Why Science is Redefining the Kilogram

Le Grand K | International Bureau of Weights and Measures

The way we define a kilogram is due for an update, and scientists around the world are happy to oblige.

What do you think it would do to the field of education if we rarely updated our textbooks?

Imagine how far behind everybody would be. Textbooks may be boring, but they are a basic educational tool. They provide a basis from which to learn.

And they need to be updated, obviously, because if they aren’t then they become fairly useless fairly quickly.

Chief of quantum measurement Jon Pratt says we need a new kilogram.Click To Tweet

And there are plenty of other things that run by the same axiom of update-or-be-left-behind. For example, you have desktop computers and antivirus software. But, these are things that are built to be outdone eventually.

With the measurement of mass, you might think that one definition is fine for all time.

Yet, not so. The way we measure mass needs an update, and that kind of thing doesn’t happen every day. It’s a task that is both monumentally large and fairly small.

And if you listen to scientist Jon Pratt, it’s about time.

Replacing Le Grand K

Most people know what a kilogram is, but I think it’s likely that few people have ever wondered why and how.

See, the standard for measurement for all kilograms is a small cylinder called Le Grand K, crafted circa 1889. The cylinder lies deep within a vault in the Bureau International des Poids et Mesures (BIPM) in Paris.

It sounds like a likely target for a James Bond villain. Then again, if you listen to Jon Pratt, it’s outdated. And I’d listen, because Pratt is the chief of quantum measurement at the National Institute of Standards and Technology (NIST).

Given a bit longer than a century, scientists have much better ways to determine the mass of a kilogram than le grand K. There are a few good reasons for this, too, including potential changes in the metals over time.

Most scientists are in accord with Pratt; it’s time to get rid of le grand K. It had a good run, but Industry 4.0 is about progressing our understanding of the universe.

Le grand K had a good run, but it's time for an update. #redefinethekilogramClick To Tweet

Like understanding a kilogram using quantum physics and revolutionizing our practical understanding of mass.

It’s an enormous achievement, even if it isn’t likely to be felt across the world. According to Pratt, “It’s not obvious that this is a big deal, but it’s a big deal.”

But it isn’t just going to happen. Believe it or not, there is an International Committee for Weights and Measures (ICWM), and they have the ultimate say. Before anyone goes around messing with le grand K, standards will have to be met.

Welcome to the 21st Century, Kilogram

Since 2014, members of the ICWM have been hard at work redefining the kilogram based on Planck’s constant. Planck’s constant is a value from quantum mechanics, and scientists think they can use it to calculate a kilogram.

It’s quantum physics, so nothing about it is simple, but Pratt and his team at the NIST have been hard at work with the problem.

So far, they have been using a tool called a Kibble balance, which uses electromagnetism to balance a scale with extreme precision.

And make no mistake, extreme precision is warranted. According to NIST, the calculation of Planck’s constant is 6.626069934 x 10−34 kg∙m2/s.

Since most of us won’t know what to make of that number, let me try to simplify. The important thing to note is that the number is within 0.0000013 of the actual value of Planck’s constant. You could also express that as having an uncertainty of 13 parts per billion.

Now, when the ICWM last got together, they announced their requirements for redefining the kilogram. They needed three measurements with uncertainties below 50 ppb, and one below 20 ppb.

So far, the NIST measurement joins three others below 20 ppb, which means that the effort is off to a good start.

The ICWM meets this month, and they will likely set a global value for Planck’s constant. NIST’s results join those from labs all over the world for this monumental achievement in the world of measurement.

And come 2018, when the next general conference of the ICWM occurs, we’ll see the first draft of the redefinition of the kilogram.

How will Planck’s constant improve our ability to measure the kilogram?

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