An old artifact kept in a vault outside Paris is no longer the standard kilogram

The other objects on which physical standards are based, such as the standard meter, were already replaced years ago, but the kilogram — generally known as the kilo for short — turned out to be a harder unit to define in absolute terms. Explaining how the other basic units have been defined through basic physical constants is a bit more straightforward than it is for the kilo: The second, for example, is defined as a specific number of vibrations of an atom of cesium. The new definitions have great power, Ketterle says, because “every new method to count photons or atoms or measure frequencies will further improve the measurements of mass, since mass is no longer connected to an imprecise, man-made artifact.”

Source: news.mit.edu