From: tjroberts137@sbcglobal.net   
      
   On 7/28/19 2:09 PM, Phillip Helbig (undress to reply) wrote:   
   > In article <85b8886b-41e2-43fd-bb70-953712d8fbb2@googlegroups.com>,   
   >> 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.   
   >   
   > By "purely illusory effects" I mean those which arise solely from the   
   > relative, unaccelerated motion.  These are well documented, easily   
   > understood in SR, and no mystery at all.   
      
   Yes, "time dilation" does not affect the moving clock being observed,   
   but it is not really "illusory", because it can have real and measurable   
   physical consequences.   
      
   	For instance, charged pions have a proper lifetime of   
   	26 nanoseconds, which at 0.999999 c corresponds to   
   	traveling only 7.8 meters. Fermilab and CERN have had   
   	pion beamlines over a kilometer long because the "time   
   	dilation" of high-energy pions permits such long   
   	beamlines to work -- this is no "illusion".   
      
   >> 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. The clock does NOT "run slow",   
   it runs at its usual and natural rate -- the first postulate of SR would   
   be violated if this were not so.   
      
   	Such incorrect statements are all too likely to confuse   
   	non-experts, including some participants in this newsgroup.   
      
   Yes, in the twin paradox the traveling twin's clock accumulates less   
   elapsed proper time than the earthbound twin's clock. But it does NOT   
   "run slow" -- rather it travels a shorter path through spacetime.   
      
   	Please remember how English works -- in the sentence   
   	"That clock runs slower than this one", only the clocks   
   	are mentioned; in SR this is simply false because the   
   	laws of physics that govern both clocks' ticking are the   
   	same in the rest frames of both clocks. To make a correct   
   	statement about this you MUST mention how its tick rate   
   	is measured. A clock moving relative to inertial frame S   
   	does not "tick slow", but S will measure it to tick   
   	slower than identical clocks at rest in S.   
      
   >>> 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. [...]   
   >>> 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.   
      
   This is an example of the confusion created by incorrect statements   
   about "clocks running slow". The resolution is simple: the clocks do NOT   
   "run slow", so no "mechanism" is needed.   
      
   > 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.   
      
   	I hesitate to call that a "cause", but it certainly   
   	is an explanation.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
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