Mental Floss

The World's Most Accurate Clock Has No Hands

Bess Lovejoy

Like it or not, the international language of commerce and science is primarily conducted not in inches and pounds but in the modern version of the metric system, officially known as the International System of Units (abbreviated SI based on its French name, Système International d'Unités). The core of the SI is made up of seven base units, covering length (the meter), mass (the kilogram), time (the second), electric current (the ampere), temperature (the kelvin), amount of substance (the mole), and luminous intensity (candela). For scientists, those are the key building blocks used to measure and define the rest of our world.

But how do you define the units themselves? That question has kept scientists busy for well over a century, ever since the first ill-fated attempts to define the meter in the terms of the earth’s meridian. But as science and technology have called for ever more precise measurements, the keepers of the SI have undertaken a redefinition of several of the base units that ties them to the fixed values of natural constants (things like the speed of light or the charge of electrons). For instance, instead of the kilogram being defined as a platinum-iridium cylinder created in the late 19th century and locked in a vault in Sèvres, France (as it is now), the proposed redefinition of the kilogram would be tied to the exact numerical values of the Planck constant h.

The unit of time—the second—was already redefined back in 1967, when the International Committee of Weights and Measures defined it based on vibrations of the cesium atom. If you want to get technical, the full definition of one second is “the duration of 9,192, 631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” Try bringing that up at your next trivia night. 

And the world’s most accurate keeper of that standard is located in Boulder, Colorado. That’s where the National Institute of Standards and Technology (NIST) keeps an atomic clock known as the NIST-F2, which tosses cesium atoms in the air in a routine repeated thousands of times an hour. NIST-F2 is one of the clocks that keeps the U.S. civilian time standard, and one of the clocks around the world that sends data to the International Bureau of Weights and Measures to produce Coordinated Universal Time. According to NIST, the F2 is so accurate it “would neither gain nor lose one second in about 300 million years.”

While the clock itself might not look like much, as Dylan Thuras of Atlas Obscura explains in the video above, it’s essential to everyday applications like Global Positioning Systems (GPS), as well as telecommunications and the internet. And if the redefinitions of the SI continue, it’s contraptions like these that will underpin much of how we define the world.

Header image via National Institute of Standards, Flickr