From: richalivingston@gmail.com   
      
   On Friday, March 25, 2022 at 5:57:18 AM UTC-5, Mike Fontenot wrote:   
   ...   
   > I think a sizeable number of physicists DO believe that simultaneity at   
   > a distance is a meaningless concept, and I've even noticed a trend of   
   > trying to de-emphasize talking about simultaneity-at-a-distance in   
   > introductory special relativity courses. But I think that is a big   
   > mistake. It certainly wasn't Einstein's view, at least for inertial   
   > observers. (I've seen a quote from Einstein somewhere where he said he   
   > hardly recognizes his theory when he reads some modern descriptions of   
   > it.)   
   >   
   > My above question of Tom Roberts was prompted mostly by his use of the   
   > word "Convention" in describing simultaneity at a distance for an   
   > inertial observer. To me, the term "Convention" implies that there is   
   > more than one possible answer to the question "How old is that distant   
   > person (she), right now", when asked and answered by a particular   
   > inertial observer. "Convention" implies that one can pick from among   
   > multiple alternatives, all equally good.   
   >   
   > But I don't believe that is true, given my above description of how an   
   > array of synchronized clocks (all permanently stationary with respect to   
   > the given inertial observer) can be set up, creating a common "NOW"   
   > instant for him and for all of the HF's ("helper friends") co-located   
   > and co-stationary with the clocks. At any given instant "tau" in the   
   > life of the given inertial observer, it's clear that there is just a   
   > single answer to the question "How old is that particular distant person   
   > (she) right now (at the given time "tau" in the life of the inertial   
   > observer): it is what the particular HF (he) who happens to be   
   > momentarily co-located with the distant person (she), says it is, at the   
   > instant when he is age "tau". The only way there could be any other   
   > allowable answer is if the synchronization of the clocks isn't valid,   
   > and that is impossible if the velocity of light in that inertial   
   > reference frame is equal to the universal constant "c".   
      
   I think I mostly agree with you, but still think that the problem with   
   "now" at a distant location is that people take it as something   
   real and meaningful, and I think an argument can be made that it   
   is not very meaningful.   
      
   What is useful for physics is establishing a coordinate framework   
   that allows us to describe physics in a consistent way.  For this   
   purpose "now" at a distant location makes sense.  But our   
   mathematics is in a sense all knowing in that we keep track of   
   events at all locations and times, before they are able to interact   
   with other objects in the future.   
      
   For an individual, however, what is "now" at a distant location is   
   not something that has a consistent answer.  Two different   
   observers at the same event can have very different ideas about   
   what is happening "now" someplace else.  What is indisputably   
   real, however, is what these observers can see on their past   
   light cone.  All observers at an event will see the same things   
   on their past light cones.   
      
   Rich L.   
      
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