Approved the new universal definition of the kilogram

Approved the new universal definition of the kilogram

The value of the unit of mass depends not already on an object, but a constant of nature.


A kilo of oranges, sugar or powder trees, weighs, by definition, the same as the platinum-iridium cylinder kept under several protective bells and enclosed with three keys in the basement of the Breteuil Pavilion on the outskirts of Paris. This International Kilogram Prototype (IPK), used to calibrate the official mass unit patterns, announces its retirement on Friday after 129 years of service. At the last session of the twenty-ninth General Conference of Weights and Measures held on Friday in Versailles, the 60 Member States voted unanimously to redefine the kilogram: from next year, mass unity will not be a physical object, but a value derived from a constant of nature. This change will have no involvement in the shopping basket or be noticed in the day to day, but it can be very important in scientific fields such as the development of medicines.

"Science history is being made. This will be counted in textbooks," said José Manuel Bernabé, director of theSpanish Metrology Centreand spanish delegate at the conference. Met metrologists, experts in the field of magnitude measurement, carrychange yearsto the International System of Units, which will include redefinitions of the mole, kelvin and amperium so that these units are also based on universal constants.

The kilogram receives special attention because it is the last fundamental unit whose definition still depends on the magnitude of a physical object. And thatis a problemscientists point out, because the object is not immutable. In the last century, the mass of the IPK has fluctuated. It is still a kilo, since by convention there can be no uncertainty in its value, but with respect to the mass of other kilo patterns, it has varied by values of at least 50 micrograms (millionths of the gram). This is because the cylinder can be soiled with air particles and loses small amounts of material when cleaned.

"We are sorry, above all, to be relieved that the decision is made," says Stuart Davidson, head of mass metrology at the UK's National Physical Laboratory (NPL), one of the centres most involved in redefining the kilogram

Micrograms do not affect the purchase of fruit or powders, but should be taken into account during the synthesis of new drugs, for example. In physical research, such fluctuationsthey are "intolerable". As Barnabas explains, this decision "is laying the foundation for the new science, one with less uncertainty for the development of technology."

"We are relieved, above all, that the decision is made," says Stuart Davidson, head of mass metrology at the UK's National Physical Laboratory (NPL), one of the centres most involved in redefining the kilogram. "There are people who have been working on this all their professional lives. Now we can focus on improving the technology and accuracy of our measurements," he adds.

Another incentive to remove the IPK has been the danger of the cylinder being damaged or deformed. The subway, which used to be the length of a platinum bar, was already redefined in 1983 precisely to avoid these problems. By setting the speed of light—constant in the vacuum—with a universal numerical value, metrologists agreed to call the meter "the distance that light travels in 1/299.792.458 seconds". Any lab capable of accurately measuring the passage of time can calibrate its own meter bar.

Revolution in measurement

The value of kg will be derived from Planck's constant thanks to a balance of power

The kilogram will do the same, when the approved changes take effect on May 20, 2019, the anniversary of the 1875 Metro Treaty. "On May 20, 2019, the biggest measurement revolution will be experienced since the French Revolution," Nobel laureate Bill Phillips said from the stage. Instead of the speed of light, the stationary figure chosen to define the unit of mass isPlanck's constant, a value that describes the energy packets emitted in the form of radiation. The approval of this definition of the kilogram has taken years to arrive because until recently there were no technological means to implement it. Now, thanks to a device called the Watt balance (sometimes Kibble balance or power balance), the value of the Planck constant can be calibrated.