observer, will the observer see both ends of the reference rod?   
   IMO the most obvious answer should be: yes. That means (and that is   
   important) in this way you have an experiment which demonstrates that   
   the length of the second rod or moving rod is physical different as the   
   reference rod. The next step is to make the second rod longer (but still   
   smaller as the reference rod) and or increase the speed of the second   
   and try to find out what the limits are that the observer can still   
   observe both ends of the reference rod. The important point is that you   
   have an experiment at hand to test under which physical conditions, the   
   physical length of a moving rod versus a rod at rest are different.   
      
   The final step is to perform this experiment with two rods, with the   
   same length at rest. We know from the first experiment it is principle   
   possible to test if rods are of different length. The question here is   
   if it is in principle possible that the outcome of this experiment shows   
   that the two rods have a different length. I have my doubts. The problem   
   is not so much the practical limitations to perform this experiment, but   
   the physical explanation.   
      
   > 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 an assumption you are making, and   
   > is an essential part of the measurement.   
      
   You are correct. But there are two issues involved: The physical issue   
   if the moving rod (or train) becomes shorter and the second issue how to   
   demonstrate that this is the case (or not). This second part requires   
   observations i.e. involve the speed of light, but light is almost of no   
   importance related to the physical issue. The same is also the case if   
   you want to understand celestial mechanics (or the behaviour of all free   
   moving objects, like bullets). The first step is observations, which   
   involve the speed of light and the second step is to investigate the   
   physical behaviour in space, which does not involve the speed of light,   
   but the speed of gravitation. The fact that the two are quantitative the   
   same is of less importance than the physical differences between the   
   two.   
      
   > Moreover, you are ignoring optical diffraction....   
      
   Nicolaas Vroom   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)