How fast is the speed of light?

1 Answer
Dec 16, 2017

The speed of light in a vacuum is exactly 299,792,458m/s.

Explanation:

Like most constants of nature, the speed of light in most units has many decimal places.

The SI unit system is striving to make units based on physical quantities rather than artifacts.

The second has been defined in terms of the frequency of radiation emitted from a Caesium atom. The metre was based on the length of a metal rod.

SI then changed the definition of the metre to be in terms of the speed of light and the well defined second. So the metre is defined to the such that the speed of light is exactly 299,792,458m/s.