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Kilogram needs a new definition, experts say

Release Date 23 February 2005

kilogram artifactIt's time to replace the 115-year-old kilogram artifact as the world's official standard for mass, even though experiments generally thought necessary to achieve this goal have not yet reached their targeted level of precision, according to a Metrologia journal article* authored by five eminent scientists from the University of Reading, the United States and France. The authors of this Metrologia paper suggest replacing the kilogram artifact — a cylinder of platinum-iridium alloy about the size of a plum — with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. The five authors, including Professor Ian Mills from the School of Chemistry, three scientists from the U.S. National Institute of Standards and Technology (NIST) and a former director of the Bureau International des Poids et Mesures (BIPM) near Paris, conclude that redefining the kilogram now in terms of an invariable property of nature rather than a material object could immediately have many benefits. For instance, it would improve the precision of certain electrical measurements 50-fold and would enable physicists to make more precise calculations in studying the fundamental quantum properties of atoms and other basic particles. The paper outlines how this could be accomplished without impairing the current international system of mass measurements. This paper reflects the authors' collective opinion rather than the official policy positions of their respective institutions. The proposal is intended to provoke discussion of what is expected to be a controversial issue, the authors say. Any decision about when and how to redefine the kilogram can be made only by an international group, the International Committee for Weights and Measures, and finally ratified by a General Conference on Weights and Measures, which meets every four years. The next meeting of the General Conference will be held in October 2007 in Paris. The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. The meter, for example, is defined as the distance light travels in a vacuum during one 299,792,458th of a second, and the second is defined in terms of the natural oscillations of the cesium atom. Even though the kilogram cylinder is housed in a special vault under controlled conditions at the BIPM, its mass can drift slightly over the years and it is subject to changes in mass because of contamination, material loss from surface cleaning, or other effects. A property of nature is, by definition, always the same and can in theory be measured anywhere, whereas the kilogram is accessible only at BIPM and could be damaged or destroyed. For the last 25 years, a small international group of measurement specialists has been conducting laboratory experiments to find a definition based in nature to represent the kilogram. Conventional wisdom in the field has held that any new definition for the kilogram should not be implemented until it could be measured with the same or better accuracy than the current kilogram mass artifact. Even though no research group has yet met this goal, the authors of the Metrologia paper argue that the time is, nevertheless, ripe for a redefinition. Due to the complex interrelationships of the kilogram with other basic measuring units, they found that for the great majority of scientific applications the benefits of using unchanging natural phenomena to define the kilogram far outweigh any drawbacks of slightly greater uncertainty in mass measurements. They propose retaining the kilogram artifact, for the time being, as the working reference for the highly precise comparisons to individual countries' national kilogram standards. "We did not think this would be our recommendation when we started writing the paper," says Ed Williams, one of the three NIST authors. "This started out as a study of the effects of redefining the kilogram. During the research, the more we discussed it, the more we thought it made sense to redefine the kilogram now. It should obviously be redefined; the question is whether it should be done in the near future or 10 or more years from now." Other authors of the paper include Terry Quinn, emeritus director of BIPM and the University of Reading's Ian Mill, who chairs the committee that would make any recommendation to the International Committee regarding the proposal. The additional NIST authors, Peter Mohr and Barry Taylor, recently headed an international effort to determine the best values for the fundamental physical constants consistent with measurements and theory. The fundamental constants are an extensive set of invariable quantities, such as the charge of the electron, which scientists use to predict a very wide range of phenomena. *I.M. Mills, P.J. Mohr, T.J. Quinn, B. Taylor, E. Williams, "Redefinition of the kilogram: A decision whose time has come," Metrologia, published Feb. 2005. End For media enquiries only, please contact Craig Hillsley, the University press officer on (tel) 0118 378 7388 or (email) c.hillsley@rdg.ac.uk

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