Making the “real” kilogram obsolete

IEE Spectrum : Last October delegates from the 55 member countries that define our basic measurement standards agreed unanimously on a tentative plan to base the kilogram on a fundamental constant of quantum mechanics instead of a lump of metal held in Paris. The move, which will also change the basis of three other core units—the ampere, the mole, and the kelvin—is the result of decades of work in trying to measure mass. One approach attempts to pin down the exact electromagnetic force needed to balance the gravitational tug on an object. The other counts the number of atoms in extremely round balls of ultrapristine silicon. For years the two approaches have produced starkly conflicting results. However, over the past few months, metrologists have been excited to find glimmers of convergence, and the effort to pin down mass once and for all is beginning to pick up steam, says IEE Spectrum‘s Rachel Courtland.