From: nicolaas.vroom@pandora.be   
      
   On Friday, 26 July 2019 17:57:12 UTC+2, Phillip Helbig  wrote:   
      
   > I think that everyone understands purely illusory effects: A sees B's   
   > clock running slower and vice versa.   
      
   I doubt if it is that simple. You must know the whole physical situation   
   from start to end of this experiment, because it involves both physical   
   (mechanical?) and optical effects.   
      
   There is a difference if A sees B's clock running slower or running   
   behind. Running behind means that the difference in clock readings is   
   constant during a certain period is constant. This can only be when the   
   distance is fixed. As soon as any clock moves (which requires a force)   
   this experiment becomes like the twin experiment with moving clocks. See   
   below.   
      
   > It is clear that these are only apparent effects and not "real".   
      
   Length contraction is simpler to study   
      
   > The question of what is observed is trickier, because the finite speed   
   > of light has to be taken into account.  There WAS some real confusion   
   > about this, but it was cleared up by Terrell and Penrose a long time ago.   
      
   What physicists should do is only to discuss measurements and how these   
   measurements are made. When I see or observe something this are also   
   measurements, but they are relatif and involve my position or my   
   opinion.   
      
   When I observe a train which moves away from me and this train all of a   
   sudden stops, then the first thing I will observe (measure) later is   
   that the back of the train stops. At that moment the front is still   
   moving away. A little later the front. This means the length of train   
   during a small period becomes longer. This is an optical effect and not   
   a physical effect. (later implies light-travel-time effects)   
      
   The same thing happens when a train approaches me. In that case I will   
   observe (see, measure) that when the train suddenly stops, that the   
   front stops first. At that moment the back is further away. A little   
   later the back stops and during that small period the train becomes   
   shorter, optical. This is I think what Terrell meant.   
      
   > [[Mod. note -- Note that in the previous paragraph, the author is   
   using > the word "observed" NOT in the technical-special-relativity   
   sense (of > measurements made by instantaneously co-located   
   sub-observers so as to > eliminate light-travel-time effects), but   
   rather in the sense of "what > a camera image would show".  That is, the   
   author's usage of "observed" > INCLUDES light-travel-time effects. > --   
   jt]]   
      
   When you use this method, and eliminate all light-travel-time effects we   
   will get instantaneous measurements for all objects involved (in one   
   reference frame) The measurements will indicate that the physical length   
   of all moving rods will be the same (and not change).   
      
   But when identical clocks are involved free moving though space, which   
   initially all show the same time at position (p,t), after a certain time   
   all will be at different positions and show different clock readings.   
   What the measurements also could show is, that the clocks do not undergo   
   any form of length contraction.   
      
   > What is difficult to understand is the twin paradox: After A goes away   
   > and comes back while B stays at home and they then compare clocks at   
   > rest, EVERYONE agrees that A's clock has ticked less.  Recent discussion   
   > here shows that acceleration is not the "cause", since the effect   
   > depends on the length of the journey, and not on the acceleration.   
   > Since all clocks (mechanical, electronic, atomic, biological, nuclear)   
   > are equally affected, it is a) hard to imagine that some mechanism   
   > affects them all equally and b) no-one has any idea what such a   
   > mechanism could be.   
      
   To explain the above each clock should intially undergo a different   
   force (in a different direction) or a temporarily acceleration which   
   will change the speed of each clock differently, including its internal   
   operation, which is based on (the direction of) the speed of the clock   
   and (the direction of) the speed of light inside the clock. The overall   
   result will be that the # of ticks will be different, which can be   
   demonstrated when they meet again at one point.   
      
   Nicolaas Vroom   
      
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