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Researchers are about redefining the kilogram of Science

Locked in a vault that requires three keys to open, in the city of Sèvres, just southwest of Paris, there…

Locked in a vault that requires three keys to open, in the city of Sèvres, just southwest of Paris, there is a kilo. In fact, it is the kilogram, the international prototype of the kilogram (IPK), the kilogram against which every other kilo must take their measure, Le Grand K . This platinum iridium alloy cylinder is housed under three protective glass blocks in a temperature and humidity controlled environment in a safe box together with six official copies in the Sèvres underwater rim.

“If you were to release it, it would still be a kilo, but the whole world’s mass would change,” said Stephan Schlamminger, a physicist with the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. just out of its vault every 40th or so, when golf ball size, just one kilogram by definition since 1

889, is used to calibrate copies shared with countries around the world. But that’s a problem. The IPK vault is six témoins or “witnesses” – the official copies. Over the years, as shown by the rare occasions when Le Grand K and its witnesses were measured, the mass of IPK ” driven. “ IPK Kilot’s International Prototype (IPK).

(Photo courtesy of the BIPM)

Most witnesses now weigh something more – a matter of micrograms or millions of grams – than IPK (although many of the copies were more massive to begin with). You can say that IPK loses mass, only you can not say that, because IPK is unchangeable and unintentionally one kilo . In addition, physicists do not even know if IPK loses mass or gets mass in the long run, only slow drives due to insignificant amounts of material aggregated from the air, or rubbed under a weighing or dirt on the silver-black surface of the IPK under one of its precise Bath.

As you can imagine, this minute drive causes the scientists a lot of headache, not to mention industries based on small and accurate mass metrics, such as pharmaceutical companies. [19659003] “At present, the kilometric is defined in terms of the mass of a particular thing,” said Ian Robinson of the National Physical Laboratory (NPL) in southern London. “And if that thing is destroyed or changed or anything, it’s awkward.”

 Kilogram masses NIST’s platinum iridium copy of IPK, K92, with stainless steel kilns in the background.

(J. Lee / NIST)

Fortunately, the world’s metrologists have a solution: redefine the kilogram in the form of a natural, universal constant. Most devices in the International System of Devices (SI) are already defined according to universal constants, such as the meter, which is the official length that traveled at the speed of light in vacuum for 1/299 792 458 seconds. Of course, this definition is dependent on the other, which is defined as the duration of 9 192 631 770 periods of a specific frequency of electromagnetic radiation (microwaves in this case) causing the external electron of a cesium-133 atom to transition ]] (Switch from a quantum of “spin up” to “spin down” or vice versa).

But kilo, the last remaining device defined by an artifact, has resisted opposition’s redefinition-so far. On November 16, at the 26th meeting of the General Conference on Weights and Measures, delegates from 60 member states will gather in Sèvres to vote for redefining the kilogram according to Planck’s constant-a number that relates the frequency of a light wave to the energy of a photos in that wave. And according to Richard Davis, a physicist with the International Bureau of Weights and Measures (BIPM), “they expect a significant majority.”

Max Planck and Albert Einstein

In 1879, IPK was made by Johnson Matthey in London, 20-year-old Max Planck defended his dissertation On the second law of thermodynamics, and Albert Einstein was born. Even though the two researchers did not know it during their lifecourses, their collective work on the basic physics of gravity and quantum mechanics would form the basis of a 21st definition of the kilogram.

So what is Planck’s constant? “At a basic level, it’s hard to say,” says Davis.

Planck’s constant is a very small number: 6.62607015 x 10 -34 to be exact, which will be officially defined at November 16 meeting. In 1900, Max Planck calculated the number to match models of light coming from stars, matching the energies and temperatures of the stars to their spectrum of electromagnetic radiation (collectively called blackbody radiation). At that time experimental data suggested that energy does not flow to any value but rather exists in bundles or quanta from which quantum mechanics take its name and Planck needs to calculate a value for these bundles to

 Einstein and Planck Five Nobel Prize Winners, from left to right: Walther Nerst, Albert Einstein, Max Planck, Robert Millikan and Max von Laue gathered for a dinner host of Laue 1931.

[Public Domain]

Five years later, Albert Einstein published his theory of special relativity, which would be expressed as the infamous equation E = mc 2mass times the speed of light squared, an epiphany like Energy is fundamentally bound in all matter of the universe). He also calculated the theoretical value of a single basic amount of electromagnetic energy, now known as a photon – which resulted in the Planck-Einstein relationship E = h v . In the equation it is stated that the energy of a photon (E) corresponds to Planck’s constant (h) times the frequency of electromagnetic radiation ( v, which is the Greek symbol now rather than a “v” 19659027] “You know you have the energy of a photon, which is h v but you also know that you have an energy of a mass, which is mc 2 . [So] E = h v = mc 2 . Here you can see how to get a lot from h [Planck’s constant] v [the wave frequency] and c [the speed of light] “, says David Newell, a physicist at NIST.

But this is not the only place The Planck’s constant manifests itself. The number is necessary to describe the photoelectric effect on which solar cells are based. It is also used in Niels Bohr’s model of the atom, and it also exists in Heisenberg’s uncertainty principle.

“It’s like saying, how’s Pi?” Davis says. “What’s Pi? Well, that’s the circle’s circumference s divided by the circle diameter. But then Pi shows up everywhere in mathematics. It’s everywhere. “

The key that connects Planck’s constant to kilos is its unit, Joule-second or J · s. The constant is given this unique device because energy is measured in joules and frequencies are measured in Hertz (Hz) or cycles per second. with a kilo multiplied by meters squared divided by seconds squared (kg · m 2 / s 2 ), so with some clear measurements and calculations one can reach kilos.

But before you can convince the world to change the definition of the standard mass unit, your measurements are better the best ever taken in the history of science. And as Newell states, it’s “measuring something absolutely badly difficult.”

Measures

] We often take for granted that one second is one second or one meter one meter. But for the majority of human history, such measures of time, length and mass were quite arbitrary, defined according to the local customs axis or rulers. One of the first decrees that national measurements have to be standardized came from Magna Carta in 1215, which states:

“Let there be a measure of wine throughout our kingdom, and a measure for beer and a measure for maize, namely the “London quarter”; and a width for fabrics, dyed, russet or halberget, namely two ells within selvedges. Let it be the same with weights as with actions. “

However, after enlightenment, as researchers began to eliminate the physical limitations of the universe, it became apparent that different units of measurement gave a serious obstacle to the emergence of the species. Researchers spread throughout the world during the 18th and 20th centuries, measuring everything from the precise form of the earth to the distance to the sun – and every time a German laughs (about two meters, depending on the region) had to be compared to an English farm (which also varied for most of its existence), uncertainties and misconceptions abounded.

 A copy of the first meter standard sealed due to a building in 36 rue de Vaugirard, Paris.

A copy of the first meter standard, sealed by a building in 36 rue de Vaugirard, Paris.

(Ken Eckert / Wikimedia Commons CC 4.0)

The French finally had a revolution, not just politics but also action. When the 18th century was finally drawn, the Kingdom of France has been expected to have quarterly different units, making it impossible to keep track of all of them. Called by the National Constituent Assembly, formed at the beginning of the French Revolution, the French Academy of Sciences was to create a new unit of length that would become the official measure for the country: the meter, defined as a ten million of the distance from the North Pole to equator.

A landing expedition led by French mathematician and astronomer Jean Baptiste Joseph Delambre and Pierre Méchain triangulated the distance of a part of that length, extending from Dunkerque to Barcelona to calculate the new meter. The survey was measured in 1798, and the new standard was soon adopted in France.

The meter came to represent a basic unit of measure, which defines liters (1000 cubic centimeters) and even kilograms (the mass of a liter of water). In 1875, the world was ready to assume the metric system and that year’s Convention of the Year saw representatives of 17 nations sign the Meter Treaty, create the International Weighing and Measuring Agency and provide new mass and longitude standards to be found in platinum iridium alloy, define the gauge and kilot for the world.

However, as a wave of 20’s researchers such as Planck and Einstein began to boil and produce on the Newtonian structure of physics, discovering new laws among the greatness of the cosmos and the foundation of the atom, the system of action must be updated accordingly. In 1960, the International System of Units (SI) was published, and countries around the world set up metrological institutions to continually refine the official definitions of our seven units of measurement: the meter (length), kilogram (mass), others), amps , kelvin (temperature), mole (amount of matter) and candela (brightness).

 An Avogado Sphere with Clean Silicon-28 atoms. By measuring the volume and volume of a single silicon-28 atom, the meteorologists can measure the mass of an individual atom in the sphere and provide a method of calculating the number of atoms in a mole called Avogado's number, which can be used to calculate Planck's constant.

An Avogado Sphere with Clean Silicon 28 atoms. By measuring the volume and volume of a single silicon-28 atom, the meteorologists can measure the mass of an individual atom in the sphere and provide a method of calculating the number of atoms in a mole called Avogado’s number, which can be used to calculate Planck’s constant.

(Photo licensed by BIPM)

From these base units, all other devices can be calculated. Speed ​​is measured in meters per second that can be converted to mph and other speeds; The volume is measured in the form of current amplifiers and ohm resistance; and the definition of the farm is now proportional to 0.9144 of a meter.

Today, as in the 18th century, it is the question of refining such measurements in advance of scientific capacity. Although the redefinition of the kilogram is unlikely to change your daily life, the ultimate effects of defining a more precise target system are often widespread and deep.

Take for example the other one. Since 1967, the definition of one second has been based on the frequency of a microwave laser, and without this precision, GPS technology would be impossible. Each GPS satellite carries an atomic clock that is critical to correct for time endless but measurable slower on our satellites as they surround the high speed earth, an effect predicted by Einstein’s relativity theory. Without the new definition, we could not correct for these small fractions in a second, and when they grew, GPS measurements would run longer and longer without doing anything from Google Maps to GPS-controlled ammunition, except for science fiction.

The relationship between the other and the GPS reveals the fundamental entwining of metrology and science: Research research requires and enables new measurement standards, and the new norms for action, in turn, enable more advanced research. When this bike will ultimately take our kind is unknown, but after the bar of the meter and the death of the other one defined by a fraction of a day, one thing is clear: IPK is next to the guillotine. [19659013] Kibble Balance The NIST-4 Kibble balance, run by the National Institute of Standards and Technology. Unlike previous Kibble batches, the NIST-4 uses a balancing wheel that acts as a pulley rather than a beam. Balance measured Plancks constantly to an uncertainty of 13 parts per billion.

Physicists have known for decades that the kilogram can be defined in the form of Planck’s constant but it was not until the metrology was sufficiently advanced to measure the number with such precision that the world would accept a new definition. In 2005, a group of researchers from NIST, NPL and BIPM, as Newell calls the “gang of five”, began to drive the issue. Their paper on the issue is the title Refinement of Kilot: a decision whose time has come .

“I think it’s a milestone paper,” says Newell. “It was very provocative – it annoys people.”

One of the most important techniques for measuring the Planck Constant identified in the paper is a wattal balance, first conceptualized by Bryan Kibble at NPL 1975. (After his death in 2016, the Wattbalance was renamed Kibble’s balance in Bryan Kibble’s honor.)

The Kibble balance is on a basic level the development of a technology that goes back over 4,000 years: balance waves. But instead of weighing one object against another to compare the two, a Kibble balance allows physicists to weigh a lot against the amount of electromagnetic force required to hold it up.

“The balance works by providing a current through a coil in a strong magnetic field, and it provides power and you can use that force to balance the weight of a mass,” said Ian Robinson, NPL, who worked with Bryan Kibble on the first watt balances from 1976 onwards.

The balance works in two modes. The first weighing or power position balances a mass against an equal electromagnetic force. The second mode, the speed or calibration mode, uses a motor to move the coil between the magnets while the mass is not in balance, giving an electrical voltage that gives you the strength of the magnetic field expressed as a measure of electric power. As a result, the force of the mass in the weighing position is equal to the electrical power generated in speed mode.

The electric power can then be calculated as a function of Planck’s constant thanks to the work of two nobel-winning physicists, Brian Josephson and Klaus von Klitzing. In 1962 Josephson described a quantum electrically related effect related to voltage, and von Klitzing revealed a quantum effect of resistance 1980. The two discoveries make it possible to calculate Kibble balance electrical power in terms of quantum measurements (using Planck’s constant), which in turn corresponds to the mass of one kilo.

In addition to the Kibble balance, the five-paper group addresses another way of calculating Planck’s continuous processing spheres of virtually pure silicon 28 atoms, the most perfect round objects ever created by humanity. The volume and mass of a single atom in the sphere can be measured, allowing metrologists and chemists to refine the Avogadro constant (the number of units is one mole), and from Avogado’s number, Planck can be calculated by means of already known equations.

“You need two ways to do this so that you get the confidence that there is no hidden problem in a single method,” says Robinson.

A white letter on NIST explains how a Kibble balance can compare a mechanical measurement (the weight of one kilogram mass) to an electrical action (the power of electrical power required to hold the kilogram, expressed as Planck’s constant).

(Jay Bennett)

In order to redefine the kilogram, a change that will take effect on May 20, 2019, the General Conference on Weights and Measures required at least three experiments to calculate the Planck constant to an uncertainty of up to 50 parts per billion, of which One has to calculate the value within an uncertainty of 20 parts per billion. The international effort for silicon specimens has been meticulous to achieve an uncertainty of only 10 parts per billion, and four Kibble balance measurements also provided the values ​​within the necessary uncertainty.

And as a result of all these actions, much more than the kilogram

The New International Device System

More than redefining the kilot, 26th meeting of the General Conference on Weights and Measures (CGPM), establishes one fixed value for the Planck Constant, and as a result, the largest transformation of the international system of units since its inception in 1960. In the past, Planck’s constant continuous measurement, on average, with other measurements worldwide, and a list of new values ​​were delivered to research institutions in a few years. [19659003] “Nobody will measure the Planck Constant when this [vote] has passed, because that’s the value will be defined,” says Davis.

In addition, two Planck constants, the avogadron constant will be set to a fixed value, like the elemental charge ( e the charge of a proton) and the triple water temperature (the temperature at which water may exist as a solid, liquid or gas defined as 273.16 degrees Kelvin or 0.01 degrees C).

By setting the Planck constant as an absolute value, scientists wave from conventional mechanical measurements and assume a series of quantum electrical measurements to define our basic units. When the constant is defined, it can be used to calculate a number of masses from the atomic level to cosmic, which means that you have to scale the IPK into less measurable parts or up to huge masses.

“If you have an artifact, you only apply your scale at one point,” said Schlamminger. “And a basic constant does not care about the scale.”

Ian Robinson with Mark II The Kibble balance. Built by NPL in the UK, Mark II was later acquired by the Canadian National Research Council, where it was used to measure Planck’s constant value within a 9-billion-per-billion uncertainty.

(Image courtesy of NPL)

The new value of Planck’s constant also changes the definitions of our power units, such as the 1948 definition of the amps. Physics have long used the Josephson and von Klitzing effects to calculate electrical values ​​with precision, but these measurements can not be included in SI to one of their variables – Planck Constant – is a fixed value.

“It’s always crazy for me if I wanted to get my SI volume or my SI ohm, I had to go through the kilogram. I had to go through a mechanical drive to get my electrical devices Newell. “It seemed very 19th century, and it was.”

Now, the electrical devices will be used to get kilos.

“People are talking about oh that’s the redefinition of the kilogram, but I think that actually missing an important point, says Schlamminger. “We will get these electrical devices back to SI.”

For all people, forever

There are more than half a dozen Kibble balances around the world, and many countries from South America to Asia are building their own – because once researchers have one, they have the tool to gain access to the kilogram and many other basic units and actions defined by nature. No longer will the mileage be limited to a vault, where few have the privilege of ever accessing it, and everyone is so scared to touch it not used but once every half of the century.

“It now means what we can do is spread the way to determine mass around the world,” says Robinson.

For the researchers working on this change, the new international system of devices is nothing but a historical opportunity.

“I’m still worried that this is a dream, and tomorrow I wake up, and that’s not true,” said Schlamminger. “I think this is the end of the arc that people began to think of before the French Revolution, and The thought was to have measurements for all times for all people. “

Stephan Schlamminger explains the Kibble balance with a working Lego model at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

(Jay Bennett)

“This has been one of the highlights of my life,” says Klaus von Klitzing of Max Planck Institute, whose own will constantly be cemented as a fixed value as a result of the new SI. “This is wonderful. We have the unification of these quantum units … with the new SIs, and this is why this is a wonderful situation.”

Such changes in our basic values ​​to describe the universe do not often come with, and it’s difficult to imagine when to happen again. The meter was redefined in 1960 and then again in 1984.

The other was redefined in 1967. “Now it was a rather revolutionary change,” said Davis. “People for eternity had spoken time through the rotation of the earth, and suddenly we changed into a vibration in a cesium atom.”

If the redefinition of the other was a more fundamental change of human understanding than the redefinition of the kilometer is not to say, but as the other is the redefined mileage undoubtedly a remarkable moment in our progress.

“Getting rid of the last artifact … that’s historic,” says Davis. “Measurement standards have been based on these artifacts, really, because someone knows. Neolithic times excavations show standard standard lengths, standard masses-it’s small pieces of chert or rock or something. And that’s how people have been doing it for millennia, and this is the last one. “

SI will change again, but primarily as a matter of reducing endless uncertainties or switching to another wavelength of light or chemicals measuring what’s ever so little more accurate. In the future we can even add units to SI for values ​​that we have yet to define. But we can never do what we do now, to leave our ancestor’s understanding and embrace a new measure.

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