From: tjroberts137@sbcglobal.net   
      
   On 7/4/19 2:41 PM, Nicolaas Vroom wrote:   
   > On Thursday, 4 July 2019 09:19:55 UTC+2, Tom Roberts  wrote:   
   >> Ultimately, the twin paradox displays the basic geometrical fact   
   >> that different paths between a given pair of points can have   
   >> different path lengths.   
   >   
   > This is a 'difficult' explanation.   
      
   Not really. It is everybody's experience in traveling different paths   
   with different lengths between a given pair of endpoints. The only   
   subtlety is that for a timelike path in relativity, proper time is the   
   path length.   
      
   > The easiest explanation is (?) that this is a physical issue which   
   > depends on the inner workings of the clock.   
      
   But that is woefully incorrect. If the "inner workings of the clock" are   
   affected by its motion, then the first postulate of SR must be violated.   
   Because, after all, the laws of physics govern the inner workings of   
   every clock. To date we have observed no such violation.   
      
   > More or less the same why moving pendulum clocks on a moving ship   
   > don't work properly (moving from A to B and back).   
      
   That is COMPLETELY different. A "pendulum clock" is not really a clock   
   until one adds the entire earth to the "clock". A moving ship (rocking   
   and pitching in the sea) disturbs the relationship between pendulum and   
   earth.   
      
   > Moving clocks working on light signals have the same problem.   
      
   No, they don't. In its rest frame, a light clock ticks happily at its   
   usual rate. Like all other clocks, this is so regardless of the inertial   
   motion of that rest frame.   
      
   Authors of popular works on science that claim "moving clocks run slow"   
   are WRONG, and are doing students no favors by make such incorrect   
   claims. Moving clocks are OBSERVED to run slow, but the clock itself is   
   unaffected by its motion.   
      
   	Remarkably, clocks are unaffected even by non-inertial   
   	motion, as long as any proper acceleration is within the   
   	clock's specifications. Muons are observed to decay with   
   	their usual proper rate, even in a storage ring under   
   	a proper acceleration of 10^18 g -- a truly enormous   
   	acceleration. Nature creates remarkably robust clocks.   
   	[Bailey et al, Nature 268 (July 28, 1977) pg 301.]   
      
   > When such a clock moves with the speed of light, the clock does not   
   > tick at all.   
      
   Cannot happen. No massive object can move at the speed of light, and the   
   mirrors of a light clock are massive.   
      
   Tom Roberts   
      
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