From: nicolaas.vroom@pandora.be   
      
   [[Mod. note -- I apologise to all for the delay in processing this   
   article, which arrived in the moderation system on 22-Aug-2021.  -- jt]]   
      
   Op woensdag 11 augustus 2021 om 18:35:46 UTC+2 schreef J. J. Lodder:   
   >   
   > > On 7/7/21 12:20 PM, J. J. Lodder wrote:   
   > > > Tom Roberts  wrote:   
   > >   
   > > Because it is an important aspect of whether the (vacuum) speed of light   
   > > varies. In our current best models of the world, nonzero photon mass and   
   > > a varying vacuum speed of light are equivalent (either both are valid or   
   > > both are invalid).   
   > >   
   > > But a) we don't do that, and b) that would make it impossible to   
   > > actually measure the speed of light, one could only measure it   
   > > approximately -- hopeless for such a fundamental aspect of the world we   
   > > inhabit, and one used in so much technology.   
   >   
   > Your points are completely beside the subject of this sub-thread,   
   > which was the question if the speed of light can vary with -time-,   
   > and if we could in principle measure this.   
   >   
   > > The correct way to deal with a nonzero photon mass is to distinguish   
   > > between the two quite different meanings of c:   
      
   When I was in high school my mathematical teacher (Mr Zweens) wrote down   
   the mathematical equation: x + y + z = 3 (approximate recapulation)   
   Does that equation make sense?   
   Yes, in mathematical space at the point x=1, y=1, z=1   
   Next he wrote down the equation: x + y + z + w = 4   
   Does that equation make sense?   
   Yes, in mathematical space at the point x=1, y=1, z=1, w=1   
   He also could have written: x + y + z + t = 4.  (x=1,y=1,z=1,t=1)   
   The reason was, because we were 'discussing' two 4D equations, and the 3D   
   solution of these equations.   
   In reality: two 4D space equations and the 3D space solution. (or objects)   
   Immediate came into my mind:   
      Yes, you can do all of that in mathematics, but does it physical make sense?   
   What I did not realize, approx 60 years ago, that I should have considerd   
   the issues involved in the reverse order:   
   First comes the question: What does physical exists? What is real.   
   Second: How can we measure the physical reality?   
   And third: are there and which are the mathematical relations between these   
   measurements?   
      
   In the text above I have the same problem:   
   First you must measure the speed of light. Or better, you have to describe   
   a general accepted way, how the speed of light is measured.   
   If you have such a recipe, you can measure and decide if the speed of light   
   is everywhere the same and if this speeed is the same in -time- at a   
   specific location   
      
   The same type of problems exists between: what is mass and how is this   
   directictly measured or calculated based on different measurements.   
      
   If you don't start with some sort of mutual agreements, similar discussions   
   will go on and on, forever.   
      
   Nicolaas Vroom   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)