A skeptic’s review of the New SI

Abstract

Proposals in draft form have been circulated for new Systeme International (SI) measurement units that are expected to be official instruments of the Treaty of the Metre by 2015. This review outlines the substance of the proposals and examines some of the consequences of the continuing evolution of the SI toward inter-dependence of base units and quantities since its introduction in 1960. The proposals in question fix at an exact value a number of inter-related fundamental natural constants such as the speed of light, the Planck constant, the elementary charge and Boltzmann’s constant. All SI units are then so defined that their magnitude is set by those fixed values. Notably, the ongoing confusions about chemical measurements and the thermodynamic ‘mole’ are exacerbated. On the big principles of the basic purpose of the SI to facilitate communication and the fixing of fundamental physical constants of nature, there are significant problems and unanswered questions. They risk: damage to the enterprise of science; wide economic loss including increased transaction costs and barriers to global trade; barriers to new technologies and to improvements in measurement accuracy; loss of measurement compatibility or consistency; and a circular global measurement system vulnerable to undetectable systematic errors with serious adverse consequences for environmental decision making among many other vital human activities. The New SI requires frank and open discussion throughout science, technology, industry, trade, and global policy well before irreversible decisions are made.