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Since 1970 lasers and caesium clocks made even more accurate measurements possible. This soon reached the point where the speed of light was known more accurately to the nearest metre per second than the definition of a metre itself. It made sense to define a standard metre by fixing the speed of light.
Systeme International define a metre in 1983
In 1983 scientists defined a meter as 1/299,792,458 the distance light travels in one second. They did this becaused they knew the distance light travels in one second more accurately than the definition of the standard meter.
So, since 1983 the speed of light is not measured in any way. It has been defined as a standard.
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Now might be an appropriate time to try out the Speed of Light Quiz.
La vitesse de lumière (French)
Geschwindigkeit des Lichts (German)
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