The one true kilo gets a hi-tech makeover

The Guardian - - NATIONAL - Ian Sam­ple Sci­ence ed­i­tor

For the band of spe­cial­ists in the much over­looked arena of metrol­ogy, it will be the most pro­found mo­ment in more than a cen­tury. Since 1889, one of the pil­lars of the trade, the kilo­gram, has been de­fined by a lump of metal held in a triple-locked vault in a lab on the out­skirts of Paris. It is the one true kilo­gram in the world.

But not for much longer. Next week, the rulers of the field will make his­tory. At the Gen­eral Con­fer­ence on Weights and Mea­sures in Ver­sailles, rep­re­sen­ta­tives from 57 na­tions will vote for change. And so the kilo­gram, the only met­ric unit still based on a soli­tary ob­ject, will have a rad­i­cal makeover. Hence­forth, the kilo­gram will be de­rived from a fun­da­men­tal con­stant, a num­ber that is wo­ven into the fab­ric of the uni­verse.

The vote is essen­tially a done deal. The de­bates have been had, the so­lu­tions agreed. But even pop­u­lar rev­o­lu­tions can be tense af­fairs. “It will be nerve-rack­ing,” said Stephan Sch­lam­minger, a physi­cist at the US Na­tional In­sti­tute of Stan­dards and Tech­nol­ogy, who will be in Ver­sailles for the vote. “I’ve been think­ing about this, or work­ing on it, for as long as I have been a sci­en­tist. Some­times I am in a state of dis­be­lief. Is this re­ally go­ing to hap­pen? Will I wake up and find this is all a dream?”

Much of the week’s meet­ing will be de­voted to rou­tine af­fairs – bud­gets and the like – with votes cast on the Fri­day. “It will be nice to know the whole thing has been re­solved,” said Stu­art David­son, head of mass metrol­ogy at the Na­tional Phys­i­cal Lab­o­ra­tory in Ted­ding­ton. “We’ll know, at least, that no one has stood up and said this is rub­bish, we’re not go­ing with it.”

The roots of mod­ern mea­sure­ment trace back to the mid-18th cen­tury. With in­ter­na­tional trade on the up, it made no sense to price rolls of fab­ric, for ex­am­ple, ac­cord­ing to the lo­cal duke’s shoe size. In the late 1700s, King Louis XVI com­mis­sioned sci­en­tists to find a more sen­si­ble ap­proach.

The group pro­posed a sys­tem in­spired by the nat­u­ral world, one in­tended “for all times, for all peo­ple”. A me­tre was de­fined as one 10-mil­lionth of the dis­tance from the north pole to the equa­tor. The kilo­gram was the mass of a litre of wa­ter. To make the units more prac­ti­cal, each was en­shrined in a phys­i­cal ob­ject, a metal bar for the me­tre, and a weight for the kilo­gram. In time, the scheme evolved into the In­ter­na­tional Sys­tem of Units with its seven base units: the me­tre for dis­tance, the sec­ond for time, the kilo­gram for mass, the mole for amount of sub­stance, the am­pere for elec­tri­cal cur­rent, the Kelvin for tem­per­a­ture, and the can­dela for lu­mi­nos­ity.

For 129 years, the world’s of­fi­cial unit of mass has been the IPK, the in­ter­na­tional pro­to­type kilo­gram, a cylin­der of plat­inum-irid­ium stored un­der three sealed bell jars at the In­ter­na­tional Bu­reau of Weights and Mea­sures (BIPM) in Sèvres, near Paris. Na­tional metrol­ogy labs hold copies of the IPK. Ev­ery 40 years or so, they are re­turned to Paris for checks against the IPK, nick­named Le Grand K.

The sys­tem is not bad. Be­cause Le Grand K de­fines the kilo­gram, its mass is never in doubt. It is al­ways, with 100% ac­cu­racy, one kilo­gram. And yet its weight goes up and down. In stor­age, the plat­inum picks up pol­lu­tants from the air and the cylin­der gets ever so slightly heav­ier. When it is cleaned, the kilo­gram loses weight. The net ef­fect is hard to gauge, but copies can put on tens of mi­cro­grams in a cen­tury. The kilo­gram that de­fines all oth­ers is not what it used to be.

It is enough to irk metrol­o­gists. “If aliens ever visit Earth what else would we talk about other than physics?” said Sch­lam­minger. “If we want to talk about physics we have to agree on a set of units, but if we say our unit of mass is based on a lump of metal we keep in Paris, we’ll be the laugh­ing stock of the uni­verse.”

If the vote pro­ceeds as ex­pected, Earth will be spared such ga­lac­tic shame. Since 1983, the me­tre has been de­rived from the speed of light in a vac­uum. The makeover will de­rive mass from the Planck con­stant, a num­ber deeply rooted in the quan­tum world.

The re­def­i­ni­tion is eas­ier said than done. Sci­en­tists first use a supremely sen­si­tive piece of equip­ment called a Kib­ble bal­ance to cal­cu­late Planck’s con­stant from a 1kg ref­er­ence mass.

1 The num­ber of true kilo­grams in the world, since 1889, de­fined by a cylin­der of plat­inum-irid­ium

129 Num­ber of years for which the in­ter­na­tional pro­to­type kilo­gram has been world’s of­fi­cial unit of mass

The ob­ject be­ing weighed is bal­anced by elec­tro­mag­netic forces. Planck’s con­stant is pro­por­tional to the en­ergy needed to bal­ance the mass. Then, armed with a pre­cise value for Planck’s con­stant, re­searchers can use the bal­ance to mea­sure un­known masses.

With a pos­i­tive vote the new sys­tem of units will take ef­fect on 20 May 2019. The rev­o­lu­tion will hardly be felt be­yond the world of metrol­ogy. No one will weigh their car­rots any dif­fer­ently. But be­hind the scenes, a more el­e­gant sys­tem will be at work.

“The great­est sat­is­fac­tion for me will be com­plet­ing the his­toric arc that started with the French rev­o­lu­tion,” said Sch­lam­minger.

“The idea was to have a mea­sure­ment sys­tem for all times and for all peo­ple. They fell short on the kilo­gram. It has th­ese prob­lems with sta­bil­ity, so it is not for all times, and it is locked in a safe, so it is not for all peo­ple. Planck’s con­stant never changes, so it is the same for all time. And its value is wo­ven into the fab­ric of the uni­verse, so it is there for ev­ery­one.”

‘If aliens visit and we say our unit of mass is based on a lump of metal in Paris, we’ll be a laugh­ing stock’ Stephan Sch­lam­minger Physi­cist

The one true kilo­gram in the world, man­u­fac­tured in the 1880s, is held in a triple-locked vault near Paris PHO­TO­GRAPH: BIPM

▲ The kilo­gram was com­mis­sioned in the late 1700s by Louis XVI

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