Redefinition of the Kilogram: A Decision Whose Time Has Come

Peter J. Mohr, National Institute of Standards and Technology

The kilogram, the base unit of mass in the International System of Units (SI), is defined as the mass of the prototype of the kilogram, a metal cylinder kept at the International Bureau of Weights and Measures (BIPM) near Paris.  Although this definition has served science and technology well during the past 115 years, the kilogram is the only base unit of the SI that still depends on a property of a macroscopic object, and as such could be changing over time. Moreover, the prototype could be damaged or even destroyed, and it must be carefully washed in a prescribed way prior to comparing its mass to the mass of other standards. This talk will review the benefits that would result from redefining the kilogram by giving a specified value to either the Planck constant or the Avogadro constant.  These benefits would include a significant reduction in the uncertainty of many of the fundamental constants used in physics. Experiments that would provide a practical implementation of such a definition will be described. Redefinitions of other SI base units currently under consideration, in particular, the ampere, the kelvin, and the mole, will also be reviewed.

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