Metre defined by the speed of light
Metro definido por la velocidad de la luz
- Date
- 1983 · confidence high
- Place
- Paris, Île-de-France, France
- Credited to
- 17th General Conference on Weights and Measures
- Field
- Knowledge & symbols
What it is
In 1983 the CGPM defined the metre as the distance travelled by light in vacuum in 1/299,792,458 s, fixing the value of the speed of light. With it, the unit of length came to depend on the unit of time and a natural constant.
It is the first unit whose definition was based on fixing the value of a fundamental constant; the 2019 redefinition extended that principle to the kilogram, ampere, kelvin and mole.
What it led to
- length measured by light time of flight (rangefinders, GPS)
- the 2019 SI revision with fixed constants
- length metrology with frequency combs