XPost: sci.physics.relativity   
   From: tjroberts137@sbcglobal.net   
      
   On 6/15/22 1:18 AM, Julio Di Egidio wrote:   
   > On 14/06/2022 17:15, Tom Roberts wrote:   
   >> Simultaneous means at the same time, and that is a   
   >> coordinate-dependent concept (because a coordinate system defines   
   >> what one means by "time") -- for a given coordinate system all   
   >> events with the same value of the time coordinate are simultaneous   
   >> with each other.   
   >   
   > No, that is fundamentally upside down:   
      
   No, it is not. It is what these words mean, and what is required for   
   determining simultaneity for spatially-separated objects. That is what   
   this (sub-)thread was about.   
      
   > The "time" we experience is *proper* time, which is the (physical)   
   > time clocks and everything tick *locally* (i.e. at and per   
   > themselves), and is *universally* (an invariant of the geometry and)   
      
   Sure, though you use an unusual and useless meaning for the word   
   "universal". Yes, the proper time of a given observer or object is   
   independent of coordinates (aka invariant). But it is not, and cannot   
   be, "universal" in the usual sense of the word: applying to everything   
   -- it only applies to the specific observer, object, of clock in   
   question.   
      
   > one and the same for every observer, clocks and everything,   
      
   No. Each observer, object, or clock's proper time is completely   
   independent of the proper time for other observers, objects, or clocks.   
   Proper time is a property of a worldline, separate for each worldline.   
      
   > i.e. not even restricted to inertial motion or observers.   
      
   Yes.   
      
   > So, it is *proper* time that is (physical) "time" and, not per   
   > chance, modulo more speculative research on time itself, it is the   
   > primary parameter of any dynamical system.   
      
   Hmmm. This depends on what one wants to do. Certainly the standard   
   relativistic equation of motion for a given object refers to its proper   
   time. The ticking of any clock displays its proper time.   
      
   But the subject was simultaneity, for objects separated spatially. For   
   that, the proper time of any object is useless -- a given observer,   
   object, or clock can only apply its proper time to events located along   
   its worldline, which does not include objects separated spatially.   
      
   For spatially separated objects, one must DEFINE what one means by   
   "simultaneous", and that means setting up a coordinate system. As I   
   said.   
      
   > OTOH, coordinate-time is an artefact of coordinate systems, i.e. of   
   > a choice of reference frame, and it is a *relative* notion   
      
   Yes. Necessarily so. "Simultaneous" is a relative concept. (Simultaneity   
   is of course only one aspect of a coordinate system.)   
      
   > that introduces distortions such as time dilation and length   
   > contraction for the trajectory of any particle that is not simply at   
   > rest in that frame.   
      
   Hmmm. This is garbled. The coordinate system does not induce "time   
   dilation and length contraction" -- they are generated by certain   
   measurement procedures applied to objects moving relative to an inertial   
   coordinate system.   
      
   Tom Roberts   
      
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