From: richalivingston@gmail.com   
      
   On Saturday, March 19, 2022 at 1:27:38 PM UTC-5, Tom Roberts wrote:   
   ...   
   > A major lesson of modern physics is to discuss only measurable   
   > (observable) quantities [#]. Both "everyday lives" and "wavefunction   
   > collapse" violate that dictum (in very different ways).   
   >   
   > [#] Interestingly, this applies to both QM and GR (for   
   > very different reasons).   
   >   
   > Tom Roberts   
      
   Tom, I would be very interested in your expanding a bit on how this all   
   applies to GR.  -Rich L.   
      
   [[Mod. note -- I too would be interested in what Tom says.   
      
   My take would be that in GR, there is no preferred coordinate system   
   and all physical quantities (= those that are measurable, at least in   
   a gedanken sense) should be independent of the coordinate system in use.   
      
   Notably:   
   * the coordinate "time" of an event, or the difference between the   
     coordinate times of two events) is merely a coordinate; it has no   
     inherent physical meaning and can be changed arbitrarily by changing   
     our coordinate system   
   * *proper* time along some (timelike) worldline is measurable (it's   
     what an (ideal) clock moving along that worldline would measure),   
     can be said to have an inherent physical meaning (as the observable   
     result of that measurement), and *doesn't* change when we change   
     coordinates   
   * similarly, the coordinate position of an object, or the (coordinate)   
     distance between two objects, is also a coordinate, has no inherent   
     physical meaning, and can be changed arbitrarily by changing our   
     coordinate system   
   * the *proper* distance along a given path is measurable (at least in   
     a gedanken sense: one can imagine laying down a sequence of standard   
     rulers end-to-end along the path), and doesn't change when we change   
     coordinates;   
   * coordinate singularities (and the set of events where they occur)   
     have no inherent physical meaning, and a change in coordinates can   
     change the set of events where there is a coordinate singularity;   
     only singularities in observable quantities like curvature invariants,   
     proper times/distances, etc, are physically meaningful   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)