![I don't care for Candela](/candela.jpg)

# SI Base Units & Constants

I recently stumbled upon a well-written (but outdated) overview of the
[base SI units](https://en.wikipedia.org/wiki/SI_base_unit) from
[Theodore Wildi's _Electrical Machine Drives and Power Systems_](https://archive.org/details/ElectricalMachinesDrivesAndPowerSystems5ETheodoreWildi/page/n5/mode/2up).

I heard about the 2019 changes to these definitions, but I totally missed the
elegance of the new model:

[![New SI: Dependence of base unit definitions on physical constants with fixed numerical values and on other base units that are derived from the same set of constants.](https://upload.wikimedia.org/wikipedia/commons/a/ab/Unit_relations_in_the_new_SI.svg)](https://en.wikipedia.org/wiki/SI_base_unit)

[^note]:
    Dependence of base unit definitions on physical constants with fixed
    numerical values and on other base units that are derived from the same set
    of constants.

Each of our SI units now roughly derives from a
[fundamental constant](https://en.wikipedia.org/wiki/Physical_constant)! Neat!

Apparently the definition of "fundamental constant" is itself a bit wishy-washy,
but [Uzan](https://pmc.ncbi.nlm.nih.gov/articles/PMC5256069/)'s summary seems
tidy:

> - the
>   [gravitational constant](https://en.wikipedia.org/wiki/Gravitational_constant)
>   G
> - the speed of light c
> - the Planck constant h
> - the 9 [Yukawa couplings](https://en.wikipedia.org/wiki/Yukawa_couplings) for
>   the quarks and leptons (equivalent to specifying the rest mass of these
>   elementary particles)
> - 2 parameters of the [Higgs field](https://en.wikipedia.org/wiki/Higgs_field)
>   potential
> - 4 parameters for the
>   [quark mixing matrix](https://en.wikipedia.org/wiki/Cabibbo–Kobayashi–Maskawa_matrix)
> - 3 coupling constants for the
>   [gauge groups](https://en.wikipedia.org/wiki/Gauge_group) >
>   [SU(3) × SU(2) × U(1)](<https://en.wikipedia.org/wiki/Standard_Model_(mathematical_formulation)>)
>   (or equivalently, two coupling constants and the
>   [Weinberg angle](https://en.wikipedia.org/wiki/Weinberg_angle))
> - a phase for the
>   [quantum chromodynamics vacuum](https://en.wikipedia.org/wiki/QCD_vacuum)

Anyway, I love this universe:

[![list of physical constants](/physical_constants.png)](https://en.wikipedia.org/wiki/List_of_physical_constants)