Redefining the kilo marks a weighty moment for metrology
By Anjana Ahuja
No drinker likes to be short-changed, as the Magna Carta recognised more than 800 years ago: “Let there be one measure for wine through our whole realm, and one measure for ale, and one measure for corn, to wit ‘the London quarter’; and one width of cloth, whether dyed, russet or halberget . . . ”

Friday will mark another weighty moment for metrology, the science of measurement. Subject to an international vote, the definition of the kilogramme will change. The standard unit of mass will no longer be the International Prototype Kilogram, a 19th-century ingot of platinum-iridium encased in three bell jars in an underground vault near Paris. Instead, the kilo will be calculated using Planck’s constant, an unbelievably tiny number that plays an outsized role in quantum physics.

It is a historic moment: the kilo is the last remaining major unit to be based on a physical object. Friday’s vote at the International Bureau of Weights and Measures in France will see the measurement, which seems so solid, benchmarked against a fundamental but intangible constant of nature.

The thing about the current kilo is that . . . it’s a thing. There are around 40 copies housed in national laboratories around the world, including at the UK’s National Physical Laboratory. Their masses have gradually drifted and diverged, by roughly the weight of an eyelash, due to surface contamination and oxidation.

These discrepancies, measured in millionths of a gramme, raise two problems. First, industries such as pharmaceuticals and microfabrication now demand more precision on mass than these antique objects can offer. Second, the IPK is used to calculate other units, such as the newton, pascal and joule (the units of force, pressure and energy respectively). A fluctuating mass, then, triggers a cascade of uncertainty elsewhere in the measurement chain.

Reinventing the kilo using fundamental constants of nature will stabilise the System International d’Unites. The SI system, a suite of seven base units drawn up in 1960, forms the backbone of metrology today. A new kilo — accompanied by three other redefined units — will be the freshest ink on an intergovernmental treaty, the Metre Convention, first signed by 17 nations in 1875.

This metrological update gives starring roles to two landmark equations in physics which knit together fundamental quantities in the universe, such as mass, energy, the frequency of radiation and the speed of light.

The first is E=mc2, formulated by Albert Einstein in 1905, where “E” is energy, “m” is mass, and “c” is the speed of light. The second comes courtesy of German scientist Max Planck, a child prodigy who eventually chose physics over music. In 1900, Planck theorised that light can only be emitted, transmitted or absorbed in discrete packets of energy, called quanta.

This truth, which underlies quantum theory, is described by the simple formula E=hf, where again “E” is energy, “f” is the frequency of radiation, and “h” is a constant. He earned the Nobel Prize for physics in 1918, and “h” became Planck’s constant.

Combining Einstein and Planck’s formulas means that mass can be defined in terms of Planck’s constant. Since 2011, scientists all over the world have worked furiously to establish a precise value for “h”, using an incredibly accurate weighing machine called a Kibble balance. They have converged on a value of 6.626070150 x 10⁻3⁴joule-seconds.

The Parisian ingot will surely fade to a quaint footnote in history, alongside the 12th-century royal decree that the yard be the “distance from the tip of the King’s nose to the end of his outstretched thumb”. Let us poignantly raise a measure of wine, or ale, to that.

The writer is a science commentator