From: nicolaas.vroom@pandora.be   
      
   Op dinsdag 29 juni 2021 om 20:49:55 UTC+2 schreef Jos Bergervoet:   
   > On 21/06/29 8:41 AM, J. J. Lodder wrote:   
   > > It is incredible how much misunderstanding there is   
   > > on such a simple subject.   
   > I think there are several reasons for it. See below..   
   > > The speed of light cannot 'really' be variable.   
   > > Why?   
      
   Why can the speed of light not be different in different places   
   in the universe   
      
   > > In order for the speed of light to be measurable at all   
   > > we need to define both a length and a time unit.   
   > > ...   
      
   The time unit is the most tricky if the method to measure time   
   involves light signals and when you want to use time to measure   
   the speed of light. This looks like circular reasoning.   
      
   > Indeed we can agree that basically this is determined by the   
   > metric of space.   
      
   Exactly what is determined by the metric of space?   
   This raises also the question how is this metric measured.   
      
   > Any massless field will have a propagation   
   > speed defined by the metric, but any measurement of speed also   
   > has to use that metric. So the result is fixed.   
      
   This also looks like circular reasoning.   
      
   In Newtonian context the first thing you have to observe the   
   objects studied (positions and speeds) during a certain period.   
   Important is: That these observations require the speed of light.   
   Secondly using Newton's Law you can calculate the masses of the   
   objects studied. That calculation does not require the speed of light   
      
   > This should make clear that a change cannot be observed using   
   > the local metric, but not everyone will agree that this means   
   > it cannot 'really' change.   
   >   
   > We know that seen from another point in space, the speed of   
   > light can be different if space-time is curved (as it usually   
   > is..)   
      
   I assume when objects are involved.   
   Tricky sentence. What means seen? Normally seen implies the speed   
   of light.   
      
   > All these things explain the ongoing discussion, I think..   
      
   The ongoing discussion involves both the speed of light and the   
   speed of a cosmic ray.   
   The issue is to describe how both are defined and how both are   
   actual measured in a clear and unambiguous manner.   
   That is not easy.   
      
   Nicolaas Vroom   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)