From: nicolaas.vroom@pandora.be   
      
   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   
      
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