The kilogram has a new definition

How much does a kilogram cost? 1000 grams. 2,20462 pounds. Or 0.0685 slugs based on the ancient imperial gravitational system. But where does this amount come from and how can everyone be sure to use the same measure?

Since 1889, the member countries of the General Conference of Weights and Measures have agreed to use a standard block of metal – kept near Paris – to define the kilogram. But although the modern block is stored in a highly controlled environment, its weight can change in small amounts because wear and tear make it lose mass and dirt, its increase. To solve this problem, scientists from all over the world spent nearly two decades discussing how the kilogram might rather be defined in relation to constant measurements of nature. And now, they finally made a decision.

The first kilogram (originally called a grave) was defined in 1793 by a commission of the French Academy of Sciences, which wanted a better standard than the fixed quantities of cereals used traditionally. The commission decided that the new measure would be the mass of a cubic decimetre of water distilled at 4 (the temperature at which water has its highest density under standard conditions). This had the advantage of allowing the best equipped laboratories to reproduce this standard. Subsequently, a prototype of this mass was cast in brass.

Unfortunately, this definition of mass depends on another variable measure, the meter. At this point, the meter has only been tentatively defined as part of the distance from the North Pole to the equator. Once the counter value and the densest water temperature were defined more accurately, the kilogram also had to be replaced. And a new prototype was cast in platinum to represent this mass.

This prototype was eventually replaced by the International Kilogram Prototype (IKP) used today, molded in a mixture of platinum and iridium to make it very hard and prevent it from reacting with the l & # 39; 39; oxygen. The IPK and six copies are kept by the International Bureau of Weights and Measures in the pavilion of Breteuil, Saint-Cloud, near Paris, France, to serve as a reference for the measures to be taken. Copies of the IPK are transported around the world to ensure that all participating countries use the same standard.

But even the modern IPK can gradually change mass. The response of the International Bureau of Weights and Measures is radically to revise the definitions of the kilogram, as well as all other basic units of measurement used in science (known as SI units, from French to international system).

Instead of measuring the kilogram against a block stored in a safe, we can define it according to precise values ​​of constants of nature. It took a long time to agree on a definition, as it needed to be able to measure these constants to rigorous standards with an uncertainty of 30 parts per billion (which means that measurements are accurate to 0.00000003 units).

Scientists have already done this for time and length. A second is no longer a fraction of the time it takes the Earth to rotate, which can change when the globe accelerates or slows down. Instead, a second is now defined by the time required for a certain amount of energy to be released in the form of radiation from cesium-133 atoms. Specifically, a second equals at 9,192,631,770 transitions in the hyperfine levels of cesium-133 in the ground state. It's the same no matter when and where it's measured.

Scientists were able to redefine the counter in relation to the second and another natural constant, the speed of light in a vacuum (c), which scientists calculated at 299,792,458 meters per second. So, one meter is now the length traveled by the light in 1 / c seconds.

The new kilogram definition uses a measure derived from another fixed value of nature, the Planck constant (h), which will be set to 6.62607015 × 10-34 joule seconds. The Planck constant can be found by dividing the amount of energy that a particle of light or "photon" carries by its electromagnetic frequency.

The constant is usually measured in joule-second, but this can also be expressed in kilograms square meters per second. We know what the second and meter of the other definitions are. By adding these measurements, as well as an exact knowledge of Planck's constant, we can obtain a very precise new definition of the kilogram.

Other units

The creation of this new definition took so long that scientists had to create very precise devices to measure the Planck constant with a sufficient degree of accuracy. The method has also been controversial as it breaks the link between the kilogram and other basic SI units, such as the mole, which measures the amount of a substance based on the number of particles of which it is composed. . Some scientists have proposed alternative methods.

Following a symbolic vote, the International Bureau of Weights and Measures and national measurement institutes around the world will use the new definition of the kilogram, as well as the new definitions of the remaining basic SI units, the mole, the kelvin (temperature) , the ampere (current) and the candela (light intensity).

For most people, everyday life will continue as usual despite redefinitions. A standard bag of sugar will contain as much sugar as ever. But some of these changes, such as Kelvin, will have practical benefits for scientists doing very specific measurements. And to answer the question "How much is a kilogram," we will not have to compare platinum blocks or worry about scratching them.

Kevin Pimbblet is a senior lecturer in physics at the University of Hull. This article was originally featured on The Conversation.

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