The original kilogram—a slab of platinum-iridium in Sevres, Paris—now registers a different mass, throwing the benchmark off. The Economist explains why this matters:
Science would thus love to be free of this awkward lump of metal, but attempts to define mass objectively—with reference to, say, the mass of a proton—have always foundered on the question: “So how do you measure that?” For all the fancy equipment that scientists now have for monitoring the behaviour of caesium atoms and the value of the speed of light, no one has come up with a more accurate way of measuring mass than taking the Parisian ingot out of its sarcophagus from time to time, and putting it on a set of scales.
Earlier Dish on the original kilogram here.
The Dish 