The definition of the kilogram has been updated for the first time in 130 years


They way we define a kilogram changes from today. Don’t worry, you won’t be heavier or lighter and you don’t have to update your favourite cake recipe – but the update has been a long time coming, and it’s a big deal for science.

Time to say bye to ‘Big K’

In Sèvres, a small commune on the outskirts of Paris, a gleaming lump of metal the size of a palm has sat for the past 130 years doing a very important job. Le Grand K, or Big K as they call the platinum and iridium alloy, is stored underground in a high-security vault. It is held under three glass bell jars, and can only be retrieved using three separate keys, each held by different individuals.

Tampering and theft isn’t the utmost concern for those who guard Big K. Instead, the artifact’s custodians have spent recent years worrying that the alloy isn’t quite living up to the reputation that it’s held for the past century — that it’s no longer exactly one kilogram in mass, but micrograms lighter.

Being off by roughly the weight of a grain of sand might seem trivial, but (for the past 130 years) Big K has been the International Prototype of the Kilogram. In other words, it’s the gold standard by which all other kilograms in the world were measured against. The tiniest discrepancy in Big K’s accuracy impacted fields such as medicine, electronics and engineering, sectors where precise measurements are paramount. But a fluctuating kilogram also had rippling effects on other phenomena — such as force, energy and luminous intensity — that use it as the building block for measurements.

Because of the wide-reaching consequences an imprecise Big K held, scientists searched for a more reliable and stable standard for the kilogram — one that doesn’t centre on a single piece of metal.

K20, a replica of the ‘Big K’ owned by the US goverment and held at the National Institute of Standards and Technology (Wikipedia)

“We are about to witness a revolutionary change in the way the kilogram is defined,” said physicist Klaus von Klitzing while speaking at CERN last October. Klaus, who won the 1985 Nobel Prize in Physics, was one of the scientists involved in the kilogram’s makeover.

The change, many argue, was long overdue. The kilogram is one of seven base units that comprise the International System of Units (SI), the most widely used measurement system in the world today. Originally both the kilogram and the metre were defined by prototypes and the time was fixed by the earth rotation, however in the meantime more and more base units are connected to physical quantities of nature that remain the same regardless of time or location.

One second, for example, is defined as the time it takes for the cesium-133 atom to complete 9,192,631,770 periods of radiation for a specified transition. One metre used to be represented by a metal bar stored alongside Big K in France, but is now defined by how far light travels in a vacuum during 1/299,792,458 of a second.

The kilogram remained the only SI unit represented by an unstable artifact. So in 2014, members of the General Conference on Weights and Measures, the international body which oversees the SI system, voted to redefine the kilogram in terms of Planck’s constant, a fundamental constant of quantum mechanics.

And so, as of today, World Metrology Day, a kilogram (kg) is now defined:

[…] by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ΔνCs.

Got it?

“The challenge now though is to explain these new definitions to people – especially non-scientists – so they understand. Comparing a kilogram to a metal block is easy,” says David Brynn Hibbert, a professor of Analytical Chemistry at UNSW.

However complicated it may sound to a non-physicist, the new definition is an important and much-needed update. It means we can switch from “a 19th century definition of mass to a more 21st or 22nd-century definition of mass,” says John Pratt of the National Institute of Standards and Technology (NIST), the body responsible for the standardisation of weights and measures in the United States.

When gold standards are unstable, as Big K proved to be, it’s a “huge inconvenience,” says Pratt.

Big K’s unaccounted weight loss meant its sister cylinders — cast from Big K and shipped around the world for calibration — are no longer identical to the gold standard. NIST’s copies, for example, differed from Big K by roughly 45 micrograms, the weight of an eyelash. That wreaked havoc several years ago, leading to NIST re-issuing certificates for its kilograms, and companies producing weights based on NIST’s standards having to manufacture new ones.

Re-defining the kilogram according to Planck’s constant will help avoid such problems altogether.

– Sandy Ong

Eliza Brockwell

Author: Eliza Brockwell

Eliza is passionate about creating content that encourages diversity of representation in STEM.


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