From: helbig@asclothestro.multivax.de   
      
   In article <384f2c33-591d-4297-934f-1013e9db0068n@googlegroups.com>,   
   Nicolaas Vroom  writes:   
      
   > Op dinsdag 31 augustus 2021 om 07:26:18 UTC+2 schreef Phillip Helbig:   
   > > In article <1peqwo1.1pvreq6wg61gwN%nos...@de-ster.demon.nl>,   
   > > nos...@de-ster.demon.nl (J. J. Lodder) writes:   
   >   
   > > > The problem with your position is that you postulate   
   > > > that what has to be shown,   
   > > > namely that there is such a thing as the speed of light,   
   > > > and that it is a constant of nature.   
   > > >   
   > > > If you want to have a 'speed of light' as a constant of nature   
   > > > you must invent new, and fundamentally different laws of physics   
   > > > in which there is such a thing,   
   > > Please explain. There are various sources of light. We can measure (1) a   
   > > distance. We can measure (2) a time. Thus, we can measure a (3) speed.   
   >   
   > Yes we can measure a speed and a distance and using both calculate a speed.   
   > This is rather straight forward when you want to calculate the speed of a car   
   > , but very difficult when you want to calculate the speed of light or the   
   > speed of a neutrino. The main problem is the reference frame.   
   >   
   > The first step is to describe exactly how (1) and (2) are measured such that   
   > we all can perform the same experiment and compare the results (at different   
   > locations or circumstances).   
   > To measure the time you can use two atomic clocks, but than you have to   
   > agree how to synchronise these clocks.   
   >   
   > To measure the distance you could start with two points A,B, a fixed distance   
   > apart. Next you can define a point C halfway between these two points AB   
   > and issue a synchronisation flash from C towards A and B.   
   > But the question is do both pulses arive simultaneous?   
   > Next you can issue a pulse from point A and measure the arriving time t1 of   
   > that pulse at point B.   
   > You can do the reverse from B to A and measure t2.   
   > The question is: Are these two arriving times t1 and t2 (durations) the same?   
   > If they are you can calculate c. If they are not you have a problem.   
      
   Leaving aside the issues that the speed of light is now constant by   
   definition (a few decades now) and that we believe that it is constant   
   in all frames (more than a century now), as far as normal measurements   
   go, there is really no problem.  Ole R=F8mer measured the speed of light   
   via timing the eclipses of Jupiter's satellites.  Fizea measured it with   
   two gears.   
      
   Bringing back the other issues, we could of course still measure the   
   speed of light with those old methods, and if it did actually change   
   with time, we would notice it, despite the facts that it is now defined   
   to be constant and that we believe that it is constant.  I don't think   
   that likely; my point is merely that we cannot prevent the speed of   
   light from changing simply by defining it to be constant.  Rather, it is   
   defined to be constant as a practical matter because we have evidence   
   that it is.   
      
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