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The key thing in a good system of units is to define units in a way that depends only on physical properties of nature.
Ideally (or basically necessarily?) the starting point generally has to be discrete phenomena, e.g.
What we don't want is to have macroscopics measurement artifacts, (or even worse, the size of body parts! Inset dick joke) as you can always make a bar slightly more or less wide. And even metals evaporate over time! Though the mad people of the Avogadro project still attempted otherwise well into the 2010s!
Standards of measure that don't depend on artifacts are known as intrinsic standards.
The key is to define only the minimum number of measures: if you define more definitions become over constrained and could be inconsistent.
Learning the modern SI is also a great way to learn some interesting Physics.
Video 1. Source.
Great overview of the earlier history of unit standardization.
Gives particular emphasis to the invention of gauge blocks.
web.archive.org/web/20181119214326/https://www.bipm.org/utils/common/pdf/CGPM-2018/26th-CGPM-Resolutions.pdf gives it in raw:
  • the unperturbed ground state hyperfine transition frequency of the caesium-133 atom 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 × J s
  • the elementary charge e is 1.602 176 634 × C
  • the Boltzmann constant k is 1.380 649 × J/K
  • the Avogadro constant NA is 6.022 140 76 × mol
  • the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,
The breakdown is:
  • actually use some physical constant:
    • the unperturbed ground state hyperfine transition frequency of the caesium-133 atom is 9 192 631 770 Hz
      Defines the second in terms of caesium-133 experiments. The beauty of this definition is that we only have to count an integer number of discrete events, which is what allows us to make things precise.
    • the speed of light in vacuum c is 299 792 458 m/s
      Defines the meter in terms of speed of light experiments. We already had the second from the previous definition.
    • the Planck constant h is 6.626 070 15 × J s
      Defines the kilogram in terms of the Planck constant.
    • the elementary charge e is 1.602 176 634 × C
      Defines the Coulomb in terms of the electron charge.
  • arbitrary definitions based on the above just to match historical values as well as possible:
    • the Boltzmann constant k is 1.380 649 × J/K
      Arbitrarily defines temperature from previously defined energy (J) to match historical values.
    • the Avogadro constant NA is 6.022 140 76 × mol
      Arbitrarily defines the mol to match historical values. In particular, the kilogram is not an exact multiple of the weight of an atom of hydrogen.
    • the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W
      Arbitrarily defines the Candela in terms of previous values to match historical records. The most useless unit comes last as you'd expect.
TODO how does basing it on the elementary charge help at all? Can we count individual electrons going through a wire? www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the NIST explains that is it basically due to the following two quantized solid-state physics phenomena/experiments that allows for extremely precise measurements of the elementary charge:
Unit of electric current.
Affected by the ampere in the 2019 redefinition of the SI base units.
Unit of mass.
Defined in the 2019 redefinition of the SI base units via the Planck constant. This was possible due to the development of the kibble balance.
Figure 1. Source.
Measures weight from a voltage.
www.bipm.org/documents/20126/28432564/working-document-ID-11315/8532173e-8bae-2bdf-b74a-cb48296b4e67
TODO appears to rely on both quantum Hall effect and Josephson effect
Named after radio pioneer Heinrich Hertz.
Mega-Hertz, i.e. a million Hertz.
Video 2. How a quartz watch works by Steve Mould (2017) Source. Mentions feedback loop loop with the quartz tuning fork for the piezoelectricity and an amplifier. Also mentions the choice of 32768 Hertz () as the first power of 2 that is outside of the human hearing range, and then how a frequency divider is used to reduce the frequency to get the second counter.
Video 3. How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012) Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger resonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.
Video 4. Inside the HP 5061A Cesium Clock by CuriousMarc (2020) Source.
A similar model was used in the Hafele-Keating experiment to test special relativity on two planes flying in opposite directions. Miniaturization was key.
Contains a disposable tube with 6g of Caesium. You boil it, so when it runs out, you change the tube, 40k USD. Their tube is made by Agilent Technologies, so a replacement since that opened in 1999, and the original machine is from the 60s.
Detection is done with an electron multiplier.
youtu.be/eOti3kKWX-c?t=1166 They compare it with their 100 dollar GPS disciplined oscillator, since GPS satellites have atomic clocks in them.
Video 5. Quick presentation of the atomic clock at the National Physical Laboratory (2010) Source. Their super accurate setup first does laser cooling on the caesium atoms.
Uses the frequency of the hyperfine structure of caesium-133 ground state, i.e spin up vs spin down of its valence electron , to define the second.
International System of Units definition of the second since 1967, because this is what atomic clocks use.
TODO why does this have more energy than the hyperfine split of the hydrogen line given that it is further from the nucleus?
Why caesium hyperfine structure is used:
Highlighted at the Origins of Precision by Machine Thinking (2017).
A series of systems usually derived from the International System of Units that are more convenient for certain applications.

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