BY ANY MEASURE: More than a century of work has reached its conclusion, with all base units of measurement now tied to defined constants rather than physical objects locked awaLfn vaults.

• Author: Adams, Amy
• Position: WEIGHTS & MEASURES

SCALES ARE NOT changing and the weather will not be noticeably different but, as of a couple of months ago, the definitions that underlie what your scale and thermometer report--along with standard definitions used in chemistry and electronics--have undergone a major overhaul. May 20 was the date that a more-than-centuries-long process of standardizing measurements reached its conclusion.

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

"Nothing has changed and everything has changed," says Marc Salit, senior scientist and adjunct faculty member at Stanford University, who helped define these measurements--known as the International System of Units, or SI units--at the National Institute of Standards and Technology (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 now is part of SLAC [Student Learning Assistance Center] National Accelerator Laboratory.

Salit indicates that the changes are invisible to most people but represent a major shift in the field of metrology. He was at the meeting in November 2018 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 equates metrology with kids trying to share a chocolate bar equally. "We each have an innate sense of justice and fairness. 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 of 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...