The Way We Measure The Universe Is About To Be Changed Forever

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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 international prototype of the kilogram. BIPM

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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