Sun, Nov 25, 2018 - Page 9 News List

How much a kilogram weighs depends on the ‘Planck constant’
一公斤到底有多重?取決於「普朗克常數」

A photo released by the International Bureau of Weights and Measures on Nov. 15 shows “Le Grand K,” a cylinder of platinum and iridium which is the official definition of the kilogram, preserved since 1879, in Sevres, near Paris, France.
由國際度量衡局於十一月十五日發布的這張照片,展示出以鉑銥合金鑄造的圓柱體「大K」,它是「公斤」的官方定義,自一八七九年後一直保存在法國巴黎附近的塞夫爾。

Photo: EPA
照片:歐新社

It may not change how you buy bananas, but scientists have voted to redefine the value of a kilogram, in what they called a landmark decision that will boost the accuracy of scientific measurements. Since 1889, a kilogram has been defined by a shiny lump of platinum-iridium kept in a special glass case and known as the International Prototype of the Kilogram. It is housed at the headquarters of the International Bureau of Weights and Measures (whose French acronym is BIPM), just outside Paris.

Members of the BIPM, which groups 60 nations, agreed on Nov. 16 after a week-long meeting at the nearby Palace of Versailles to redefine the kilogram in terms of a tiny but unchanging value called the “Planck constant.” They also voted to update definitions for the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).

All modern mass measurements are derived from the kilogram, whether micrograms of pharmaceutical medicine or gold dust, kilos of fruit or fish, or tonnes of steel. The problem is that the prototype doesn’t always weigh the same. Even inside its three glass bell jars it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.

That can have profound implications. If the prototype were to lose mass, atoms would in theory weigh more since the base kilogram must by definition always weigh a kilogram. Scientists have been trying for decades to define a constant value for the kilogram that is derived from immutable physics, in the same way they have done for other standard units (SI units) overseen by the BIPM.

For example, a meter isn’t 100 centimeters, it’s actually “the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.” The “Planck constant,” which derives from quantum physics, can be used along with a Kibble balance, an exquisitely accurate weighing machine, to calculate the mass of an object using a precisely measured electromagnetic force.

TODAY’S WORDS
今日單字

1. redefine v.

重新定義

(zhong2 xin1 ding4 yi4)

2. measurement n.

測量

(ce4 liang2)

3. constant n.

常數

(chang2 shu4)

4. mass n.

質量

(zhi2 liang4)

5. weigh v.

重量為

(zhong4 liang4 wei2)

6. immutable adj.

不變的

(bu2 bian4 de5)


“The SI redefinition is a landmark moment in scientific progress,” said Martin Milton, director of the BIPM. “Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges.”

Barry Inglis, who heads the committee for weights and measures, said the implications were immense. “We will now no longer be bound by the limitations of objects in our measurement of the world, but have universally accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement,” he said.

It is arguably the most significant redefinition of an SI unit since the second was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the Internet. The new definitions agreed by the BIPM will come into force on May 20, next year.(Reuters)

這個改變或許不會影響你買香蕉的方式。不過,在日前一項被譽為有望提升科學測量準確度的劃時代決定中,科學家表決通過對一公斤的數值重新定義。自一八八九年以來,一公斤的重量由一塊鉑銥合金定義,這塊閃閃發光的金屬稱作「國際公斤原器」,被保存在特別的玻璃櫃中,目前存放於巴黎近郊的國際度量衡局(法文首字母縮寫為「BIPM」)總部。

來自大約六十個國家的國際度量衡局會員,日前齊聚鄰近的凡爾賽宮,召開長達一星期的會議,並於十一月十六日做出決議:根據微小卻不會變動的「普朗克常數」數值,重新定義一公斤。會員們也決議更新安培(電流)、克耳文(熱力學溫度)以及莫耳(物質的量)等單位的定義。

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