In the bal­ance: Sci­en­tists vote on first change to kilo­gram in a cen­tury

Iran Daily - - Science & Technology -

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 sci­ence, 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, lead­ing fig­ures in the field are set to 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 be reborn. Hence­forth, the kilo­gram will be de­rived from a fun­da­men­tal con­stant, a num­ber that is wo­ven into the fab­ric of the uni­verse, The Guardian re­ported.

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 such as bud­gets and the like, with votes cast on Fri­day. “It will be nice to know that the whole thing has been re­solved,” said Stu­art David­son, the head of mass metrol­ogy at the na­tional phys­i­cal lab­o­ra­tory in Ted­ding­ton.

“We’ll know, at least, that noone has stood up and said this is rub­bish, we’re not go­ing with it.”

The roots of mod­ern mea­sure­ment can be traced back to the mid-18th cen­tury, when it be­came clear that na­tions might do well to share com­mon units. 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 as­sem­bled group pro­posed a sys­tem in­spired by the nat­u­ral world, one in­tended “for all times, for all peo­ple”.

A me­ter was de­fined as one ten-mil­lionth of the dis­tance from the North Pole to the equa­tor. A kilo­gram was the mass of a liter of wa­ter. To make the units more prac­ti­cal, each was en­shrined in a phys­i­cal ob­ject, a metal bar for the me­ter, and a weight for the kilo­gram. In time, the scheme evolved into an in­ter­na­tional sys­tem of units (SI) with seven base units: The me­ter 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 the past 129 years, the world’s of­fi­cial unit of mass has been the in­ter­na­tional pro­to­type kilo­gram, or IPK, a shiny cylin­der of plat­inum-irid­ium stored un­der three sealed bell jars at the in­ter­na­tional bu­reau of weights and mea­sures (BIPM) in Sèvres, west of Paris. Na­tional metrol­ogy labs hold copies of the IPK to cal­i­brate mea­sure­ments in their own coun­tries. Ev­ery 40 years or so, the copies are re­turned to Paris for checks against the IPK, nick­named Le Grand K.

It’s not a bad sys­tem. Be­cause Le Grand K de­fines the kilo­gram, its mass is never in doubt. It is al­ways, with 100 per­cent 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 as tiny amounts of al­loy are re­moved. The net ef­fect is hard to gauge, but copies can put on tens of mi­cro­grams in a cen­tury. All sci­en­tists know for sure is that 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 me­ter has been de­rived from the speed of light in a vac­uum. The kilo­gram makeover will de­rive mass from the Planck con­stant, a num­ber deeply rooted in the quan­tum world. It de­scribes the size of bun­dles of en­ergy, known as quanta, which pour out of a hot oven, for ex­am­ple.

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. The in­stru­ment is sim­i­lar to a scales, but in­stead of coun­ter­act­ing one weight with an­other, 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 do the re­verse, and use the bal­ance to mea­sure un­known masses.

With a pos­i­tive vote the new sys­tem of units, in­clud­ing up­dates to the mole, Kelvin and am­pere, comes into ef­fect on May 20, 2019. The rev­o­lu­tion will hardly be felt be­yond the world of metrol­ogy. No one will weigh their car­rots any dif­fer­ently at the su­per­mar­ket. 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.”

JOCHEN LUEBKE/EPA Arnold Ni­co­laus, of the Phys­i­cal-tech­ni­cal Fed­eral Agency, shows a sil­i­cone ball next to a copy of the kilo­gram pro­to­type in Brunswick, Ger­many.

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