Redefining Units: Reflections on the 2018 Reform of the Metric System

On November 16, 2018 the General Conference on Weights and Measures plans to announce a major revision to the International System of Units (SI; known colloquially as the 'metric system'). Four base SI units - the kilogram, ampere, mole and kelvin - will be redefined by fixing the numerical values of four fundamental constants: the Planck constant, the electron charge, the Avogadro constant and the Boltzmann constant, respectively. The culmination of three decades of debate and preparation in the metrological community, the metric reform will abolish the existing definition of the kilogram as the mass of a platinum-iridium cylinder kept at a vault near Paris, and with it the dependence of SI unit definitions on concrete artefacts. This symposium, held just two weeks before the new definitions take effect, will reflect on the ontological, epistemic and semantic implications of the metric reform. We will critically examine the rationale behind detaching unit definitions from reference to concrete objects; determine how scientific progress can be made through acts of definition; discuss the apparent circularity of testing fundamental theoretical laws with a system of measurement that presupposes those very laws; and explore the broader implications of the redefinition for the philosophy of science.