What makes the speed of light exactly 299,792,458 metres per second?

1 Answer
May 25, 2016

The definition of the metre.


The metre used to be defined as the length of a particular International Prototype Metre - a metal bar made from an alloy of platinum and iridium, measured at the melting point of ice.

In #1960#, the standard was changed to be expressed in terms of wavelengths of the red-orange spectral line of Krypton-86.

In #1983#, the definition was updated to be exactly #1/299792458# the distance travelled by light in vacuum in one second.