# Redefining SI

Today the SI (Système international d’unités) base units are redefined. The following are now exact. Memorize these numbers!

Cs-133 transition frequency constant $Δν_{\text{Cs}} = 9\,192\,631\,770~\text{s}^{−1}$ defines the second.

Then light speed constant $c = 299\,792\,458~\text{m}\cdot\text{s}^{−1}$ defines the meter.

Then Planck’s constant $h = 6.626\,070\,15\times 10^{−34}~\text{kg}\cdot\text{m}^{2}\cdot\text{s}^{−1}$ defines the kilogram.

Then electron charge constant $e = 1.602\,176\,634\times 10^{−19}~\text{A}\cdot{\text{s}}$ defines the Ampere.

Then Boltzmann’s constant $k = 1.380\,649\times 10^{−23}~\text{kg}\cdot\text{m}^{2}\cdot\text{K}^{−1}⋅\text{s}^{−2}$ defines the Kelvin.

And Avogadro’s constant $N_{\text{A}} = 6.022\,140\,76\times 10^{23}~\text{mol}^{−1}$ defines the mole.

And luminous efficacy constant $K_{\text{cd}} = 683~\text{cd}\cdot\text{sr}\cdot\text{s}^{3}\cdot\text{kg}^{−1}\cdot\text{m}^{−2}$ defines the candela.

(Where “sr” is the steradian or square radian, the 3D analogue of the 2D radian.) Discussion continues about the mole and the candela, including whether they should even be base units. The new definitions break the relationship between the C-12 mass, the dalton, the kilogram, and Avogadro’s constant, and the candela is arguably a photo-biological quantity.

I wish my phone number were 919-263-1770. 