The International System of Units (SI from the French Le Système International d’Unités) is the universally accepted method of expressing physical measurements for world commerce, industry, and science. Though officially established in 1960, the origins of the present SI can be traced back to the creation of the decimal Metric System during the French Revolution. The SI has proven to be a living, evolving system, changing as new knowledge and measurement needs arise, and once again international consensus is building to advance the SI to reflect contemporary understanding of the physical world. The new framework of the future SI will no longer be based on definitions of units such as the meter, kilogram, and second. Instead it will adopt exact values for seven fundamental constants of nature upon which all SI units will be realized. Gone are the base units and their definitions.
A brief history of measuring nature will be presented, with emphasis on the formation of the metric system and the SI. A simplistic method for constructing a system of units will be described to contrast the present and future foundations of the SI. The impact and consequences of the shift to a system of units based on exact values of fundamental constants will be highlighted along with the timeline for the possible redefinition of the SI in 2018.
David Newell received his B.S. in Physics and B.A. in mathematics from the University of Washington and his Ph. D. in Physics from the University of Colorado. He was awarded a NRC post-doctoral fellowship to work on the NIST watt balance project in Gaithersburg, MD, and became a full time staff member in 1996. He has received the NIST Stratton Award for his contribution to reducing the uncertainty on Planck's constant by a factor of 7, the Department of Commerce Silver Medal for innovations in traceable nanoNewton level force measurements and the Department of Commerce Gold Medal for a landmark measurement of the Planck constant leading toward a new definition of the SI. He joined the CODATA Task Group on Fundamental Constants in 2006 and participated in the 2006 and 2010 adjustment of the fundamental constants. He is currently preparing for the 2014 adjustment of the fundamental constants and working with a NIST team to construct a new watt balance to realize the kilogram from a fixed value of the Planck constant. He is a member of the American Physical Society and chair of the CODATA Task Group on Fundamental constants.