The kilogram just got a revamp. A unit of time might be next | Science News

Support Science Journalism

Science News is a nonprofit.

Support us by subscribing now.


The kilogram just got a revamp. A unit of time might be next

The unit of mass is now defined by a fundamental constant of nature, not a hunk of metal

7:00am, May 20, 2019
metal kilogram

HAS-BEEN  For 130 years, this metal cylinder was the definition of a kilogram. Now, it's just another hunk of metal, as scientists have instituted a new definition based on a quantity known as the Planck constant.

Sponsor Message

The new kilogram has finally arrived.

Updates to scientists’ system of measurement went into force May 20, redefining the kilogram and several other units in the metric system. The revamp does away with some outdated standards — most notably, a metal cylinder kept in a vault near Paris that has defined the kilogram for 130 years (SN: 12/8/18, p. 7).

Tinkering with units allows scientists to more precisely measure weights, temperatures, electric currents and other quantities laid out in the International System of Units used around the globe. The kilogram, the basic unit of mass, is now defined by a quantum quantity known as the Planck constant. That value, an immutable constant of nature, is the same everywhere in space and time. That’s an improvement over the Parisian artifact, which could have changed slightly if gunk or scratches marred its surface.

Also redefined, according to an agreement reached in November 2018 at the 26th General Conference on Weights and Measures in Versailles, France, are the kelvin, the unit of temperature; the ampere, the unit of electric current; and the mole, the unit for an amount of substance (SN: 12/8/18, p. 7).

Scientists now have their sights set on updating the unit of time: the second.

optical atomic clocks

Currently, the second is defined by atomic clocks made of cesium atoms. Those atoms absorb a certain frequency of light. The wiggling of the light’s electromagnetic waves functions like the pendulum on a grandfather clock, rhythmically keeping time. One second is defined as 9,192,631,770 oscillations of the light.

But a new generation of atomic clocks, known as optical atomic clocks, outdo the cesium clocks (SN: 11/11/17, p. 8). “Their performance is a lot better than what currently defines the second,” says physicist Andrew Ludlow of the National Institute of Standards and Technology in Boulder, Colo. Because those optical atomic clocks operate at a higher frequency, their “ticks” are more closely spaced, making them about 100 times more precise than cesium clocks.

Ideally, the length of a second should be defined using the most precise timepieces available. A switch might happen in the late 2020s, Ludlow says.

The change to the kilogram’s definition was carefully orchestrated so that it wouldn’t affect normal people: A kilogram of flour still makes the same number of biscuits. Any change to the second will be similarly coordinated.

So, sorry, there’ll be no chance to squeeze any extra seconds into a day.


The 26th General Conference on Weights and Measures. Versailles, France. November 16, 2018.  

Further Reading

E. Conover. It’s official: We’re redefining the kilogram. Science News. Vol. 194, December 8, 2018, p. 7.

M. Temming. New atomic clock is most precise yetScience News. Vol. 192, November 11, 2017, p. 8.

E. Conover. Units of measure are getting a fundamental upgradeScience News. Vol. 190, November 12, 2016, p. 24.

Get Science News headlines by e-mail.

More from Science News

From the Nature Index Paid Content