From: nicolaas.vroom@pandora.be   
      
   On Sunday, 1 July 2018 19:39:21 UTC+2, Tom Roberts  wrote:   
   > 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.   
      
   Correct.   
      
   > 	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.   
      
   The comparison is done at two specific moments.   
   First at moment t1 when the two clocks depart at P1.   
   Secondly at moment t2 when the two clocks arrive at P2.   
   The simplest situation is when P1 and P2 are the same, that A stayed at P1   
   and that B travelled from P1 to P3 and back to P1.   
      
   > One CAN say that between P1 and P2 clock A ticked more times than clock B.   
      
   Correct.   
   But one can also say IMO that the rate is different, without quantifying   
   what the  actual rate is.   
   (because the period between the two measurements is the same).   
   IMO one can also say that one clock runs faster.   
      
   > 	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 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.   
      
   IMO if A's clock ticks faster than this coordinate time and   
   If B's clock ticks slower than this coordinate time than   
   A's clock ticks also faster 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.   
      
   Gregor Scholten wrote:   
      
   "So, the claim that A's clock is ticking faster than twin B's clock is   
   not correct in a frame-independent sense. Depending on the frame, A's   
   clock may tick faster during the complete trip, or tick even slower than   
   B's clock temporarily."   
      
   The issue is the final outcome of one specific experiment.   
   It is either one of: A is faster, B is faster or A=B   
   When P1 = P2 this outcome is frame (observer) independent and depends   
   about the path followed.   
      
   > As I said, this is as much about English word usage as it is about physics.   
      
   IMO physics is the most important.   
   It is the physics (inner workings) of a clock that describes its beheviour.   
   If its inner workings is based on photons than the speed of these photons   
   is an issue.   
   If its inner workings involves mass/gravity than gravitation is an issue.   
   The inner workings of an atomic clock, it depends how many atoms change   
   from state A to state B   
   (see https://www.timeanddate.com/time/how-do-atomic-clocks-work.html)   
   Also here the issue is in so far the # of oscillations is a function   
   of the speed v of the atomic clock.   
      
   > Tom Roberts   
      
   Thanks   
      
   Nicolaas Vroom   
   http://users.pandora.be/nicvroom   
      
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