From: tjroberts137@sbcglobal.net   
      
   On 6/29/21 1:41 AM, J. J. Lodder wrote:   
   > [...] The speed of light cannot 'really' be variable. [...]   
      
   You make far too many assumptions to be reasonable.   
      
   Certainly the (vacuum) speed of light COULD vary, it's just that in the   
   world we inhabit, with current technology, it is observed to not vary   
   significantly (when measured using standard clocks and rulers at rest in   
   some locally inertial frame). But it certainly is possible that in the   
   future we will develop technology with greatly improved resolution and   
   discover that it actually does vary in the world we inhabit. It is also   
   possible we will never find it varies -- science is a JOURNEY, not a   
   destination.   
      
   Yes, c is really a units conversion factor, IN THE SPACETIME MODEL. And   
   yes, it is not possible for the symmetry speed of Lorentzian manifolds   
   to vary. But the restriction you imagine is on that symmetry speed, and   
   not really on the (vacuum) speed of light -- there is no fundamental   
   reason why light must propagate at the symmetry speed.   
      
   Note that it is an historical accident that we call the symmetry speed   
   of Lorentzian manifolds by the name "the speed of light" (with "vacuum"   
   implied). Light really has nothing to do with it.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)