For millennia, humans have turned to the sky to tell time. Our planet rotates once on its axis, and we’ve lived another day. We divide that day into smaller fractions: the hour, minute, and second. But Earth is an imperfect clock. Its orbit and rotation vary slightly from year to year—the kind of wiggle room that makes it incredibly difficult for scientists to do their calculations, or for satellites to send their precisely-coordinated communications.
That’s why, since 1967, the scientific second has had nothing to do with celestial bodies. Instead, it’s calculated according to the properties of a cesium atom. When illuminated with laser light under specific conditions, a cesium atom emits microwave radiation—rapid waves of light that crest and dip. That year, the General Conference on Weights and Measures (CGPM, after its name in French) officially defined the second as 9,192,631,770 cycles of that wave.
Cesium seems like a random choice. But scientists use it because the atom—one of the smallest units of matter—is about as well-behaved as they come. Its radiation won’t change, no matter where or when you observe it. It’s the ultimate calibration: syncing Earth’s timepieces to the universe’s clock.
Since then, scientists have defined the meter based on the speed of light, a similarly unflinching universal value. “It makes sense to have your measurement system based on true invariants of nature,” says physicist David Newell of the National Institute of Standards and Technology. And scientists are now working on re-standardizing four more units—the ampere, Kelvin, kilogram, and mole—by measuring fundamental constants more precisely than ever before.
Researchers worldwide submitted their final measurements in July to a group of scientists working with CGPM, and they will meet next week to discuss them. By late next year, the plan is for every single unit but one, the candela, to be based on a fundamental constant of nature. Scientists want…