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.

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About John F. Lindner

John F. Lindner was born in Sleepy Hollow New York and educated at the University of Vermont and Caltech. He is a professor of physics and astronomy at The College of Wooster. He has enjoyed multiple yearlong sabbaticals at Georgia Tech, University of Portland, University of Hawai'i, and North Carolina State University. His research interests include nonlinear dynamics, celestial mechanics, and variable stars.