The Kilogram Is No Longer Valid, U.S. Argues

A computer-generated image of the International Prototype kilogram (IPK), which is made from an alloy of 90% platinum and 10% iridium and machined into a cylinder. (Wikipedia)

For 130 years, the kilogram has weighed precisely one kilogram. Hasn't it? The U.S. government isn't so sure.

The precise weight of the kilogram is based on a platinum-iridium cylinder manufactured 130 years ago; it's kept in a vault in France at the International Bureau of Weights and Measures. Forty of the units were manufactured at the time, to standardize the measure of weight.

But due to material degradation and the effects of quantum physics, the weight of those blocks has changed over time. That's right, the kilogram no longer weighs 1 kilogram, according to the National Institute of Standards and Technology (NIST). And it's time to move to a different standard anyway.

Once upon a time there were seven artifacts in the weights and measures family. Along with the kilogram, there was the candela, which measured luminous intensity, the kelvin (temperature), the meter (length), the ampere (electric current), the mole (substance) and the second (time).

Science has found ways to quantify all the other base units in terms of mathematical constants -- the kilogram is the only material "artifact" still left in the current system of units, NIST points out. And strangely enough, it affects three of the base units of measure -- the mole, ampere and candela. They're are all linked to the weight of those 40 blocks of platinum-iridium.

For example, a mole is currently defined as the number of carbon-12 atoms whose total mass is 12 grams, NIST stated, so the kilogram's tendency to lose weight is affecting our ability to measure quantities.

The U.S. has two official copies of the original 40, which were made in France of 90 percent platinum and 10 percent iridium alloy and measure 39mm high by 39mm across. We've kept them safe since 1884, but now NIST plans submit what amounts to a formal complaint at next October's General Conference on Weights and Measures -- along with a proposal to define a new kilogram according to something called a Planck value.

The proposed revision would remove the final connection to that physical bit of matter, said Ambler Thompson, a NIST scientist involved in the international effort. “We get rid of the last artifact.”

Physicists may scoff at the thought people allowed to walk among the living who don't know what a Planck value is. But all you need to know is, they're using it to determine the mass of one mole of silicon atoms.

From there on, they'll theoretically be able to deduce a perfect kilogram and it won't have anything to do with lumps of metal ever again.

Spoilsports.

News.com.au contributed to this report.