From: tjroberts137@sbcglobal.net   
      
   On 6/13/22 10:43 AM, Richard Livingston wrote:   
   > Events that are on each others light cone are effectively   
   > simultaneous (the metric separation is zero), and that is true for   
   > all observers.   
      
   Yes, for such pairs of events the metric separation is zero, and that   
   applies for all observers  (it is really independent of observer and   
   coordinates).   
      
   But that is not at all what "simultaneous" means. Rather, that is a   
   lightlike interval.   
      
   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. In an inertial   
   coordinate system, for each such value the locus of events is   
   necessarily a 3-D spacelike hypersurface. In general, for different   
   coordinate systems these loci are completely different.   
      
   Tom Roberts   
      
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