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If the IPK drifts further with time then its value will change, but any mass calibration will have an uncertainty of the order of 20 parts per billion.” “So from today, one kilogram will stay the same. “The Plank constant has been fixed at 6.626070150 × 10 −34 kg⋅m 2/s using the IPK as standard,” explains Eichenberger. But does it affect the value of the kilogram itself? Not really. In 2018, the Kibble balance in Canada measured the Planck constant with the necessary ultrahigh precision, allowing a combination of measurements from around the world to help fix its value. “I do not know if the redefinition of the kilogram has a direct impact on the experiments at CERN, but the past teaches us that there are many new advancements which, at their initial moment, may not reveal their whole potential.”
“I am extremely proud to have participated in this adventure,” says Davide Tommasini from CERN’s Magnets, Superconductors and Cryostats group, who was directly involved in the project. Knowing CERN’s expertise in magnet systems, Eichenberger and Baumann reached out to the Laboratory to help prepare the required magnets. METAS, the Swiss Federal Institute of Metrology, has been working on their Kibble balance project for almost two decades, under the leadership of Ali Eichenberger and Henri Baumann. Several Kibble balances around the world were constructed to compare measurements, including one in Switzerland. Once the precision was achieved, the Planck constant’s value could be fixed and the definitions inverted, removing the kilogram’s dependence on the IPK. In 1975, British physicist Bryan Kibble proposed a device, then known as a watt balance and now called the Kibble balance in his honour, which would allow the Planck constant to be measured precisely based on the IPK. Since the latter two units are already defined by constants of nature, the value of a kilogram can be obtained without relying on comparison with a physical reference block.īut measuring the Planck constant to a suitably high precision of ten parts per billion required decades of work by international teams across continents, and CERN played a small part in the endeavour. From now on, it will be defined based on the most precise measurement ever made of the Planck constant, which can be expressed in terms of the SI units kilogram, metre and second. On the occasion of World Metrology Day, which commemorates the signing of the Metre Convention back in 1875, the kilogram has been given a new definition. This changes today, and metrologists – those who study measurement – are excited. While all the other base units of the International System of Units (SI) had been redefined over the years based on fundamental constants of nature or atomic properties, the kilogram had remained since the late 19th century the only one to rely on a human-made artefact. Until today, a kilogram was defined as the mass of the International Prototype Kilogram (IPK), a platinum–iridium cylinder stored in Paris, France.
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