When is a kilogram not a kilogram?

Here’s a funny headline that caught my eye today: Kilogram Mysteriously Loses Weight. Interesting. I always thought a kilogram was defined as an unchangeable measurement of mass. How can a kilogram be less than a kilogram? Isn’t that like saying that the mile has inexplicably just gotten shorter?

Well, not exactly. What the article was really talking about was International Prototype Kilogram (the IPK), the reference cylinder at the International Bureau of Weights and Measures in Sevres, southwest of Paris. The cylinder itself, used to define what a kilogram really is, seems to be shrinking.

Hold on a second - if this cylinder itself is the definition of a kilogram, it’s shrinking in relation to what? The IPK was made in 1879, made from a platinum and iridium alloy, and is 1.54 inches in diameter and height. For years, it has been used to define the measurement of “kilogram.” Duplicates of this reference point were made and shipped around the world. And since then, the original object lives in an ultra-secure vault in a chateau in Sevres, rarely coming out to see the light of day. In fact, the only time it comes out is to be compared back to the duplicates, just to make sure that everyone is following the same metric standard. Recently, the IPK weighed in at 50 micrograms less than all of the other kilograms. (In case you’re wondering, 50 micrograms is roughly equivalent to the weight of a fingerprint. In terms of everyday life, this changing reference won’t matter very much. However, if you are a physicist calculating something complex like electricity generation, it will matter a little bit more.)

This is quite a puzzle for scientists. Apparently, these reference objects are made from the same material, at the same time, and they are kept under the same conditions. And yet their masses are drifting slowly apart. To be honest, physicists at the International Bureau of Weights and Measures don’t know whether it’s because the IPK is shrinking, or the duplicates are getting heavier. However, scientifically speaking, only the original IPK in Sevres defines the kilogram. The duplicates are defined as incorrect.

Because of this problem, scientists would like to have a better definition of a kilogram. So it’s likely that the kilogram will have to undergo a facelift in the near future. There is plenty of precedence for this. Many other standards of measurement have changed over the years. For example, take the meter. Over the years, the definition has gone through the following permutations:

1/10,000,000th of the distance from the pole to the equator (1793)

the distance between 2 marks on a platinum reference bar (1799)

1000000 / 0.64384696 wavelengths in air of the red line of the cadmium spectrum (1906)

the length traveled by light in vacuum during 1 / 299 792 458 of a second (1983).

Keep in mind that these changes have not altered the distance of a meter, merely the precision by which it is measured. Maybe soon we’ll have a newer, more precise definition of what makes a kilogram.

Until then, I think we just have to concede – a kilogram just isn’t what it used to be.