Is c, the speed of light in vacuum, constant?
At the 1983 Conference Generale des Poids et Mesures the following SI (Systeme International) definition of the metre was adopted: 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. This defines the speed of light in vacuum to be exactly 299,792,458 m/s. This provides a very short answer to the question “Is c constant”: Yes, c is constant by definition! However, this is not the end of the matter. The SI is based on very practical considerations. Definitions are adopted according to the most accurate known measurement techniques of the day and are constantly revised. At the moment you can measure macroscopic distances most accurately by sending out laser light pulses and timing how long they take to travel using a very accurate atomic clock (The best atomic clocks are accurate to about one part in 10^13.) It therefore makes sense to define the metre unit in such a way as to minimise errors in such a measurement. The SI d
