Future Tense

How Much Does 1 Kilogram Weigh?

It’s actually about to change—slightly.

A replica of the prototype of the kilogram at the Cité des Sciences et de l’Industrie, Paris, France.
A replica of the International Prototype Kilogram at the Cité des Sciences et de l’Industrie in Paris.

Japs88/Wikimedia Commons

What does a kilogram weigh? Middle school science classes often teach that the unit is based on the weight of water—specifically a cube of water, a tenth of a meter on each side, at just above freezing. This used to be the case, but it isn’t actually true anymore—since 1875, the kilogram has been defined by one specific platinum cylinder, known affectionately as “Le Grande K” and officially as “the International Prototype Kilogram,” or IPK. It stands stands an inch-and-a-half high and wide and is housed in a vault outside Paris, inside three concentric glass containers to protect it from dust and other weight-altering debris.

Every scale in the world—even those that measure in pounds—is ultimately based on the IPK, which was commissioned by the General Conference on Weights and Measures. But the IPK’s uniqueness may be its downfall. “The problem with the kilogram in Paris is that it’s so precious that people don’t want to use it,” said Stephan Schlamminger, a physicist with the National Institute of Standards and Technology, in an interview with the American Institute of Physics. Even handling the model kilogram with your fingers will leave oil, changing its weight ever so slightly. It’s rarely removed from its enclosure, and never transported to other areas. Most people who care about exactly how much a kilogram weighs (chemists and physicists, mostly) calibrate their most precise instruments using replicas of the IPK, not the real thing. Problematically, these replicas vary slightly in weight when compared to one another. That’s why in 2005, the International Committee for Weights and Measures proposed that the kilogram be slightly redefined, anchored not to a physical object but to some fundamental property of nature that could be easily replicated in labs across the world.

After several deferrals, the international community of metrologists (scientists who study measurements; yes this is a real thing) has decided to use theoretical math to help redefine the kilogram. They are doing this via the “Planck constant,” the number that relates a particle’s frequency, meaning how fast it goes through its wavelike motion, to its energy. From there, using Einstein’s famous E=mc² equation, we can convert that calculated energy into mass. So in the end, we can discern the precise mathematical relationship between particle frequency and weight, which means that we can define the kilogram by particle frequency instead of by an object. And particle frequency is something that we can use in multiple places, without compromising anything.

The problem is the Planck constant is very, very small. That makes it very, very difficult to complete the above calculation—it is so difficult, in fact, that scientists don’t expect to complete it until 2018. One major part of the problem is that we still don’t have a precise measurement for the Planck constant. So a bunch of researchers are trying to calculate one.

For instance, the National Institute of Standards and Technology, based in Maryland, has developed a new generation of its machines that are specifically designed to measure the Planck constant. Like many values in quantum physics and relativity, the Planck constant is measured with a measure of uncertainty. The newest machine, dubbed the NIST-4, recently collected its first data set. The data could calculate a value with an uncertainty of 34 parts per billion, according a paper that was published last week in the Review of Scientific Instruments. That’s on par with the level of precision at other institutions, but by refining their process, the team hopes to soon get that number down to 20 parts per billion.

Globally, other researchers are working to refine their measurement of the Planck constant. A team from the National Research Council in Canada recently derived a figure with an uncertainty of 19 parts per billion using similar methods to the Maryland team. Measurements from researchers at other institutions will be announced in the coming months.

Once all groups have submitted their best estimate of the Planck constant, a computer program will compile all the figures using complex statistical methods and produce a best guess of the Planck constant, a figure which will then be used to define the exact size of a kilogram. As for the IPK, Schlamminger said, “It’s such a symbol, and it has such a rich history of measurement. I don’t think people will just throw it in the garbage,”

The redefinition will have rippling implications; over 20 units in the metric system—including units of pressure, magnetism, and electrical charge—are based on the kilogram. So once the kilogram is redefined, these measurements’ exact definitions will change, too, though the average citizen will almost certainly not notice.

“It’s the frustrating part about being a metrologist; if you do your job right, nobody should notice,” Schlamminger said.

This article is part of Future Tense, a collaboration among Arizona State University, New America, and Slate. Future Tense explores the ways emerging technologies affect society, policy, and culture. To read more, follow us on Twitter and sign up for our weekly newsletter.