The Race to Define the Kilogram

In August, scappuccino reported on a BBC radio story about the changing weight of the kilogram. That is, THE kilogram: an apple-sized mass of platinum-iridium protected behind glass that defines how much a kilogram weighs. There are replica masses all over the world, but this is the queen bee. If she loses weight, everyone feels it. And she has been. Over the past few decades her weight has fluctuated. It's also possible that the comparison weights have been gaining mass, but no one can be sure because the change is happening on such a small scale. It's only by a few micrograms - nothing that would affect general consumers like you and me - but enough to affect precision measurements done by scientists. Needless to say, there's a bit of a panic to come up with a solution so that the weight of the world, so to speak, won't continue to rest on Le Grand K's shoulders.

There are currently two key efforts to successfully redefine the kilogram in a way that would not rely on a hunk of metal sitting in Paris, accumulating contaminants or losing mass each time it is cleaned. While each has been in the works for many years, the race seems to be heating up, and things may come to a head in the next five years.

So how would YOU define a standard of weight?

The simplest, perhaps most obvious method, would be to find a mass and keep it as the standard. But we see now that that method has it's potential flaws.

So how about counting up something? Lets say 500 black beans equals one kilogram. Again, this might work for general consumption, but it still has its flaws. We'd have to assume that black beans all weigh the same, when in reality they fluctuate just a little. So what other standard mass could we use?

Scientists believe that atoms should have standard mass. One atom of silicon should have the same mass as an identical atom of silicon. The problem there is that atoms are difficult to count. There could be 50 septillion atoms in a kilogram of silicon! (That's 50 trillion trillion). Atoms are small, and there's a lot of them. But some scientists believe this is the very best way to define the kilogram, and they are working on what's called the Avogadro project to determine the number of atoms in one kilogram of silicon crystal.

Before they can count the number of atoms in a kilogram of silicon, scientists must arrange those atoms into an ideal body that they can study. After more than a decade of work, the result so far are a family of grapefruit-sized spheres made up of 99.99% silicon 28. The raw materials alone cost about $700,000 per sphere. They are reportedly the most perfectly spherical objects ever made by man, having been measured in millions of locations to make their surfaces perfectly smooth. Leading the Avogadro project, scientists at the National Physics Laboratory in England and the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, the German national metrology institute, are now trying to count the atoms in these spheres, but even that can't happen until the spheres are absolutely perfect. Their website proclaims, "As to counting, there is no end in sight!"

If those scientists can measure the atoms in a sphere of silicon crystal to an acceptable level of uncertainty, it would also redefine Avogadro's constant, which is linked to another international base unit of measurement, the mole. A mole is the amount of substance which contains as many particles as there are carbon atoms in 12 grams of monoisotopic carbon 12. It's also used as a comparison value for calculating the exact amount of any material (chemistry students remember it all too well). It's a massive number (there are a lot of atoms in even a gram of carbon) that has an uncertainty of 5x10^8. The Avogadro project will have to beat that uncertainty to be adopted. Scientists working on the Avogadro project announced in a feature article for the Comité International des Poids et Mesures (CIMP, the International Committee for Weights and Measures) that they hope to reach a redefinition of the kilogram by 2011.

Competing with the Avogadro project is an effort known as the watt balance, which would define the kilogram in terms of current and magnetic field. If you send a current through a wire in the presence of a magnetic field, the wire will feel a force. If you can measure a current and magnetic field which generate a force equal to the weight of a kilogram, you can define it by the values of the current and magnetic field.

There is a little bit of mathematical manipulation with this measurement that would end up providing a more precise value of the Planck constant, "h". This quantum mechanical constant relates energy to the frequency of a photon, and it pops up in a lot of mathematical equations, so the impact could ripple through many areas of physics.

Right now, the race is on. Scientists on both sides have reason to believe their method will work best, that the other may not be precise enough or easy enough to replicate, while others are waiting to see if either side will deliver what they promise. Neither has achieved the needed precision to compete with the current system, but both claim they are well on their way. In another year or so things may have headed up, or be delayed indefinitely once again.

Here are a few other great reports on this story from BBC News and the Backreaction blog.