Standards

Everything Stays the Same: Standard Measurement Units Redefined

More than a century of work reached its conclusion this week on May 20, when the kilogram was redefined, making all base units of measurement now tied to defined constants rather than physical objects locked away in vaults. Above is a picture of a kilogram weight under a bell jar. The National Institute of Standards and Technology's (NIST's) platinum-iridium kilogram K92 (front) with stainless-steel kilogram masses in the background. (Image credit: J.L. Lee/NIST)

Scales aren't changing and the weather won't be noticeably different, but on May 20 the definitions that underlie what your scale and thermometer report - along with standard definitions used in chemistry and electronics - experienced a major overhaul. That's the date that a more-than-centuries-long process of standardizing measurements reached its conclusion.

Beginning on May 20, the way we define an amount of light or electrical current - along with the more familiar measurements of volume and mass - are all be based on descriptions that could be replicated by anyone not just on Earth but in galaxies far, far away.

"Nothing changes and everything changes," said Marc Salit, who helped define these measurements - known as the International System of Units, or SI units - at NIST for 28 years. He now directs an effort to apply similar rigor to definitions of biological measurements. That effort, called the Joint Initiative for Metrology in Biology, or JIMB (pronounced Jim-Bee), was founded by NIST and Stanford in 2014 and is now part of SLAC National Accelerator Laboratory.

The seven SI base units on the left are now all derived from a set of definitions linked to the laws of physics. Those base units are used to calculate other measurements like force, energy and electric charge. (click on image to enlarge) Image credit: National Institute of Standards and Technology

Salit said Monday's changes will be invisible to most people but represent a major change in the field of metrology. He was at the meeting in November where the assembled international representatives voted to adopt the proposed changes.

"It was cool to see the U.S. voting yes, Russia voting yes, Ukraine voting yes and China voting yes. All voting yes to adopt the resolution that redefines the SI," Salit said.

He equates metrology with kids trying to equally share a chocolate bar. "We each have an innate sense of justice and fairness," he said. "This redefined SI is how we share. It's the basis for trade. It's the basis for equity. It's the basis for knowledge that is quantitative and interoperable and communicable."

The international effort to standardize measurements began with a treaty in 1875, when countries recognized the need for a consistent way of measuring products for trade. One early outcome was a standardized kilogram created in 1889 and housed in an underground bunker outside Paris, known as Le Grand Kilo or Le Grand K. Copies exist around the world, but Le Grand K, protected with three sets of keys, set the standard.

Now, a kilogram, like each of the seven base units of measure, will hinge on unchanging properties of nature - the speed of light, the wavelength of known atoms - and all other measurements build on those.

"This represents centuries of work bringing us together to share in a quantitative and repeatable, reproducible, precise manner," Salit said. "That redefinition makes it so that those units are coupled through a harmonious system that could be beamed into space."

To promote their effort, NIST has created superhero personas for each base unit of measure, all united around a common villain - Major Uncertainty (its symbol: the question mark).

Salit's work at JIMB focuses on trying to bring the same absolute specificity to biology. "How do you sequence something in Canada and I sequence it here and how do we compare? How do we make sure we got it right?" he asked.

Salit helped devise standardized controls for measuring the activity of genes and is looking at standardizing descriptions of how scientists measure protein levels.

As with the base units of measure, he said, the goal is to help scientists communicate, share knowledge and ultimately support fair partnership in trade.

National Institute of Standards and Technology , Stanford University
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