From: whit3rd@gmail.com   
      
   On Saturday, October 5, 2019 at 1:17:53 AM UTC-7, RichD wrote:   
   > On another board, during some discussion of relativity,    
   > it was claimed that a test of relativistic Doppler shift    
   > could be performed via frequency combs (a recent tool)    
   > and diffraction gratings; i.e. v = fλ   
   >    
   > Then, someone objected, dismissing the argument as    
   > circular, since the diffraction grating model itself    
   > depends on assuming a constant speed of light, c, as    
   > per Einstein's second postulate.   
      
   A 'diffraction grating' is a physical object, held together by electrical   
   forces.   
   If you alter the laws of electricity and magnetism on a cosmological scale,   
   it could change the object in question.   
   The speed of light in vacuum is derived from those laws of electricity and   
   magnetism.   
      
   Yes, there's a certain circularity involved.   
      
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