From: tjroberts137@sbcglobal.net   
      
   On 9/6/17 12:55 AM, Nicolaas Vroom wrote:   
   > On Sunday, 3 September 2017 16:31:18 UTC+2, Gary Harnagel  wrote:   
   >> A proper test would have to be done with both clocks in space.  They would   
   >>  be synchronized when both are together and then one would be accelerated   
   >> (gently) to a different trajectory and then brought back later and   
   >> compared.   
   >>   
   >>> Also here the question is: is the duration the same? I doubt this.   
   >>   
   >> So does every scientist who understands relativity.   
   >   
   > 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.   
      
   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.   
      
   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".   
      
   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.   
      
   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.   
      
   > Part of the problem is the more I try to read, the more I try to understand   
   > and discuss the issues involved are becoming less clear. Sometimes the   
   > responds indicate that Einstein had it wrong. Also many books have it wrong.   
      
   Yes, relativity is subtle. And all too many popular books do indeed get it   
   wrong, in that they use shortcuts to avoid complex discussions that would   
   confuse non-experts. Clearly you are an instance of that.   
      
   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_.   
      
   > What is "Length contraction"? Is it physical?   
      
   Depends on what you mean by "physical". Is the moving object affected? -- NO   
   [#]. Does it have physical consequences? -- YES [@].   
      
   	[#] Just like "time dilation" (TD), "length contraction" (LC)   
   	is a geometrical projection, not any effect on the object.   
      
   	[@] Rotating a ladder to fit thought a doorway does not   
   	affect the ladder itself, but certainly does affect whether   
   	it can be carried through; what matters is the geometrical   
   	projection of the ladder's length onto the doorway's width.   
   	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.   
      
   LC: A current-carrying wire remains electrically neutral in its rest frame, but   
   the difference in LC between the electrons and ions in the wire yields an   
   electrostatic force in a moving frame that is interpreted as a magnetic force   
   in   
   the wire frame.   
      
   TD: Pions traverse kilometer-long beamlines at Fermilab and CERN because for   
   such high-energy pions the projection of their lifetime onto the beamline is   
   significantly longer than the beamline's length.   
      
   > What is "Time dilation"? Is it physical?   
      
   See above. the answer is the same as for "length contraction".   
      
   > What are twin type experiments? Are they physical?   
      
   Certainly. But they don't really display "time dilation". They demonstrate a   
   different geometrical property: different paths can have different path length   
   between intersections. For timelike paths of the twins, path length is elapsed   
   proper time, and that is what their clocks display.   
      
   > [...]   
      
   Understand the above before trying to go on.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)