Is the kilogram getting smaller?

copy of the
international prototype kilogram

The kilogram is currently defined by an object, the IPK. Definitions of basic units would seem to need a couple of criteria, constancy and precision. The unit should be stable over time. It should also be a highly accurate measure, as much as reasonable for other units that derive from it. But apparently the platinum-iridium cylinder is losing atoms, so think some.

The official kilogram, a cylinder of platinum and iridium maintained by the International Bureau of Weights and Measures, is more than 130 years old. It is stored under three glass domes in a safe in a basement in Sèvres, France, and can be accessed only with three independent keys.

Technically, the cylinder is not even the official kilogram until it has been cleaned in an authorized manner to remove contaminants. It is the only remaining international standard in the metric system that is still a man-made object. Some scientists now believe the official kilogram may be losing mass, which defeats its only purpose: constancy. This adds new urgency to a longstanding search for a new official kilogram, based, like the meter, on one of nature’s fundamental numbers, called constants.

One would also assume that such a unit is accessible, which this cylinder is not; kept under glass. Though there are several replicas around the world which are periodically checked with the IPK.

So we may see a future definition based on fundamental constants rather than objects. Of course this inhibits us measuring whether or not these constants are in fact constant over space and time.