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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Recently (and by that I mean over the last decade) the scientific community has moved away from physical standards and have moved to definitions of units of measure based on universal constants. The most recent was the kilogram which was redefined in May 2019 to be based on the Planck constant with other comparable measurements given. [Source](https://www.npl.co.uk/news/science-loses-a-kilogram)
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