Wednesday , January 27 2021

Universal Units Reflect Worldly Origins

Scientists, surgeons, transport companies and shoppers have been relying on a common measurement system over the past 2 centuries: the metric system. The basic units of this system have their origins in spatial dimensions, time scales and material masses of the Earth. As our ability to measure becomes more sensitive than ever, these units are redefined one by one in terms of basic physical constants, rather than material objects, except for kilograms.

In an age where the units are increasingly defined by the properties of atomic physics, the properties of the Earth remain important in terms of metrology and measurement science.

This basic mass unit remains attached to a 139-year-old metal cylinder about the size of a plum sitting in the air under three pottery in France. However, starting from November 16, this link will be cut when the kilogram is new Physics-based description has been formally recognized.

Scientists in the world rely on the International System of Units as a common basis for recording and reporting their findings. Units used in this system; It is based on meters, kilograms, seconds, kelvin, amps, moles and candelas. Although all scientific fields, including geology, use these units every day, they may not realize how important the features of many geostivist worlds are in developing these units, and how the new definition maintains links to terrestrial dimensions.

The change in definition shows that the properties of the Earth are important in terms of metrology and measurement science in an age where units are increasingly defined with the properties of atomic physics.

Measuring the World

Over time, the world, which was thought to represent life and stability, was the starting point of the International System of Units (Système International) as mass, time, and indirect mass. Geodesy, the science of the Earth's shape, its orientation in space, and the field of gravity played a key role in the definition of the metric system during the French Revolution.

In 1791, the French Academy of Sciences, the counter, defined the length of a quarter of the Earth's meridian as 1 / 10,000,000. However, since 1983, the meter is defined as the length of the road passing by light in a vacuum within a range of 1 / 299,792,458 & lt; RTI ID = 0.0 & gt; Therefore, cThe speed of the light in a vacuum was fixed to a certain value, and the counter definition now comes from the second one.

The transformation of this concept into practical measures requires a precise and reproducible method. Several methods compete for the practical realization of the counter (also called mise en pratique). Today, the SI meter is usually derived from the wavelength of the red helium-neon laser stabilized with iodine.

The SI unit of time, the second, was initially defined as 1 / 86,400 of the average sun day. Then, the clocks showed a sensitivity that allowed the monitoring of irregularities in the rotation and revolution of the Earth. In 1967, the Conference on the General Discussion and Measures (CGPM) changed the definition of the latter to ler the time of the radiation corresponding to the transition between the levels of the two hyperfines of the earth's condition ler to the time of 9.192.631.770;. Cesium-133 atom. "

According to this definition, the above frequency (abbreviated as rν in metrology)133cs)-hfsis equal to 9,192,631,770 passes per second and the latter is defined accordingly.

Drifting Kilogram

The kilogram is unique because the standard is still based on a manufactured object (for now) rather than a physical constant.

in 1799 kilograms were defined as 1 cubic decimeter water body at a temperature of 4 ° C. This unit is unique in that the standard is still based on an object (for now) manufactured instead of a physical constant. From 1799 (IP kilogram of archives prot), the prototype and the work available since 1875 (international prototype kilogram or IPK) were produced to be compatible with this definition. IPK is a cylinder with a diameter of 39 millimeters and diameter consisting of 90% platinum and 10% iridium. Currently IPK is being held in Bureau International des Poids et Mesures in Sèvres, France.

Forty replicates of IPK were produced in 1884 and distributed to the signatories of the Meter Convention. Prototypes 4 and 20 were allocated to the United States; Belgium received 28 and 37 numbers, and Switzerland was 38 and finally 89. These copies have been used as national standards ever since.

The kilogram is planned to be added to other SI units and the prototypes are planned to become museum items when the kilogram is redefined officially in terms of the planck constant. This change in definitions is necessary because the use of IPK, which is a physical work, poses various problems. There is no way to ensure long-term stability, it can be destroyed or damaged, and there are logistical problems when it is to be compared with copies in other national metrology institutes (NMIs).

The comparisons of the IPK mass to official copies and national prototypes in 1889, 1948, 1989, and 2014 showed that the IPK lost 50 micrograms in 100 years (five parts per 100 million). It is also possible for all prototypes to show combined mass deviation; Therefore, we are facing a strange situation: By definition, the mass of IPK is invariant, but there is no way to control its stability by using an absolute reference!

The imbalance of IPK is spread to other kilograms connected to kilograms, such as candela (light intensity), moles (number of atoms in a mass of material) and amperes (electric current). It also affects derived quantities such as force, density and pressure. As a result, over the past 25 years, many NMIs have been working to change the IPK to a definition based on a fundamental nature constant.[[[[Richard et al. 2016].

In Physics, Measured in Geodesy

Although the features of the world were not stable enough to form a basis for the SI, the geodesy did not say the last word. New definitions of the second and meter, which were previously derived from geodesy, are now based on laboratory physics experiments. However, the new definitions should be consistent with their predecessors and therefore still be relevant to the shape and movement of the Earth.

The Smurfs try their hands to measure the acceleration of gravity.
(left) The Smurfs try to gauge the gravitational acceleration, perhaps (right), inspired by the absolute gravimeter of FG5 # 202 near Membach in Belgium's underground geophysical observation station. Credit: (left) © Peyo – 2018, Lic. Lafig Belgium –; (right) Kris Vanneste, Royal Observatory in Belgium

On November 16, 2018, the 26 CGPM will approve the revised SI based on seven constants: the frequency of hyperfine cleavage frequency of the ground state of the cesium-133 atom 16ν (133cs)-hfs, the speed of light in vacuum cPlanck sabs constant hbasic charge toBoltzmann's constant k, Avogadro’s constant N-oneand bright activity KCD. These constants exist independently of our ability to measure them, and therefore, the definition and practical implementation of the units are parsed.

In short, this means that the practical derivatives of the mass can be generated and reproduced with continuous increasing accuracy through different experiments;

Thereafter, the weight (kg) will be obtained from the value of the mass unit, Planck constant (h = 6.62607015 × 10-34 Joule-seconds; see Fischer and Ullrich [2016]Used in Einstein's energy formula TO = mc2nd = HV. Because a joule is a kilogram square meter per second (kg m2nd/ s2nd), the kilogram standard will already rely on the SI units of the pre-standardized length and time.

A New Balance

How do we obtain only kilograms from known quantities?

After the redefinition, a first way to standardize the kilogram will consist of counting the number of atoms in a silicon 28.28Si) X-ray crystal density approach using single crystal sphere. This is also known as the un Avogadro experiment olarak because it is used to give an accurate value for the number of carbon-12 atoms which is exactly 12 grams for the constant of Avogadro. (The lightest element, the lightest element in Avogadro, weighs 1 gram, but carbon is easier to handle than hydrogen).

Another way of kilogram is based on Kibble equilibrium[[[[Robinson and Schlamminger, 2016]. In this diagram, the mechanical power in the mass of gravity is balanced by the electrical power of the balance. The kilogram depends on the constant of Planck, which appears in quantum cases used to determine the current and voltage of the balance.

Knowing the current, voltage, length and time to measure the speed of the bobbin moving within a magnetic field and the local acceleration of gravity define one as the amount of substance required to balance a certain amount of electrical power. In order to allow this kind of kilogram to be obtained, the gravitational acceleration should be set to 10.-8 Level with absolute gravimetry, free falling of an object or cold atoms continuously into a vacuum chamber[[[[Van Camp et al. 2017].

The new definition of kilograms gives the measurements using a Kibble balance and absolute gravimeter.
The new definition of kilogram, Kibble balance (left) and absolute gravimeter (in the background with a black tripod and transparent, cylindrical drip chamber) measurements made using the Federal Metrology Institute's measures such as these instruments. METAS), Switzerland. Credit: André Roth, Roth und Schmid Fotografie for Mettler Toledo

Practical from Conceptual

Since 1967, geodetic metrology is no longer necessary for the definition of the meter and the latter. However, in the new realization of the kilogram, gravity will still be the key. A measure of the constant of Avogadro can be converted to a measure of Planck's constant. h (and vice versa) with the constant of Rydberg, which links the atomic and macroscopic properties of matter.

The new constants will not be broken away from their historical ties. Number selected for numerical value h At the time the definition is accepted, the re-defined kilogram will be such that the value of the Planck constant is equal to the mass of the IPK in the uncertainty of the combined best estimates. Same as current c and Δν (133cs)-hfs, which one you like hIt is historically relevant to the size and rotation of the Earth.

Accurate gravity measurements required to determine the kilogram by using the kibble balance would not be possible without measuring the acceleration of gravity and monitoring and understanding the variations in time and space.[[[[Van Camp et al. 2017]. Since Galileo Galilei explored the movements of the falling masses in the 16th century and showed that the acceleration from the gravitational field of the Earth was the same for all masses (and therefore could not be used to describe any specific masses, including kilograms), the geospheres worked on this problem. .

However, using the Kibble equilibrium method requires not to reduce objects, to measure their mass, but to determine an accurate value for gravitational acceleration. Thus, monitoring of the free fall of an object or cold atoms, as obtained in absolute gravimeters, is still a fundamental tool in geology and metrology.

After November, our counters, kilograms and seconds will be defined by the motions and energies of electrons, atoms and photons. However, these criteria are based on daily measurements, based on measurements from our home planet.


This article died in May 2018 in memory of Jean O. Dickey, a geodeticist in the Jet Propulsion Laboratory of NASA. Using satellite data, gravity was measured and became a fan of the Smurfs. The authors thank Miguel Diaz Vizoso and IMPS for the beautiful cartoon.

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Citation: () Eos, ,
doi: 10,1029 /.
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