Abstract
The International System of Units (the SI). The official name of the modern metric system has recently undergone a revolutionary change in which the definitions of four of the base units have changed in dramatic ways. The kilogram, ampere, kelvin, and mole are now defined by assigning fixed values to natural constants: Planck’s constant, the elementary (electron) charge, Boltzmann’s constant, and Avogadro’s number. This reform eliminates artifacts in measurement standards, unifies legal and SI electrical units, and connects units to the atomic and quantum nature of the physical world. This talk will describe how and why these reforms were made and discuss the future re-definition of the SI unit of time, the second.
