Thursday , August 5 2021

Universal units reflect their earthly principles

For the past 2 centuries, scientists, surgeons, shipping companies and buyers have relied on a common measurement system: the metric system. The basic units of this system are their origins in the spatial dimensions of the earth, the timetables and the masses. As our ability to make measurements becomes more accurate, these units have been redefined, one by one, in terms of the fundamental physical constants and not the physical objects – apart from the kilogram.

At a time when units are increasingly defined by the properties of atomic physics, the properties of Earth remain important in metrology, the science of measurement.

This basic unit of mass remains attached to a metal cylinder of about 139 years old the size of a plum sitting in the air under three jars with a bell in France. But as of November 16th, this tie will be cut off when the new kilogram The definition based on physics is formally adopted.

Scientists around the world rely on the International System of Units as a common basis for recording and reporting their findings. The units used in this system are based on the meter, kilogram, second, Kelvin, white, mole, and candela. Although all science fields, including geosciences, use these units every day, many geoscientists may not realize how important Earth properties are in the development of these units and how the new definition maintains its links with land-based dimensions.

The change in the definition shows that at a time when the units are increasingly determined by the properties of atomic physics, the properties of Earth remain important in metrology, the science of measurement.

Measurement of the Earth

The Earth, which at that time was considered to represent innocence and stability, was the initial basis for the International System of Units (SI) of length, time and indirect mass. Geodesy, the science of Earth's form, its orientation in space, and its gravitational field were critical to defining the metric system during the French Revolution.

In 1791, the French Academy of Sciences set the meter as 1 / 10,000,000 the length of a quadrant of the Earth's meridian. However, since 1983, the meter has been defined as the length of the path traversed by light in a gap during a 1 / 299,792,458 interval of one second. Hence, do, the vacuum velocity was stabilized at a given value, and the meter definition now comes from the definition of the second.

The translation of this concept into practical measurements requires a method that is both accurate and repetitive. Several methods compete for the practical implementation of the meter definition (also called mise en pratique). Today, the SI meter is often derived from the wavelength of a iodine-fixed red sun-neon laser.

The SI unit of time, the second, was initially set as 1 / 86,400 of the average solar day. Thereafter, the watches reached a precision that allowed for the observation of irregularities in the rotation and revolution of the Earth. In 1967, the General Conference on Limits and Measures (CGPM) changed the definition of the second to "during the period of 9.192.631.770 radiation periods corresponding to the transition between the two overpopulations of the basal state the cesium-133 person."

By this definition, the above frequency (abbreviated to metric in ΔV (133Cs)hfs) is exactly equal to 9.192.631.770 transits per second, and the second is set accordingly.

The Kilogram Drifting

The kilo is unique since the standard is still based (currently) on a manufactured object rather than a physical constant.

In 1799, the the kilogram is defined as the mass of one cubic decade of water at a temperature of 4 ° C. This unit is unique since the model is still based (for now) on a manufactured object rather than on a physical constant. The original kilo from 1799 (the "kilogram of records") and the present artifact from 1875 (the international kilo or IPK) were constructed to be consistent with this definition. The IPK is a 39mm height and 39mm cylinder made of 90% platinum and 10% iridium. Currently, IPK is kept at Bureau International des Poids et Mesures in Sèvres, France.

Forty copies of IPK were manufactured in 1884, and 34 of them were distributed to signatories to the Meter Convention. In the United States, original numbers 4 and 20 were granted, Belgium received numbers 28 and 37 and Switzerland received 38 and, recently, 89. These copies have been used as national standards since then.

The kilogram is planned to fit into the other SI units and its prototypes are programmed to become museum objects when the kilogram is officially redefined to the Planck constant. This change in definitions is necessary because the use of IPK, a natural artifact, creates several problems. There is no way to ensure long-term stability, it can be destroyed or destroyed and creates logistical problems when compared to copies in other National Metrology Institutes (NMIs).

IPK's comparisons with official copies and national prototypes in 1889, 1948, 1989 and 2014 have shown that IPK appears to have lost about 50 micrograms in 100 years (five parts per 100 million). It is also possible that all prototypes show a common mass slip, which can not be detected by inter-assemblies. So we have a strange situation: By definition, the IPK's mass is unchanged, but there is no way to test its stability using an absolute reference!

IPK instability spreads to other base units associated with the kilogram, such as candela, mole (number of atoms in a mass of material) and ampere (electricity). It also affects the quantities produced such as strength, density and pressure. Consequently, over the past 25 years, many EMMs have worked to replace IPK with a definition based on a fundamentally stable nature[[[[Richard et al., 2016].

It is based on physics, measured in geodesy

Although Earth's properties are not stable enough to serve as a basis for SI, geodesy has not said its last word. The new definitions of the second and the meter, which originated from geodesy, are now based on laboratory physics experiments. The new definitions, however, must be consistent with the previous ones and thus continue to be related to the form and movement of the Earth.

The Smurfs test their hand to measure gravitational acceleration.
(left) The Smurfs test their hand in measuring gravitational acceleration, perhaps inspired by (absolute) FG5 # 202 absolute gravimeter at the Geophysical Observation Station of the Royal Observatory of Belgium near Membach. Credit: (left) © Peyo – 2018, Lic. Lafig Belgium –; (right) Kris Vanneste, Royal Observatory of Belgium

On November 16, 2018, the 26th CGPM will ratify the revised SI based on the seven constants: the frequency of the superabrasive split of 133 Cesium-133 ΔV (133Cs)hfs, the speed of light in the vacuum do, Planck's constant h, the elementary load mi, The Boltzmann constant K, The Avogadro constant NONE, and light efficacy KCD. These constants exist regardless of our ability to measure them and thus the definition and practical implementation of the units will be disconnected.

In short, this means that the practical variations of the mass can be determined and reproduced with different experiments with increasing accuracy, while the definitions remain unchanged.

From now on, the size of the kilogram (kg), the mass unit, will come from the value of the Planck constant (h = 6.62607015 × 10-34 joule-seconds. I see Fischer and Ullrich [2016]), used in Einstein's energy formula mi = mc2 = hv. Because a joule is a square kilogram squared per second square (kg m2/small2), the kilogram standard will now be based on SI units of length and time, which were already standard.

A new balance

Just how do we export the kilogram of these known quantities?

After redefinition, a first path to standardize the kilo will consist of calculating the number of atoms in a silicon-28 (28Si) monocrystalline sphere using X-ray crystal X-ray approach. This is also known as the "Avogadro experiment" because it was initially used to give an accurate value for the Avogadro constant, the number of carbon atoms-12 gathered to just 12 grams. (The number of hydrogen atoms of Avogadro, the lightest element, will weigh 1 gram, but carbon is much easier to weigh than hydrogen).

Another path to the kilogram is based on the Kibble balance[[[[Robinson and Schlamminger, 2016]. In this figure, the mechanical strength of a mass in a gravitational field is balanced by the electrical power of the balance. The kilogram is dependent on the Planck constant, which occurs in the quantum phenomena used to determine the current and the trend of the balance.

Knowing the current, voltage, length and time to measure the velocity of the coil moving within a magnetic field and the local acceleration of gravity, we define the mass as the amount of matter needed to balance a given amount of electrical power. To allow this kilogram to be produced, acceleration of gravity must be determined at 10-8 level with absolute gravity, monitoring the free fall of an object or cold people who repeatedly fall into a vacuum chamber[[[[Van Camp et al., 2017].

The new kilogram definition is offered for measurements using a Kibble yoke and an absolute gravimeter.
The new kilogram definition is offered for measurements using a Kibble yoke (on the left) and an absolute gravimeter (in the background, with a black tripod and a transparent, cylindrical dropping chamber), as these instruments at the Metas Federal Institute of Metrology, Switzerland. Credit: André Roth, Roth und Schmid Photo for Mettler Toledo

From Conceptual to Practice

Since 1967, geodetic metrology is no longer required to define the meter, and the second. However, gravity will continue to be the key to the new implementation of the kilogram. A measure of Avogadro's constant could be transformed into a measure of Planck's constant h (and vice versa) through the Rydberg constant, which links the atomic and macroscopic properties of matter.

The new constants will not completely dissolve from their historical ties. The number selected for its numeric value h will be such that upon adoption of the definition, the redefined kilogram will be equal to the IPK mass within the uncertainty of the combined best estimates of the Planck constant value. The same applies to do and ΔV (133Cs)hfs, which, like h, remain historically linked to the dimension and rotation of the Earth.

The exact gravity measurements required to determine the kilogram using the rest of Kibble would not be possible without research to measure and monitor gravitational acceleration and to understand variations in time and space[[[[Van Camp et al., 2017]. Geoscientists have worked on this problem since Galileo Galilei explored the mass movement in the 16th century and showed that acceleration due to the gravitational field of the Earth is the same for all masses (and therefore can not be used to determine a specific mass,.

However, the use of the Kibble Balance method requires the objects to fall, not their mass measurement, but to determine an exact value for gravitational acceleration. Therefore, monitoring the free fall of an object or cold atoms, as achieved in absolute gravity, is still a fundamental tool in geosciences and metrology.

After November, our measures, pounds and seconds will be determined by the movements and energy of electrons, atoms and photons. However, the benchmarks by which we introduce these definitions into daily use remain rooted in the measurements coming from our planet.


This article is dedicated to the memory of Jean O. Dickey, a geostationary at NASA's Jet Propulsion Laboratory, who died in May 2018. He measures gravity using satellite data and was a big fan of Smurfs. The authors thank Miguel Diaz Vizoso and IMPS for the beautiful animation.

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