Something I’ve always been curious about – how do we keep units of measurement like centimeters, kilograms, seconds, etc the exact same over time? If we use measurement tools to create other measuring tools over the span of centuries, how do we prevent slight deviations from the original measurements in the long run? For example, when we measure a kilogram today, is that still the precise mass of the original kilogram?
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It’s interesting that you use the Kilogram as your example, as that’s recently become a point of contention.
THE Kilogram is a physical entity, kept under lock and key and is the definition of how much mass a kilogram is. BUT since that’s existed for so long it’s actually starting to lose mass on a very small scale. So it’s driving the scientific community to make more choices redefining units of measurement to some known quantity, instead of basing it on some physical standard.
So rather than defining the Kilogram as “the SI unit of mass equivalent to the international standard kept at Sevres near Paris”, we redefined it using the Planck constant. Since the Planck constant is an unchanging value (hence “constant”), we can use it to derive the value of a kg using: h = 6.62607015 × 10^−34 kg ⋅ m^2 ⋅ s^−1
https://puu.sh/Ff2OD/f8ee85d3df.png
By defining 7 primary constants of nature, we can use these constants to define all other units of measurement with unchanging definitions, independent of some man-made prototype or model that could be limited, or unstable.
(the ground state hyperfine structure transition frequency of the caesium-133 atom) ΔνCs = Δν(133Cs)hfs = 9192631770 s−1
(The speed of light) c = 299792458 m⋅s−1
(The Planck Constant) h = 6.62607015×10−34 kg⋅m2⋅s−1
(The elementary charge) e = 1.602176634×10−19 A⋅s
(The Boltzman constant) k = 1.380649×10−23 kg⋅m2⋅K−1⋅s−2
(The Avogadro constant) NA = 6.02214076×1023 mol−1
(The luminious efficacy of monochromatic radiation of frequency 540 X 10^12 Hz) Kcd = 683 cd⋅sr⋅s3⋅kg−1⋅m−2
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