Well knock my socks off and file this under: ‘things I never thought about until I was a researcher.’ Today, the definitions of metric units (including the kilogram, second, and metre) have officially changed.
Metric units are defined within the International System of Units (SI for short, which stands for the French title Système international). The SI uses 7 ‘base units’: the ampere (electricity), kelvin (temperature), second (time), metre (length), kilogram (mass), candela (illumination), and mole (amount of a chemical substance).
All but three countries around the world have signed up to this system, and every four years, member countries send their delegates to the General Conference on Weights and Measures (or CGPM for the French title Conférence générale des poids et mesures). At their 2018 meeting, the CGPM agreed to make some key changes to the definitions of some units; and those changes came into effect today.
Even if you use lots of measurements in your research, this probably won’t affect you. The definitions haven’t changed drastically – we are talking about incredibly subtle shifts. But if you use very, very precise measurements in your research, you might need to look into the changes.
Most notably, the SI no longer uses a physical object as a benchmark for the kilogram. As of yesterday, a kilogram equaled the weight of a cylinder of platinum iridium (nicknamed ‘Big K’) stored in the International Bureau of Weights and Measures in Sevres, France. The downside of that system was that if Big K ‘lost weight’ in some way – if it got worn down, for instance – the entire global system of measurement had to adjust accordingly. (This actually happened in the 1980s, when Big K was measured as having lost a few micrograms. People were not happy.)
Now, a kilogram is defined in relation to Planck’s constant. This is a quantum physics solution to a practical problem. The value of Planck’s constant was determined to an acceptable degree of accuracy in 2017, and the CGPM adopted it as the preferred basis for the definition of the kilogram.
There have also been a number of other changes, including amendments to the definitions of the ampere, kelvin, and mole; plus an overall shift to a measurement model called the ‘explicit constant’ formulation.
These changes will only affect researchers working with extremely precise measurements. If you think your research may be affected, you can check out all the technical details here.