THE HUNT FOR A SPECIAL ‘K’
An Ottawa lab has redefined the kilogram
There was only one official kilogram weight handed down from the time of Napoleon, and by the time of France’s centennial in 1879 it was considerably bunged up.
That year, as the Eiffel Tower was going up, France commissioned several new cylinders made from mixed platinum and iridium, and chose one of these as the new official kilogram. Le Grand K, it’s called today. It defines all mass, everywhere.
Locked in a vault near Paris, it has been taken out only four times since its creation.
But there’s trouble: It is losing weight.
Interaction with the surrounding air adds traces of hydrocarbons and water, which do weigh something. The metal surface has been gently cleaned periodically to fix this.
So today Le Grand K weighs a tiny bit less than its original 1889 “sister-copies,” which in turn have changed weight in different amounts, leaving no hint about which is closest to the original.
“We don’t know which of them is any good, if any of them,” says Barry Wood, a National Research Council scientist whose specialty is measurement.
All of which means the kilo isn’t what it used to be, and the world needs a replacement. One that won’t change, and one that won’t be sealed in a vault where no one can use it as a standard. To do that, scientists need a mathematical definition of a kilogram, based on numbers taken from the real world that do not change with time.
This means using a “mathematical constant” such as pi, or the speed of light.
To aid this search, Wood’s lab at the NRC has now made the world’s most precise measurement of a constant that will define the kilogram of the future, called Planck’s constant. Starting one year from now, the international science community is expected to use this mathematical definition of mass and weight, leaving Le Grand K as a lovely, shining relic.
“Probably only a couple of people in Canada have seen the (official French) kilogram,” Wood says.
“It would be nice to have our own mass standard, and that is what we are trying to do.” So, what’s Planck’s constant? This is a number that defines something about a photon — the relationship between its energy and its wavelength. It is more than 30 digits long, expressed in a unit called the joule-second.
But more to the point, it doesn’t change. And if it is used to define the kilogram, the kilo will have a definition that does not change.
To pin down its value the NRC needed special equipment, combined in an apparatus called the Kibble balance, after its creator: A balance with two arms, like a teeter-totter. A vacuum chamber. A really heavy magnet. Tools that aren’t made of steel. An electrical coil that carries a current. Counterweights. And computers that can work out the ever-changing pull of gravity, which goes up and down with the moon, the tides and even the weather.
Wood’s group experiments with a half-kilogram cylinder made of polished silicon.
An electrical current runs through the coil, which is inside the magnet, “and it pushes up and it balances the mass (the silicon weight),” Wood says.
Once the silicon is literally hanging in the balance, the team measures all the variables at work — the current involved, resistance, gravity, voltage, time, position and velocity, and all this leads to Planck’s constant.
“So we go from a hunk of material, in this case silicon, and we can now relate that to fundamental constants, like frequencies and quantum numbers and Planck’s constant.
“And that’s what the kilogram really is,” Wood said. “We’re getting away from a lump of metal being a standard, and we are moving to fundamental constants.”
While 14 teams round the world have been trying to do this, it was the NRC that came up with the greatest precision, a number with only a few parts per billion of “uncertainty.”
“We trust that these fundamental constants are truly fundamental — that means the same here, and in France, and in outer space, and on Alpha Centauri, but also not changing with time. And available anywhere, always.” Who needs such precision? The economy, for starters. It’s not serious if the sliced meat from your deli is off by a milligram, but this becomes serious for someone selling gold in large amounts.
And is this the final answer, forever? Probably not.
“You might say we’re going to learn new physics 20 years from now or 50 years from now. For this to be overturned by new physics, we’re going to need something that is more complicated than quantum mechanics. And that’s quite possible,” Wood said.
“My guess is this new system is going to last 100 years and maybe much longer.”