Author: Stephen Ornes / Source: Science News for Students
November is election time, and not only for voters in the United States.
On November 16, hundreds of scientists will gather in Paris, France, for the most unusual election this year, if not this century. Maybe ever. The voting scientists won’t be choosing new leaders or passing new laws.They’ll be voting on something more elementary: a definition.
If the vote passes, they’ll permanently change the meaning of the word kilogram. The change will help scientists and engineers measure more precisely. Experts have been preparing for this vote for a decade. It they have done their job, they’ll fix a looming problem — and the rest of the world won’t even notice.
“I will go, and I will vote,” says Peter Mohr. He’s a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. He is also to blame, at least partially, for the vote. Twenty years ago, Mohr helped write papers arguing that the kilogram needed a new definition.
He expects the vote to pass with flying colors. “Hopefully there won’t be surprises,” he says. “But you never know.”
To understand why changing a kilogram is a big deal, you have to consider what the word means now. And to understand that, you have to visit a secure vault in Europe. This quiet, small room is opened only once a year.
The kilogram
The underground vault is in Sèvres, France, close to Paris. Inside lies an unusual treasure nicknamed Le Grand K. It’s also known as the International Prototype of the Kilogram, or the IPK. It’s a squat cylinder made of polished metal. It stands about as tall as someone’s thumb, and has a diameter equal to its height. It sits inside a vacuum-sealed bell jar and no bare hands ever handle it!
To unlock the vault requires not one but three keys, each in the possession of a different person. Hao Fang doesn’t have a key, but she has been inside the room. She’s a physicist at the International Bureau of Weights and Measures, which takes care of Le Grand K. Every year, she’s part of a small group of scientists who enter the vault to visit the cylinder.
“It’s a small room, with lots of sensors,” she says. “We only stay a short time because we don’t want to disturb the environment. We check that everything is okay.”
That may seem like a lot of bother, but Le Grand K isn’t just any ordinary cylinder. Every time you weigh something or measure its mass, the accuracy of your measurement depends on Le Grand K.
That’s because this curious and shiny little doodad is the exact definition of a kilogram — at least until November 16.
Kilograms are used to describe the mass of an object, which tells you how much matter the object contains. A bowling ball has a mass of about 5 kilograms. A basketball has a mass of about 0.5 kilogram. If you compare those two numbers, you can confirm what you already knew from picking them up. A bowling ball contains more matter than a basketball.
If you live somewhere that measures mass in pounds, the new change will affect you, too. That’s because a pound is defined as 0.4536 kilogram. That bowling ball is 11 pounds. And the basketball is 1.1 pounds. And this means that if the definition of a kilogram changes, so does the definition of a pound.
By our current definition, noting that a bowling ball has a mass of 5 kilograms is the same as noting that it contains exactly five times as much matter as Le Grand K. (But of course, we don’t learn it that way, or say that. We don’t say that because it’s a lot of words, and because it sounds ridiculous.) Every mass measurement, no matter where it’s done or what it’s done on, no matter how big or how small, traces back to Le Grand K.
And that’s a problem, say experts. If you define a unit with a physical object like Le Grand K, you have to consider some strange problems. For example: What if something happened to the cylinder?
“If someone dropped the IPK, the mass of the world would change,” says Stephan Schlamminger. He’s a physicist at NIST who is working on the new definition. In a split second, he notes, “everyone will have the wrong measurement.”
But this situation should change soon. Scientists are expected to vote to no longer define a kilogram as the mass of Le Grand K. Instead, a kilogram will be defined according to a mathematical formula that uses a natural constant of the universe. It’s called Planck’s constant. A constant is a number that doesn’t change. Planck’s constant is a number that shows up when scientists study quantum mechanics. These are the rules that describe the behavior of subatomic particles.
Is this the first time scientists have decided on a new definition for the kilogram?
No. The quest for a precise unit of mass has a long history. Historians know the Romans had a system of weights and measures, for example. But at the end of the 18th century, scientists all over the world were becoming frustrated because units had different values from country to country.
In 1799, during the French Revolution, scientists adopted a specially-designed copper metal bar as the definition of one meter (3.3 feet). A small platinum cylinder became the definition of one kilogram (2.2 pounds). The cylinder represented the mass of one cubic centimeter of water near its freezing point.
Those two objects represented the beginning of the metric system. In 1889, scientists from all over the…
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