Weights and Measures

Can a kilogram not weigh a kilogram? On the face of it: no. If something weighs a kilogram then, trivially, it weighs a kilogram. It is a necessary and analytic truth and knowable a priori, as philosophers would say. But where does the kilogram come from? The answer is: Paris. In 1889, a lump of metal cast in London was taken to the International Bureau of Weights and Measures in Paris as the exemplar of a kilogram. Being the kilogram, it surely weighs a kilogram; and so does everything else that weighs as much as it.

Yet the metal lump has lost some 50mg over the course of the last 120 or so years. So, do we say that a kilogram is not what it used to be? No. We say that the original lump was used to fix the reference of a kilogram. Once fixed, it could in principle be discarded. In practice, of course, it could not be. For us to determine whether something is a kilogram, we need some set of scales which must be calibrated by a standard weight. So, it's important to keep an eye on your lumps of metal.

Scientists understandably want to use more robust entities to define physical constants. This article from the Guardian introduces the current project to re-define - or re-establish, perhaps - the kilogram. In another article, some brief details are given of how the metre and the second have been re-established over the years.

On a philosophical note, we can answer the original question by saying: it depends. If by "a kilogram" we mean the measure - that which was defined by the original kilogram - then, no. If we mean an object used to define the measure, then yes.