Are any problems caused by having the kilogram defined in terms of a physical artefact?
Yes there are. The current definition is quite limiting, essentially for three reasons: the International Prototype Kilogram is not perfectly stable (its mass changes with time), the amount it changes cannot be known perfectly (there is no ‘perfect’ reference against which to judge it) and the values of the national copies cannot be monitored at the highest level of accuracy without being compared directly with it. As said in a previous FAQ, the kilogram is unique among the seven base SI units in that it is the only one still defined in terms of a physical artefact – and this brings many unique practical problems in realising ‘the kilogram’ (ie reproducing it) and disseminating (passing on) its mass value throughout the technological World. By contrast, the units of the other six base quantities (length, time, electric current, thermodynamic temperature, amount of substance and luminous intensity) are all now defined in terms of natural constants [1]. (One unofficial, tongue-in-cheek b
