When a standard isn't so standard any more

It seems the International Bureau of Weights and Measures, near Paris in France, has a problem. They keep the 'standard' example of a one-kilogram weight there - but its weight appears to have changed! NPR reports:

More than a century ago, a small metal cylinder was forged in London and sent to a leafy suburb of Paris. The cylinder was about the size of a salt shaker and made of an alloy of platinum and iridium, an advanced material at the time.

In Paris, scientists polished and weighed it carefully, until they determined that it was exactly one kilogram, around 2.2 pounds. Then, by international treaty, they declared it to be the international standard.

The international standard one-kilogram weight

Since 1889, the year the Eiffel Tower opened, that cylinder has been the standard against which every other kilogram on the planet has been judged. But that's creating problems. According to scientists, the cylinder's mass appears to be changing.

. . .

As it stands, the entire world's system of measurement hinges on the cylinder. If it is dropped, scratched or otherwise defaced, it would cause a global problem. "If somebody sneezed on that kilogram standard, all the weights in the world would be instantly wrong," says Richard Steiner, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

For that reason, the official kilogram is kept locked inside a secured vault at the International Bureau of Weights and Measures near Paris. Scientists are so paranoid that they've only taken it out on three occasions: in 1889, 1946 and 1989. Each time, they've compared it to a set of copies. In 1889, the copies and the kilogram weighed the same, but by 1989, they had drifted apart. Based on the data, the kilogram appears to weigh slightly less than the copies.

The real crux of this problem is that it's impossible to tell what has changed over the past 120 years. The copies may have grown heavier over time by absorbing air molecules. But it's equally possible that the kilogram is getting lighter. Periodic washings, for example, may have removed microscopic quantities of metal from its surface.

Or it could be that both the copies and the kilogram are changing, but at different rates. There is no way to tell what's happening because mass is always calibrated against another mass, says Peter Mohr, a theoretical physicist at NIST who is working on the kilogram problem.

That's the bad news. The good news is that the change is extremely small, around 50 micrograms (billionths of a kilogram). "The actual ramifications for somebody going to the store will be negligible," Mohr says. But "for scientific work, it makes a difference."

For that reason, scientists have embarked on a quixotic quest to redefine the kilogram in terms of a fundamental constant. Constants are used by physicists to describe the natural world. They are both precise and unchanging — the perfect instruments for setting standards.

. . .

Some researchers believe the best hope for redefinition comes from a new kind of scale called a watt balance. "It's basically just a very highly calibrated bathroom scale," says Steiner, who is in charge of the watt balance at NIST.

Prof. Steiner adjusts a watt balance

Rather than using another mass, watt balances measure the mass of a kilogram in terms of electrical and magnetic forces. Those forces can be translated into a number that is related to Planck's constant.

The scale is so sensitive that it can detect changes as small as ten-billionths of a kilogram. "If you pulled a hair out of a person's head and then weighed them, we could tell the difference," Steiner says.

There's more at the link.

The geek in me is fascinated by this. I mean - if the international standard one-kilogram weight has already changed in the one hundred and twenty years since it was made, how will you calibrate a watt balance to the original value of a kilogram? How is it to be established in the first place?

I foresee an interesting debate . . .

Peter