speed of light in vacuum

SI defining constant

Name	Symbol	Base units
speed of light in vacuum	c	m s^-1
The speed of light in vacuum, symbol c, is a fundamental physical constant. It is the speed at which all electromagnetic radiation travels in vacuum. The numerical value of the speed of light in vacuum, symbol c, is defined to be exactly 299 792 458 when expressed in the unit metre per second, m s⁻¹.
$c = 299 \mspace{4mu} 792 \mspace{4mu} 458 \mspace{6mu} \text{m} \mspace{4mu} \text{s}^{-1}$

Relativity

The speed of light in vacuum is constant, regardless of the motion of the source or the inertial reference frame of the observer.

Speed

The speed of light in vacuum, c, forms the basis for the definition of the unit of speed, the metre per second.

The fixed numerical value of the speed of light in vacuum, c, is defined exactly:

$c \mspace{4mu} = 299 \mspace{4mu} 792 \mspace{4mu} 458 \ \text{m} \ \text{s}^{-1}$

Inverting this relation gives an exact expression for the metre per second in terms of the speed of light in vacuum, c :

$1 \mspace{4mu} \text{m} \mspace{4mu} \text{s}^{-1} \mspace{4mu} = \dfrac{1}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{4mu} c\\$

Length or distance

The speed of light in vacuum, c, together with the hyperfine transition frequency of the caesium 133 atom, Δν_Cs, forms the basis for the definition of the unit of length, the metre.

The definition of the metre implies the exact relation:

$\dfrac{c}{\Delta \nu _{Cs}} \mspace{4mu} = \dfrac{299 \mspace{4mu} 792 \mspace{4mu} 458}{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770} \ \text{m}$

Inverting this relation gives an exact expression for the metre in terms of the speed of light in vacuum, c, and the caesium frequency, Δν_Cs :

$1 \mspace{4mu} \text{m} \mspace{4mu} = \dfrac{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}$