XPost: sci.physics.relativity   
   From: julio@diegidio.name   
      
   [Cross-posted: please follow-up as appropriate.]   
      
   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:   
      
   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) one and the same for   
   every observer, clocks and everything, i.e. not even restricted to   
   inertial motion or observers.  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.   
      
   OTOH, coordinate-time is an artefact of coordinate systems, i.e. of a   
   choice of reference frame, and it is a *relative* notion 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.  It   
   is indeed overall "just" related to *simultaneity*, i.e. a class of   
   procedures to *measure* things (distances, speeds, etc.) that are not   
   local.  But, indeed, as we should very well know and never forget: time   
   dilation is not actually happening on board that ship...   
      
   Julio   
      
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