From: tjroberts137@sbcglobal.net   
      
   On 8/22/19 1:44 PM, Nicolaas Vroom wrote:   
   > On Wednesday, 14 August 2019 21:40:49 UTC+2, Tom Roberts  wrote:   
   >> All is resolved when you use the ACTUAL postulates from Einstein's 1905   
   >> paper:   
   >> 1. The laws by which the states of physical systems undergo change are   
   >>      not affected, whether these changes of state be referred to the one   
   >>      or the other of two systems of co-ordinates in uniform translatory   
   >>      motion.   
   >   
   > This sentence is not very clear. Which laws are meant?   
      
   Just what it says implicitly: all of them.   
      
   > Physical systems don't undergo changes as a result of any law.   
      
   You have to read a paper written in 1905 in the context of 1905. That's   
   how they spoke and wrote back then. Today we would replace "laws" with   
   "models" and slightly re-phrase the sentence to be modeling the change   
   of state, rather than determining it.   
      
   > The evolution of the universe is not controlled by any law.   
      
   No. But in today's nomenclature he is discussing a model of how changes   
   of state occur.   
      
   > The evolution of the universe partly is a cooling process which   
   > involves different stages.   
      
   He is not discussing that at all. Implicitly he is describing an   
   abstract world that is very much simpler.   
      
   	As did Newton, and essentially every physicist outside   
   	of cosmology.   
      
   > All of this from a physical point of view has nothing to do with any   
   > co-ordinate system.   
      
   Yes. That's why the model must not depend on one's choice of coordinate   
   system. In today's nomenclature this postulate requires models to agree   
   with this rather obvious property of the world we inhabit. (Though he is   
   limiting the discussion to inertial frames.)   
      
   >> 2. Any ray of light moves in the "stationary" system of   
   >> co-ordinates with the determined velocity c, whether the ray be   
   >> emitted by a stationary or by a moving body.   
   >   
   > IMO the simplest situation is to consider a "stationary" system and a   
   > stationary body both at rest.   
      
   Hmmm. This explicitly says that the emitting body need not be at rest,   
   but its emitted light will still travel with speed c relative to the   
   "stationary" system.   
      
   	Remember that earlier he defined the "stationary" system   
   	as an ARBITRARY inertial frame, with "stationary" being   
   	just a label with no semantic content.   
      
   >> Note in particular that Einstein's statements are more precise than   
   >> any of the paraphrases.   
   > Specific the first postulate is very difficult to understand, what is   
   > meant.   
      
   One must read it in the context of 1905. See above.   
      
   >> As I keep saying: physics is subtle and to understand it one must   
   >> be precise in thought and word.   
   >   
   > I fully agree with you. The problem it is easy to write that the   
   > speed of light is every where the same. It is very difficult to   
   > explain why that is true.   
      
   Today science does not attempt such "explanations", because we now   
   recognize that we are making MODELS, and such "explanations" are valid   
   only in the context of the model. Attempting to discuss "why" the model   
   works is outside the scope of physics, and enters into metaphysics and   
   philosophy.   
      
   > Experiments to demonstrate the same are even more 'complex'   
      
   Not really. See the relevant sections of:   
        http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html   
      
   Tom Roberts   
      
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