The way we measure things will radically change in 2019. Today in Versailles, representatives of the 60 member states of the Bureau International des Poids et Mesures have voted unanimously for the adoption of a revised system of units. Now, all seven fundamental base units will no longer be defined by comparing them to a physical object but by the fundamental laws of the universe. The change will come into effect on May 20, 2019.
The International System of Units, or SI, is historically based on seven quantities from which all the others are defined. SI units include the meter, the second, and the kilogram, which most people are familiar with. Quite well known are the kelvin, the official unit of temperature, and the ampere, which is used in measuring electrical current. Then there are the two more unusual ones: the mole, which is the amount of substance of a system, and the candela, which is the unit for luminous intensity.
The vote will have the kilogram, the kelvin, the ampere, and the mole finally defined by physical constants, and the meter, the second, and the candela will have their definition only slightly edited based on the bigger changes. The seven constants will from now on be defined as single numbers with no uncertainty. The numbers we have are exact values and they won’t change ever again.
“The importance of changing the four units is about the foundation of measurement. Because these standards are used in so many places, we take a very conservative attitude towards changing anything about the way they are defined,” Dr Michael de Podesta, a metrology expert from the National Physical Laboratory, told IFLScience.
“We are trying to put in new foundations which are indistinguishable in size, so there should be no change in the way ‘the building’ is supported. They are made of new materials so they’ll never crumble or crack.”
How will it affect us? “People won’t notice,” he added. “If people notice, we’ve screwed up!”
However, this is not a simple revolution in metrology. It’s an epochal shift. Since the dawn of civilization, humanity has tried to standardize measurements. After all, measurement is a quantitative comparison of one thing against another. You can make a universal stick or pick the length of your king’s foot. Base your temperature on boiling water or freezing brine. There are lots of ways to create scales. But as long as you use a physical object, your measurements will have fundamental limitations. The kilogram is the perfect example.
Every weighing scale around the world is set indirectly against the international prototype kilogram, a platinum-iridium cylinder kept under vacuum in Sèvres, France. The choice of material was to make sure it was durable and unchanging, but it turns out that it’s not exactly true. The prototype has 40 exact copies. Measurements in the 20th century have shown that the masses of all the copies are diverging, some gain weight and some are losing it. Which means we have to assume that the prototype itself is also not what it used to be. But since that one is the true kilogram, every mass measurement since its creation has changed.
From next May, the prototype will become an artifact for museums and the kilogram will be defined in terms of Planck’s Constant, a quantity key to quantum mechanics. The mole also had a physical connection although not quite as unique as the kilogram. It was defined as the number of atoms in 0.012 kilograms of carbon-12. From now on, 1 mole will contain exactly 6.022 140 76 x 1023 elementary entities. This value is known as Avogadro’s Number.
The motion voted today also fixes the value for the elementary electrical charge. Since the current is defined as the flow of charge over time, this will make its unit, the ampere, only dependant on the elementary charge and a second. The previous definition used two straight parallel conductors of infinite length placed 1 meter apart in a vacuum. The entire setup could not be used for a real measurement.
Last but not least, the unit of temperature. The redefinition of the kelvin is the culmination of 200 years of history. Over the last two centuries, the temperature slowly shifted from being a macroscopic property of an object we establish with a thermometer to being linked to the thermal motion of molecules. The redefinition will move away from comparison to a specific temperature (known as the triple point of water) and it will only depend on the actual energy of the molecules in a system.
“The new definition will be in terms of the Boltzmann constant and that tells you how much energy molecules have corresponding to a particular temperature,” Dr de Podesta explained. “It’s a much simpler definition. It’s basic physics. It links temperature to the energy of molecular motion and it doesn’t take any particular temperature or particular material as being special.”
The SI is a great human achievement. It is used in science and engineering all around the world, and every country but Liberia, Myanmar, and the United States use it as their official system of measurement. The approved changes just strengthen its universality. This is truly an international effort that has taken many decades to be achieved. From now on, with the right apparatus, everyone in the whole cosmos could measure and quantify according to the SI.
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