SI REDEFINITION
NIST is producing a series of special reports on the worldwide consensus plan to redefine four of the seven basic units of measurement in the International System of Units in terms of in-variants of nature
On November 16, 2018, in Versailles, France, a group of 60 countries made history. With a unanimous vote, they dramatically transformed the international system that underpins global science and trade. This single action finally realized scientists’ 150-year dream of a measurement system based entirely on unchanging fundamental properties of nature.
On that day, the International System of Units, informally known as the metric system—the way in which the world measures everything from coffee to the cosmos—changed in a way that is more profound than anything since its establishment following the French Revolution.
It was a turning point for humanity.
Too often, we’re blissfully unaware of the enormous number of precision measurements that make modern life possible. For example, every component of a smartphone — its processing chip, memory, microphone, and camera optics — depends on an infrastructure of meticulously measured and tested scientific principles, materials, tools and processes that combine to ensure that phone can reliably make calls, send texts, access the internet and use GPS to help us navigate. Measurements make the world go round.
This worldwide measurement infrastructure grew out of the original French metric system, which was conceived in 1790 to be “for all times, for all people” because its units are ultimately based on nature itself. The kilogram was the mass of one liter of water. The meter was defined as one ten-millionth of the distance from the North Pole to the equator. But those units had to be embodied in physical objects, such as the meter bar and a piece of metal that serves as the kilogram: objects that could wear out, and were certainly not available “for all times, for all people.”
In November, the world’s measurement experts voted to revise the SI, approving a system that does not depend on physical objects. Instead, it’s based entirely on the speed of light and other “constants” of physical science, resulting in a measurement system that might truly and finally be for all times and for all people.
These constants are central to a set of well-established scientific principles. They are the backbone of our ever-expanding knowledge of natural laws, such as Einstein’s well-known E = mc2 , which describes how mass and energy behave throughout the universe.
These scientific principles are the same ones we’ve already used to create a modern world where we watch flat panel TVs, navigate deep space and explore quantum computing. And this revised measurement system promises to help lead to technological innovations we cannot yet imagine.