The kilogram to be redefined for the first time in 130 years

In an underground vault located in a suburb of Paris, is a small metal cylinder rarely seen called Le Grand K.

For 130 years, this piece of the size of a golf ball, composed of 90% platinum and 10% iridium, served as a prototype of the kilogram. This means that it was the only physical object with which all other kilograms on the planet were measured.

If the microscopic contaminants present in the air have made the Big K a little bigger, the kilogram itself has slightly increased. If a rigorous cleaning or small scratches made it slightly lighter, the kilogram itself also became lighter. Indeed, it is estimated that The Big K lost 50 micrograms of mass during his life.

But the long reign of The Big K is coming to an end.

As of Monday, the kilogram will be redefined not by another object, but by a fundamental property of nature called Planck's constant. Like the speed of light, the value of Planck's constant can not fluctuate – it is constructed with exquisite precision in the very fabric of the universe.

"Unlike a physical object, a fundamental constant does not change," said Stephan Schlamming, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. "Now, a kilogram will have the same mass if you are on Earth, on Mars or in the Andromeda galaxy. "

Researchers who have dedicated their life to the science of measurement explain the new definition of the kilogram and similar modifications made to the mole (measurement of very small particles), to the ampere (measurement of the electric charge) and to the kelvin ( temperature measurement). ) – represents a profound turning point for humanity.

"The ability to measure with increasing accuracy is part of the progress of our species," said Walter Copan, director of NIST.

Most of us, ordinary people, will hardly notice the change. A 4 pound chicken (1.81437 kg) at the grocery store or a pound of coffee beans (0.453592 kg) at Starbucks will stay exactly the same.

"We do not want to shock the system," said Schlamminger.

The decision to redefine four basic units of the International System of Units was taken in November at the 26th General Conference of Weights and Measures in Versailles, France. Delegates from 60 Member States gathered in a large auditorium for the historic vote. It was unanimous. A standing ovation and a champagne toast followed.

"The meeting itself was an electrical experience," said Copan, the US representative. "The trip was long to get to this point".

The origins of the metric system date back to the French Revolution in the late 1700s. At the time, it was estimated that about 250,000 different units of measure were used in France, making trade difficult. The new system was designed to be rational and universal, with units based on natural properties rather than by royal decree or by the whims of dukes and local magistrates.

"The idea was that these measurements would be eternal and identical for everyone, everywhere," said Ken Alder, a science historian at Northwestern University in Evanston, Illinois.

The base unit of the system was the meter, supposed to represent a ten millionth of the distance separating the north pole from the equator along the meridian of Paris. (Scientists of the time made a slight mistake in their measurements and the meter is about 2 millimeters longer than it should be.)

At the same time, the kilogram was defined as the mass of 10 cubic centimeters of water at 4 degrees Celsius.

These units were adopted by the French Republic in 1795, although in practice people continued to use their own local measures for decades.

"It's not as if everyone had taken the bandwagon as soon as the metric system was formalized," said Barry Taylor, NIST scientist emeritus. "This was really not the case."

Countries in Europe and South America adopted the metric system throughout the 19th century. In 1875, delegates from the United States and 16 other countries signed the Treaty of the meter in Paris. He established a universal system of units based on meter, kilogram and second, which would rationalize trade between nations. (The second was defined as[6400foisletempsmoyennécessaireàlaTerrepoureffectueruneseulerotationsursonaxe)[6400oftheaveragetimeittakesforEarthtocompleteasinglerotationonitsaxis)[6400foisletempsmoyennécessaireàlaTerrepoureffectueruneseulerotationsursonaxe)[6400oftheaveragetimeittakesforEarthtocompleteasinglerotationonitsaxis)

Although the meter and kilogram are based on the size of the Earth, they were officially defined by metal artifacts, including The Great K, which were melted in London in 1889 and kept in a vault located in the sub -sol of the new International Bureau of Weights. and measures in Sèvres, France. Member countries received one of 40 precise replies.

The Meter Treaty also established the General Conference on Weights and Measures (GFCM), an international group charged with studying and voting on proposed changes to measurement units for all Member States.

"Metrology is a living science," said Schlamminger.

The GFCM approved three other basic units in 1954: the ampere for the electric current, the kelvin for the thermodynamic temperature and the candela for the luminous intensity.

In 1967, he redefined the second based on the oscillations of a cesium 133 atom, a much more accurate and reliable pendulum than the slightly flickering rotation of the Earth.

In 1983, the meter became the first metric unit linked to a fundamental property of the universe when it was redefined as the distance traveled by light in a vacuum at 99,792,458 seconds.

"Today, we can measure the distance between the Earth and a satellite located at 6 km with an exquisite accuracy of 6 millimeters," said Schlamminger. "Try that with a meter."

And yet, the kilogram remained attached to the mass of The Big K, an object so precious that it was removed from its triple-lock vault only every 40 years for cleaning and lightening. ;calibration.

Metrologists have long been keen to update the definition of the kilogram since the early 1900s, but the ability to measure the Planck constant with the necessary precision has only recently materialized.

The Planck constant is a number that relates the energy and frequency of light, much like how pi connects the circumference and the diameter of a circle. The technological advances that have fixed the value of the constant have come in all.

In the 1970s, scientists at the British National Physical Laboratory developed a new type of scale that links mass to electromagnetic force. It was named Kibble Balance in the honor of its inventor, Bryan Kibble, and although it has not yet been specific enough to redefine the kilogram, he suggested a way forward.

In 2005, measurements made with the Kibble scale were sufficiently improved for a group of researchers known as Gang of Five among metrologists to have written an article entitled "Redefining the kilogram: a decision whose time has come. "

"This newspaper really started this whole odyssey," said Schlamminger.

In 2013, the experts agreed that, to change the definition, national metrology institutes should measure the Planck constant to an accuracy of 20 parts per billion and show that two different measurement methods would provide the same answer. .

"An experiment might have a hidden flaw, but if you have two absolutely different approaches and they agree, the chances that you are completely wrong are very small," said National Researcher Ian Robinson Physics Laboratory.

Croquette sales provided value. The other measure involved a sphere of pure silicon enriched the size of a bullet-to-bullet. The structure of the 1 kilogram sphere and the atoms it contains allowed scientists to accurately measure the Avogadro constant, which links the number of atoms or molecules of a substance to its mass. This was used to determine the Planck constant with the help of well understood equations.

"The silicon sphere has helped control Kibble's approach to balance," Taylor said.

A similar philosophy of using fixed constants underlies the new definitions of mole, kelvin and ampere. After Monday, the mole will be defined by the value of the Avogadro constant, the kelvin by the value of the Boltzmann constant (which connects the temperature to the energy) and the ampere by the value elementary charge, the smallest observable charge in the universe. .

"Everyone has access to these fundamental constants," said Schlamminger. "They do not distinguish between rich and poor, all you need is a little physics."

Nor do they distinguish between earthlings and beings elsewhere in the universe. Just as the first iteration of the metric system has streamlined communication and trade among nations, newly-defined units may one day help humanity communicate with aliens, scientists said.

"If we come in contact with extraterrestrials, what are we going to talk to them about? Physics. There is nothing else," Schlamminger said. "But if you tell the aliens that our units of measurement are based on a piece of metal, you will be the laughingstock of the galaxy."

Scientists do not know how the new units will affect future discoveries, but it is certainly possible that they do it. For example, the second can now be measured with such precision that researchers can detect small changes in the gravitational field of the Earth, as time moves a little faster as it moves away. from a center of gravity.

Lord Kelvin, one of the leaders in the field of metrology, said: "Measuring, is knowing," said Copan. "As we can measure with increasing precision, we can learn more about the fundamentals of our universe and the fundamentals of life. "

Robinson said the new definitions will allow scientists to open their imagination about the possibilities of measurement.

"From now on, they will not have to think about this mass of platinum and iridium in Paris, they will just have to think about physics," he said.

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