# Kilogram on yo-yo diet

A United States agency is calling for the abolition of the kilogram.

That’s not to say the National Institute of Standards and Technology wants to get rid of the unit of measurement itself. Instead it believes it’s time to stop using the physical object (or “artifact”) that determines the precise mass that makes up a kilogram.

The kilogram is the base unit for mass in the International System of Units (or “SI” from the French, which is the main international system for seven key measures: mass, length (base unit the metre), time (the second), electric current (the ampere), thermodynamic temperature (the kelvin), luminous intensity (the candela) and amount of substance (a measure used in particle studies, for which the unit is the mole). In each case, all other units of measurement are decimal derivatives of the base unit.

What makes the kilogram — and thus mass as a whole — unique is that it is the only SI unit still based on a physical object. Everything else is based on a physical process that remains constant: for example, one meter is the distance that light travels in a vacuum in one second, divided by 299,792,458. (That number does stem from the original, physical definition, but by being fixed it’s not affected by physical changes.)

It’s also believed that all of the kilograms are gradually declining in mass over time through material decay. Unfortunately, and ironically, there’s no way to verify this under the current system as, by definition, the original kilogram always has a mass of, well, one kilogram, regardless of any physical changes.

To make things slightly worse, although the ampere, mole and candela are all based on physical processes, they all involve a measure of mass, meaning they are in turn reliant on the stability of the kilogram.

There has been serious talk of switching the definition of the kilogram for the past five years or so. The most common proposal’s precise definition involves calculations that frankly I don’t dare try to summarize for fear that I get things wrong and somebody tries to recreate them only to end up tearing the fabric of space and time.

The key, though, is that the definition of the kilogram would be based on the speed of light and a fixed amount of energy, the unit thus deriving ultimately from E=MC2.

The National Institute of Standards and Technology has filed a resolution to bring about the change and hopes it will be discussed and adopted at an international meeting next October.