The International System of Units (SI)

 

 

 

From 20 May 2019 all SI units are defined in terms of constants that describe the natural world. This assures the future stability of the SI and opens the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The SI is the system of units in which:

• the unperturbed ground state hyperfine transition frequency of the caesium-133 atom Δν_Cs is 9 192 631 770 Hz
• the speed of light in vacuum c is 299 792 458 m/s
• the Planck constant h is 6.626 070 15 x 10^–34 J s
• the elementary charge e is 1.602 176 634 x 10^–19 C
• the Boltzmann constant k is 1.380 649 x 10^–23 J/K
• the Avogadro constant N_A is 6.022 140 76 x 10²³ mol^–1
• the luminous efficacy of monochromatic radiation of frequency 540 x 10¹² Hz, K_cd, is 683 lm/W

 Source:www.bipm.org