From: tjroberts137@sbcglobal.net   
      
   On 3/25/22 5:57 AM, Mike Fontenot wrote:   
   > 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.   
      
   Right, that's what it means: there are an infinite number of coordinate   
   systems from which to choose, and in GR (and modern physics in general),   
   all are equally good for describing what happens. (Note that none of   
   them actually affects what happens, they only describe it differently.)   
      
   > 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, [...]   
      
   As I said before, you simply applied the convention of using Einstein   
   synchronization in an inertial frame. There is no compulsion to do so,   
   it just so happens that it is (usually) the easiest and simplest system   
   of coordinates to use in the case where the observer is at rest in the   
   inertial frame.   
      
   IOW the frame does not determine the coordinates, the human observer   
   does so. We humans generally select the coordinates in which   
   calculations are simplest, but that is all it is. Don't confuse an   
   ordinary human desire for simplicity (laziness) with anything more profound.   
      
   Richard Livingston then wrote:   
   > 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.   
      
   Hmmm. It's not merely an "argument", it's an inescapable aspect of   
   modern physics: "now at a distant location" does not contribute to any   
   current model of the world. In GR such spacelike intervals are outside   
   the past lightcone and so cannot affect what happens at the event in   
   question (i.e. now at the speaker's location); in QFT, fields evaluated   
   at such a spacelike interval always commute (and thus do not contribute   
   to any amplitudes).   
      
   > What is useful for physics is establishing a coordinate framework   
   > that allows us to describe physics in a consistent way.   
      
   Yes.   
      
   > For this purpose "now" at a distant location makes sense.   
      
   For describing what happened, AFTER THE FACT, based on reports received   
   from distant 'helper friends', sure. But for modeling the world, it is   
   not useful at all (see my previous paragraph).   
      
   Mike Fontenot then wrote:   
   > IF those clocks are synchronized (according to the given observer),   
   > then he can't help but conclude that the current age of that distant   
   >  person IS completely meaningful TO HIM.   
      
   Given the CONVENTION used, sure. But such clock synchronization is only   
   a convention. For an inertial observer, using the coordinates of their   
   rest frame is the simplest way to interpret "now" at distant locations,   
   but they are well advised to remember it is merely a convention to do it   
   that way, one based on simplicity TO HUMANS, not any physical basis.   
   Don't confuse an ordinary human desire for simplicity (laziness) with   
   anything more profound.   
      
   > But what about a non-inertial observer?   
      
   That is what you have spent a lot of time writing nonsense about. To an   
   accelerating observer, "now" has no definite meaning, and attempting to   
   give it one is just foolish, and no sensible person would ever believe   
   it. They can calculate whatever they like, but no sensible person would   
   think that just by turning their spaceship around their distant friend   
   gets younger -- they would KNOW that this is purely an artifact of the   
   math used in the calculation and their current acceleration, and that   
   has nothing whatsoever to do with their friend. Any astronaut in a   
   spacecraft capable of traveling at an appreciable fraction of c would   
   already know that their time is completely divorced from that of their   
   friends back home.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)