From: nicolaas.vroom@pandora.be   
      
   On Wednesday, 6 September 2017 17:32:34 UTC+2, Tom Roberts  wrote:   
   > On 9/6/17 12:55 AM, Nicolaas Vroom wrote:   
   > >   
   > > That means that all scientist agree that moving clocks (once synchronised)   
   > > run slower?   
   >   
   > No. As I keep saying, moving clocks do NOT "run slow" -- that is a shortcut   
   > phrase used in some popular writings that glosses over the actual situation.   
   >   
   > 	If moving clocks actually did "run slow", then when multiple   
   > 	observers are involved the clock would have to "run slow"   
   > 	at a different rate for each observer, which is manifestly   
   > 	impossible -- a clock can tick at just one rate.   
      
   In any experiment I should try to minimize the number of observers.   
   The clocks are the observers.   
   The preferred number of real observers is 1.   
   In a twin type experiment you also need one observer. The pilots are not   
   part of the experiment. The pilots follow a strict flight plan. (in fact one)   
   The only thing that the observer does is to reset both clocks at the beginning   
   of the experiment and perform the readings of both clocks at the end.   
      
   > How could an observer who is just passing by possibly affect a clock's tick   
   > rate??? -- from the clock's perspective, the phrase "moving clocks run slow"   
   > would imply that they do.   
      
   Observers are not supposed to touch the clocks during the experiment and are   
   not physical part of the experiment. The immer workings of the clock   
   (light rays) affect the ticking (rate).   
      
   > One can say that a stationary observer will MEASURE a moving clock to tick   
   > slower than an identical clock at rest. But that is quite different from   
   > the moving clock actually "running slow".   
   Why ? I would add: actually "running slow" compared to a clock at rest.   
      
   What you are doing is comparing the ticking rate of a clock at rest versus   
   a the ticking rate of a moving clock both in the same frame at rest.   
      
   > Bottom line: clocks tick at their usual rate, regardless of how they might   
   > be moving or where they might be located (relative to anything).   
   > This includes gravity.   
      
   Maybe I use the wrong wording, but if my understanding is correct than the   
   laws of physics are the same in each reference frame.   
   Using that same reasoning the physical speed of light is the same in each   
   reference system.   
   But that says nothing about the value of the speed of light in each frame   
   which in a moving frame should be measured by moving rods and clocks.   
   I agree with you that clocks tick at their usual rate each in their own   
   reference frame, but that does not say anything.   
   What you should do is compare (the same and different clocks) in the same   
   reference frame. First all at rest and decide which type is the most stable   
   and then moving clocks and unravel how moving clocks influence their behaviour.   
      
   > However, clocks that follow different trajectories through spacetime can   
   > experience different elapsed proper times between meetings, and observers   
   > moving relative to a clock can measure different values for its tick rate.   
   > This is all just geometry, and there is no effect on the clock itself.   
      
   I think we use different wordings for the same observations.   
   I think what you call proper time is the same as clock reading.   
   If I start an experiment and and I reset my clock and at the end of the   
   experiment my clocks shows 10 counts than something mechanical or physical   
   has caused this counting process.   
   If you do the same and when we meet your clock shows 8 counts than also   
   something mechanical or physical has taken place.   
   However because the results are not the same the mechanical, chemical or   
   physical processes involved are not the same. This is not geometry.   
      
   > Get a GOOD book on Special Relativity:   
   > 	Taylor and Wheeler, _Spacetime_Physics_.   
   > When you understand that, get a GOOD book on General Relativity:   
   > 	Misner, Thorne, and Wheeler, _Gravitation_.   
      
   Discussed separately   
      
   > > What is "Length contraction"? Is it physical?   
   >   
   > 	[@] Rotating a ladder etc   
   > 	That is a rotation in a space-space plane; both LC and TD   
   > 	are similar rotations, but in a space-time plane; all can   
   > 	have physical consequences.   
      
   IMO both LC and TD should be handled separately.   
   Rotating a ladder is a physical process but has no long-term consequences   
   on the length of the ladder.   
   Moving a clock is a physical process but can have long-term consequences   
   i.e. the temporarily moved clock runs behind a clock at rest.   
      
   Nicolaas Vroom   
      
   --- SoupGate-Win32 v1.05   
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