Abstract
Air Canada managed to have a passenger aircraft run out of fuel in mid-air due to confusion about metric units (Stephenson in Mars climate orbiter mishap investigation board phase I report, NASA, 1999), and NASA lost an entire spacecraft due to a misunderstanding amongst engineers about the units used in the propulsion system design (Witkin in Jet’s fuel ran out after metric conversion errors, The New York Times, 1983). Measurements only make sense if the units are correct and well-defined. A unit of measurement is a definite magnitude of a quantity, defined by convention or law. Any other quantity of that kind can then be expressed as a multiple or submultiple of the unit of measurement. The Egyptians used the Farao as definite magnitude, while many years later, the french revolutionists introduced the earth as a reference and laid the foundations for the modern decimal system. Since recently, we have a truly universal and stable system that uses physics’s natural constants and laws to define the base units of measurement. This paper explains how this new concept works and how it is implemented in practice.
Highlights
From 20 May 2019, to allow the international system of units to exploit both the advances in our understanding of Nature and recent technological developments, all units are implicitly defined in terms of seven fundamental constants, without distinctions between primary and derived units (Stock et al 2019)
This means that the natural constants serve as definite magnitude, which, together with the laws of physics, allow to construct a system of units of measurement that is truly universal, accessible to everyone, and that does not require physical objects or “artefacts” as reference
The foundations were laid for the metric system, where natural quantities were used to establish a system of units for all time, for all people
Summary
From 20 May 2019, to allow the international system of units (the SI) to exploit both the advances in our understanding of Nature and recent technological developments, all units are implicitly defined in terms of seven fundamental constants, without distinctions between primary and derived units (Stock et al 2019). What constants to fix was determined by trading off between the depth and breadth of the theories that they tag and the need to ensure continuity to the everyday metrology, which continuity, in turn, correlates to how well we are able to measure them Their values have been conventionally chosen by agreement to minimise the differences between the sizes of the new units and the old ones (Mohr et al 2018), which took a considerable technical effort of the metrology community before the redefinition in 2019 could take place. In this paper we show how the constants’ definitions and units depend on the measurement technologies and the theoretical framework used to explain the experimental observations
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
