Metrology is the study of measurements and the accuracy thereof. While the profession is essential to how we live, we never give much though to how a standard is set. Thankfully scientists and engineers all around the world do that job for us. International standard units are important in setting out just how our formulas will work for critical things. Mass, time, gravity and even chemical composition. Get it wrong and the consequences are enormous. Just this week, the standard for the kilogram has been redefined.
The kilogram is pretty well understood. You need it to know how much meat and vegetable you’ve just bought at the supermarket. Or how much mass there is in a satellite so that you can calculate the fuel required to get it into the right orbit. For about 130 years, a kilogram has been defined by a standardised piece of metal and a number of copies. The original is locked up tight in Paris. However, even in its sealed chamber, the mass of the standard kilogram is changing ever so slowly. Thus, the unit varies over time, and metrologists agree this cannot go on.
The change in the way a kilogram is defined doesn’t change how much mass there is. It is simply to ensure that the kilogram is now based on physical laws rather than a number of physical objects that degrade differently over time. Even the current standard metal blocks of a kilogram have changed over time (micrograms here and there). Not something we’d notice in everyday measurement, but when precision is required, the subtle differences can have big effects.
The International Bureau of Weights and Measures (BIPM) has met and redefined not just the kilogram, but the kelvin (temperature), the ampere (electrical current) and the mole (number of molecules in a mass). These will all be tied to physical laws and fundamental constants rather than a variable measure such as a physical object. This will make the units more precise and, more importantly, constant for the rest of time. Unless the universe randomly changes the laws.
For its part, metrologists have agreed that the kilogram will now be defined via the Planck Constant. The other three units under consideration have also been redefined by electrical charge (ampere), the Boltzmann constant (the kelvin) and the Avogadro constant (the mole). David Newell, a researcher at the National Institute of Standards and Technology, told Science News that : “It’s about as excited as you’re going to see metrologists get.” We thank you for your work.