If you're familiar with the basics of metric conversion, you know that 2.2 pounds are equivalent to 1 kilogram. But how much does a kilogram weigh, exactly? How does one define an abstract concept like a unit of measurement? Since 1875, the exact weight of a kilogram has been defined by the International Prototype of the Kilogram, a cylinder made from an alloy that's 90 percent platinum, 10 percent iridium, and sits locked in an environmentally regulated vault outside Paris. Every 40 years, it's removed and compared to a half-dozen copies around the world, which in turn are used as models for replica weights around the world.

That system was OK in the late 19th century. But today, with super-precise measurements needed for pharmaceutical production, chemistry, astrophysics and other endeavors, it no longer works so well. Add to that another problem — after nearly a century and a half, it's gained weight, thanks most likely to contaminants that gradually have accumulated on its surface.

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That's why the International Committee for Weights and Measures, the governing body that controls what it means to weigh a kilogram, is interested in switching from a physical standard to one based on some fundamental standard in nature — something that scientists can replicate in laboratories across the planet.

Researchers for the U.S. government's National Institute of Standards and Technology (NIST) say that they've made a breakthrough that could solve the problem. Using a new machine called the NIST-4, they've determined a more precise value for Planck's constant, a really tiny number (and a fundamental physical quantity) used in quantum physics to calculate the amount of energy transmitted by a single photon.

Planck's constant also can be used to define the kilogram, because it too is a fundamental constant — something that doesn't change in our universe — and scientists can conduct experiments to determine what it is. "We can fix the unit and ask nature, 'What is the numerical value of the fundamental constant using these units?'" said NIST physicist Stephan Schlamminger in an email. "Or else, we can fix the numerical value and use the experiment to determine the size of the unit."

To calculate Planck's constant more precisely, NIST researchers employed a device called a Kibble balance, named for the British physicist who invented the technique. The device "compares electrical power to mechanical power," said Schlamminger. "Power is measured in units of the watt. Mechanical power is given by force times velocity. In the balance the force is the weight of a mass standard. Electrical power can be measured as a product of two frequencies and the Planck constant. These have to do with two quantum mechanical effects that are being used to measure voltage and resistance. Hence, the unit of mass can be linked to the Planck constant."

Here's a video in which NIST physicist Darine Haddad uses a cup of coffee and sugar cubes to explain the significance of Planck's constant:

In 2016, NIST scientists published a measurement of Planck's constant with an uncertainty of just 34 parts per billion, which is, well, pretty darn precise. But they kept experimenting, and now that they've got several years with of data to work with, they've honed the tiny number even further. The new NIST measurement sets Planck's constant at 6.626069934 x 10−34 kg∙m2/s, with an uncertainty of only 13 parts per billion.

The NIST researchers are one of numerous scientific teams in countries ranging from Canada to South Korea who are vying to come up with the most precise value for Planck's constant. Some also are performing experiments with the Kibble balance, while others are using the X-ray crystal density method, in which they infer the number of silicon atoms in an extremely round silicon sphere, Schlamminger said.

The NIST physicist noted that it might be possible to reduce the degree of uncertainty in their measurement of Planck's constant even further than the published value, down to 11 parts per billion. But they're hoping that the published number is sufficient to redefine the kilogram, he said.

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