by Patrick J. Abbott
The International Prototype Kilogram (IPK) realized the unit of mass in the International System of Units (SI) for over 130 years. On May 20, 2019, all fundamental quantities of the SI were tied to defining constants, with mass now defined in terms of the Planck constant, the second, and the speed of light. For several years prior to the revision, the world engaged in measuring the Planck constant to very high precision, and with the Committee for Data on Science and Technology (CODATA) adjustment in 2017, the value of the Planck constant was forever fixed with no uncertainty. However, the world is still not ready for independent realizations of mass by National Metrology Institutes (NMIs), and the IPK continues to hold its position as an internationally agreed-upon realization of the kilogram. How did this happen? What does it mean for mass metrology, and how long will this situation endure? This article will answer these questions and present the ongoing international effort to finally enable the kilogram to be realizable and available “for all times and all people.”
