From: nospam@de-ster.demon.nl   
      
   Nicolaas Vroom  wrote:   
      
   > Op dinsdag 29 juni 2021 om 08:51:03 UTC+2 schreef Nicolaas Vroom:   
   > > Op zondag 27 juni 2021 om 00:57:19 UTC+2 schreef Phillip Helbig:   
   > >   
   > > Nicolaas Vroom.   
   > >   
   > > [[Mod. note -- This topic is a bit tricky, because to measure a speed   
   > > in meters/second, we need to know what a meter is, and what a second is.   
   > That is correct   
   >   
   > > "The metre is the length of the path travelled by light in vacuum   
   > > during a time interval of 1/299 792 458 of a second."   
   > >   
   > > So with this definition, the speed of light is necessarily exactly   
   > > 299 792 458 meters/second. Experiments to "measure the speed of light"   
   > > (e.g., by timing a light pulse over a measured distance) are actually   
   > > measuring a *length* (in meters). E.g., if your measurement shows   
   > > that it takes light 100 nanoseconds to travel a certain distance,   
   > > then what you've really done is measure that distance to be   
   > > (as 100e-9 seconds * 299 792 458 meters/second) = 29.9792458 meters.   
   > That is the measurement by person "A" in vacuum.   
   > What that means that "A" first places two markers a certain distance away   
   > and then sends a light signal between those two markers.   
   > What "A" measures is that it takes 100 nanoseconds to travel that distance.   
   > His conclusion is that the distance is 29.9792458 meters.   
   >   
   > Suppose "B" does 'exactly' the same, but "B" measures that it takes less than   
   > 100 nanosecs and his is conclusion is that the distance is 29.9792458 meters.   
   >   
   > Is that physical possible?   
   > In order for "B" to perform the experiment he has to rely on a very detailed   
   > description (supplied by "A" or ?), on how to perform this experiment.   
   > For example it should tell you how to measure the time (everywhere in the   
   > universe) and give a clear definition exactly what a vacuum is.   
   > This type of information is of critical importance to calculate the distance   
   > travelled by a light pulse and secondly to establish if that distance is   
   > everywhere the same.   
   > Implying that the speed of light is a physical constant and also everywhere   
   > the same. (Personally I doubt that)   
   >   
   > The same type of description is also required if you want to measure   
   > the speed of an electron or a cosmic ray.   
   > In that case you first have to measure the 'fixed' distance using a light   
   > pulse, secondly you have to measure the time t2 it takes for the cosmic ray   
   > to travel that same 'fixed' distance.   
   > Dividing the 'fixed' distance by t2 gives you the speed of the cosmic ray.   
   >   
   > Nicolaas Vroom   
      
   Eh, I don't want to increase your confusions further,   
   but all accurate laboratory 'speed of light' measurements   
   were done using --standing waves--.   
   No propagation timing involved,   
      
   Jan   
      
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