From: nospam@de-ster.demon.nl   
      
   Phillip Helbig (undress to reply) :   
      
   > 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.   
      
   What Ole Romer really measured was the astronomical unit,   
   aka the AU, in terms of light seconds.   
   (that means 'really' in an operational sense)   
      
   You may have noted that he can no longer do that.   
   The AU too has been given a defined value of 149597870700 m (2012)   
   What a new Romer will be measuring nowadays   
   is where the Earth really is, with respect to Jupiter.   
      
   > 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.   
      
   There you go again.   
   You start with the (Platonic) assumption that there 'really'   
   is some 'speed of light' in some absolute sense,   
   independently of measurements of it.   
   (and that we can then 'measure' it in some unproblematic way)   
      
   And yes, of course we can define the speed of light to be constant,   
   and then it really is constant because we defined it to be.   
   That merely implies that we absorb the changes,   
   if any, somewhere else.   
   (so somewhere in our choices about units)   
      
   Let me take a conceptually simpler example to make the point clear.   
   The last CGPM defined Boltzmann's constant, k, to have a fixed value.   
   That means that the triple point of water now needs to be measured.   
      
   There is no point in saying that we cannot define the value of k,   
   because it 'really' might be changing.   
   Conversely there is no point in saying that the triple point of water   
   'really' is at 273.16 kelvin in some absolute unchangeable sense.   
   There is no physical reality involved,   
   it is just definitions versus definitions.   
      
   Digressing into philosophy: your 'real speed of light'   
   is an example of a Kantian 'Ding an sich' ,   
   and as such unknowable.   
   It has no place in physics.   
      
   Jan   
      
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