From: tjroberts137@sbcglobal.net   
      
   On 6/29/18 2:27 AM, Nicolaas Vroom wrote:   
   > On Tuesday, 26 June 2018 12:13:16 UTC+2, Tom Roberts  wrote:   
   >> As I said to Mr. Vroom: a clock's tick rate can only be measured   
   >> by a co-located and co-moving observer; anything else involves   
   >> signals, not just the clock.   
   >   
   > Consider a co-located and co-moving observer A which clock moves a   
   > long a certain path from P1 to P2. Consider a second co-located and   
   > co-moving observer B which clock moves a long a certain but   
   > different path from P1 to P2.   
      
   You are using words in confusing and unusual ways. I assume you mean   
   that observers A and B each carry a clock, so they are each co-located   
   and co-moving WITH THEIR OWN CLOCK (but not with each other or the   
   other's clock). A and B travel different paths from P1 to P2.   
      
   	("Co-located" is a RELATION, not an adjective; one cannot   
   	 have a "co-located observer". one can only have an   
   	 observer co-located with some other object.  Ditto for   
   	 co-moving.)   
      
   > When they compare clocks at P2 A's clock has counted 40 ticks and   
   > B's clock has counted 30 ticks. What is wrong by claiming that A's   
   > clock (for some reason) has ticked faster during the time that both A   
   > and B travelled from P1 to P2?   
      
   It is wrong because the phrase "A's clock ticked faster than B's clock"   
   references ONLY THE TWO CLOCKS. We know that A's clock ticked at its   
   usual rate, and B's clock ticked at its usual rate; presuming the two   
   clocks are identical, this means they ticked at THE SAME RATE (their   
   usual rate).   
      
   	Note that physics is subtle and requires precision in   
   	thought and word. That phrase is insufficiently precise,   
   	as it does not specify how the comparison is performed.   
      
   One CAN say that between P1 and P2 clock A ticked more times than clock B.   
      
   	A major lesson of 20th-century physics is to not attempt   
   	to discuss things which were not measured. Here you   
   	measured the number of ticks of each clock, but did NOT   
   	measure either clock's tick rate.   
      
   You are implicitly attempting to use some sort of "absolute" or "God's   
   eye" view of the situation, and comparing the clocks via "God's eye",   
   without actually saying so. That's invalid, as there is no such thing in   
   modern physics. You could construct a coordinate system, compare the two   
   clocks to coordinate time along their paths, and conclude that A's clock   
   ticked faster RELATIVE TO THIS COORDINATE TIME than B's clock. But this   
   is QUITE different from what you said -- in particular it explicitly   
   says how the comparison is performed. Moreover, it is possible to   
   construct different coordinates in which B's clock ticks faster relative   
   to coordinate time for at least part of the trips -- Gregor Scholten did   
   that explicitly.   
      
   As I said, this is as much about English word usage as it is about physics.   
      
   Tom Roberts   
      
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