From: tjroberts137@sbcglobal.net   
      
   On 8/4/19 7:14 PM, Nicolaas Vroom wrote:   
   > On Thursday, 1 August 2019 21:34:33 UTC+2, Tom Roberts  wrote:   
   >>>> There is a difference if A sees B's clock running slower or   
   >>>> running behind.   
   >>> It runs behind because it runs slow.   
   >>   
   >> This is just plain not true, and I wish physicists who OUGHT to   
   >> know better would stop repeating such errors. [... further   
   >> discussion]   
   >   
   > In the book 'Subtle is the Lord' by Abraham Pais at page 145 we can   
   > read: "(g) Einstein rather casual mentioned that if two synchronous   
   > clocks C1 and C2 are at the same initial position and if C2 leaves A   
   > and moves along a closed orbit, then upon return to A, C2 will run   
   > slow relative to C1, as often observed since in the laboratory.   
   > [...]   
      
   Einstein did not know better when he wrote, as this had not yet been   
   fully understood. Pais is not a physicist and is writing for a general   
   audience, so it is no surprise that he does not know better.   
      
   	The observation mentioned is really that C2 showed less   
   	elapsed proper time than C1; the tick rates of the two   
   	clocks were NOT compared, and "running slow" was NOT   
   	actually observed.   
      
   This is merely another example of insufficiently precise wording: the   
   notion that C2 "runs slow" compared to C1 implicitly assumes a) using   
   the inertial frame of A (and C1), and b) the difference in final   
   displayed times depends ONLY on the clocks' tick rates. But a) if you   
   are talking about the tick rate of C2 you MUST use its own rest frame,   
   and b) the final difference also CLEARLY depends on the clocks' paths.   
      
   Bottom line: C2 does not "run slow". But C2 does run slow RELATIVE TO   
   THE FRAME OF C1. Note carefully the difference in English wording, which   
   reflects an essential difference in meaning.   
      
   >>> In the classic twin paradox, only one clock is accelerated. While   
   >>> it is clear that the accelerated clock runs slower [...]   
   >>   
   >> Again, stop making such erroneous and confusing claims. The clock   
   >> does NOT "run slower"; rather, it accumulated less elapsed proper   
   >> time (and did so while ticking at its usual and natural rate). The   
   >> reason for this is geometrical: the traveling twin followed a   
   >> shorter path through spacetime than did the earthbound twin.   
   >   
   > See the text by Einstein above.   
      
   Einstein did not know better -- he had the excuse that this had not yet   
   been fully understood. Today there is no such excuse.   
      
   I repeat: if a given clock actually does "run slow", then the first   
   postulate of SR would be violated, and the entire edifice of modern   
   physics would be overthrown. But no such violation has ever been   
   observed.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
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