2019 redefinition of SI base units

The SI system after the 2019 redefinition: Dependence of base unit definitions on physical constants with fixed numerical values and on other base units.

The SI system after 1983, but before the 2019 redefinition: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants of nature and artefacts used to define them (such as the mass of the IPK for the kilogram).

The 2019 redefinition of the SI base units came into force on 20 May 2019, ^{[1] [2]} the 144th anniversary of the Metre Convention. After the redefinition, the kilogram, ampere, kelvin, and mole were defined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (N_A), respectively. The second, metre, and candela are already defined by physical constants and are subject to correction to their present definitions. The new definitions aim to improve the SI without changing the value of any units, ensuring continuity with existing measurements. ^{[3] [4]} In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, ^{[5] [6]} which the International Committee for Weights and Measures (CIPM) had proposed earlier that year. ^{[7] :23}

The previous major change of the metric system occurred in 1960 when the International System of Units (SI) was formally published. At this time the metre was redefined; the definition was changed from the prototype metre to a certain number of wavelengths of a spectral line of a krypton-86 radiation, ^{[Note 1]} making it derivable from universal natural phenomena. The kilogram remained defined by a physical prototype, leaving it the only artefact upon which the SI unit definitions depend. At this time the SI, as a coherent system, was constructed around seven base units, powers of which were used to construct all other units. With the 2019 redefinition, the SI is constructed around seven defining constants, allowing all units to be constructed directly from these constants. The designation of base units are retained but are no longer essential to define SI measures.

The metric system was originally conceived as a system of measurement that was derivable from unchanging phenomena, ^[8] but practical limitations necessitated the use of artefacts—the prototype metre and prototype kilogram—when the metric system was introduced in France in 1799. Although it was designed for long-term stability, the masses of the prototype kilogram and its secondary copies have shown small variations relative to each other over time; they are not thought to be adequate for the increasing accuracy demanded by science, prompting a search for a suitable replacement. The definitions of some units were defined by measurements that are difficult to precisely realise in a laboratory, such the kelvin, which was defined in terms of the triple point of water. With the 2019 redefinition, the SI became wholly derivable from natural phenomena with most units being based on fundamental physical constants.

A number of authors have published criticisms of the revised definitions; their criticisms include the premise that the proposal failed to address the impact of breaking the link between the definition of the dalton ^{[Note 2]} and the definitions of the kilogram, the mole, and the Avogadro constant.

Background

Main article: History of the metric system

The basic structure of SI was developed over about 170 years between 1791 and 1960. Since 1960, technological advances have made it possible to address weaknesses in SI such as the dependence on a physical artefact to define the kilogram.

Development of SI

During the early years of the French Revolution, the leaders of the French National Constituent Assembly decided to introduce a new system of measurement that was based on the principles of logic and natural phenomena. The metre was defined as one ten-millionth of the distance from the north pole to the equator and the kilogram as the mass of one thousandth of a cubic metre of pure water. Although these definitions were chosen to avoid ownership of the units, they could not be measured with sufficient convenience or precision to be of practical use. Instead, realisations were created in the form of the mètre des Archives and kilogramme des Archives which were a "best attempt" at fulfilling these principles. ^[9]

By 1875, use of the metric system had become widespread in Europe and in Latin America; that year, twenty industrially developed nations met for the Convention of the Metre, which led to the signing of the Treaty of the Metre, under which three bodies were set up to take custody of the international prototype kilogram and metre, and to regulate comparisons with national prototypes. ^{[10] [11]} They were:

• CGPM (General Conference on Weights and Measures, Conférence générale des poids et mesures) – The Conference meets every four to six years and consists of delegates of the nations that had signed the convention. It discusses and examines the arrangements required to ensure the propagation and improvement of the International System of Units and it endorses the results of new fundamental metrological determinations.
• CIPM (International Committee for Weights and Measures, Comité international des poids et mesures) – The Committee consists of eighteen eminent scientists, each from a different country, nominated by the CGPM. The CIPM meets annually and is tasked with advising the CGPM. The CIPM has set up a number of sub-committees, each charged with a particular area of interest. One of these, the Consultative Committee for Units (CCU), advises the CIPM on matters concerning units of measurement. ^[12]
• BIPM (International Bureau for Weights and Measures, Bureau international des poids et mesures) – The Bureau provides safe keeping of the international prototype kilogram and metre, provides laboratory facilities for regular comparisons of the national prototypes with the international prototype, and is the secretariat for the CIPM and the CGPM.

The first CGPM (1889) formally approved the use of 40 prototype metres and 40 prototype kilograms made by the British firm Johnson Matthey as the standards mandated by the Convention of the Metre. ^[13] One of each of these was nominated by lot as the international prototypes, the CGMP retained other copies as working copies, and the rest were distributed to member nations for use as their national prototypes. At regular intervals the national prototypes were compared with and recalibrated against the international prototype. ^[14]

In 1921 the Convention of the Metre was revised and the mandate of the CGPM was extended to provide standards for all units of measure, not just mass and length. In the ensuing years, the CGPM took on responsibility for providing standards of electrical current (1946), luminosity (1946), temperature (1948), time (1956), and molar mass (1971). ^[15] The 9th CGPM in 1948 instructed the CIPM "to make recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention". ^[16] The recommendations based on this mandate were presented to the 11th CGPM (1960), where they were formally accepted and given the name "Système International d'Unités" and its abbreviation "SI". ^[17]

Impetus for change

There is a precedent for changing the underlying principles behind the definition of the SI base units; the 11th CGPM (1960) defined the SI metre in terms of the wavelength of krypton-86 radiation, replacing the pre-SI metre bar and the 13th CGPM (1967) replaced the original definition of the second, which was based on Earth's average rotation from 1750 to 1892, ^[18] with a definition based on the frequency of the radiation emitted between two hyperfine levels of the ground state of the caesium-133 atom. The 17th CGPM (1983) replaced the 1960 definition of the metre with one based on the second by giving an exact definition of the speed of light in units of metres per second. ^[19]

Mass drift over time of national prototypes K21–K40, plus two of the International Prototype Kilogram's (IPK's) sister copies: K32 and K8(41). All mass changes are relative to the IPK.

Since their manufacture, drifts of up to 2×10⁻⁸ kilograms per year in the national prototype kilograms relative to the international prototype kilogram (IPK) have been detected. There was no way of determining whether the national prototypes were gaining mass or whether the IPK was losing mass. ^[21] Newcastle University metrologist Peter Cumpson has since identified mercury vapour absorption or carbonaceous contamination as possible causes of this drift. ^{[22] [23]} At the 21st meeting of the CGPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artefact.

Independently to the identification of this drift, the Avogadro project and the development of the Kibble balance, which was known as the "watt balance" before 2016, promised methods of indirectly measuring mass with very high precision. These projects provided tools that enable alternative means of redefining the kilogram. ^[24] A report published in 2007 by the Consultative Committee for Thermometry (CCT) to the CIPM noted that their current definition of temperature has proved to be unsatisfactory for temperatures below 20 K and for temperatures above 1300 K. The committee took the view that the Boltzmann constant provided a better basis for temperature measurement than did the triple point of water because it overcame these difficulties. ^[25]

At its 23rd meeting (2007), the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artefacts that were then in use. The following year this was endorsed by the International Union of Pure and Applied Physics (IUPAP). ^[26] At a meeting of the CCU held in Reading, United Kingdom, in September 2010, a resolution ^[27] and draft changes to the SI brochure that were to be presented to the next meeting of the CIPM in October 2010 were agreed to in principle. ^[28] The CIPM meeting of October 2010 found "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. ^{[Note 4]} For this reason the CIPM does not propose a revision of the SI at the present time". ^[30] The CIPM, however, presented a resolution for consideration at the 24th CGPM (17–21 October 2011) to agree to the new definitions in principle, but not to implement them until the details had been finalised. ^[31] This resolution was accepted by the conference, ^[32] and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014. ^{[33] [34]} At the 25th meeting on 18 to 20 November 2014, it was found that "despite [progress in the necessary requirements] the data do not yet appear to be sufficiently robust for the CGPM to adopt the revised SI at its 25th meeting", ^[35] thus postponing the revision to the next meeting in 2018. Measurements accurate enough to meet the conditions were available in 2017 and the redefinition ^[36] was adopted at the 26th CGPM (13–16 November 2018).

Redefinition

The numerical values adopted by the CGPM ^[36] are identical to the published CODATA 2017 values. ^[37]

Following the successful 1983 redefinition of the metre in terms of an exact numerical value for the speed of light, the BIPM's Consultative Committee for Units (CCU) recommended and the BIPM proposed that four further constants of nature should be defined to have exact values. These are:

• The Planck constant h is exactly 6.62607015×10⁻³⁴ joule-second (J⋅s).
• The elementary charge e is exactly 1.602176634×10⁻¹⁹ coulomb (C).
• The Boltzmann constant k is exactly 1.380649×10⁻²³ joule per kelvin (J⋅K⁻¹).
• The Avogadro constant N_A is exactly 6.02214076×10²³ reciprocal mole (mol⁻¹).

These constants are described in the 2006 version of the SI manual but in that version, the latter three are defined as "constants to be obtained by experiment" rather than as "defining constants". The redefinition retains unchanged the numerical values associated with the following constants of nature:

• The speed of light c is exactly 299792458 metres per second (m⋅s⁻¹);
• The ground state hyperfine structure transition frequency of the caesium-133 atom Δν_Cs is exactly 9192631770 hertz (Hz);
• The luminous efficacy K_cd of monochromatic radiation of frequency 540×10¹² Hz (540 THz) – a frequency of green colored light at approximately the peak sensitivity of the human eye – is exactly 683 lumens per watt (lm⋅W⁻¹).

The seven definitions above are rewritten below with the derived units joule, coulomb, hertz, lumen, and watt) expressed in terms of the seven base units; second, metre, kilogram, ampere, kelvin, mole, and candela, according to the draft ninth SI Brochure. ^[4] In the list that follows, the symbol sr stands for the dimensionless unit steradian.

• Δν_Cs = Δν(¹³³Cs)_hfs = 9192631770 s⁻¹
• c = 299792458 m⋅s⁻¹
• h = 6.62607015×10⁻³⁴ kg⋅m²⋅s⁻¹
• e = 1.602176634×10⁻¹⁹ A⋅s
• k = 1.380649×10⁻²³ kg⋅m²⋅K⁻¹⋅s⁻²
• N_A = 6.02214076×10²³ mol⁻¹
• K_cd = 683 cd⋅sr⋅s³⋅kg⁻¹⋅m⁻²

As part of the redefinition, the international prototype kilogram was retired and definitions of the kilogram, the ampere, and the kelvin were replaced. The definition of the mole was revised. These changes have the effect of redefining the SI base units, though the definitions of the SI derived units in terms of the base units remain the same.

Impact on base unit definitions

Following the CCU proposal, the texts of the definitions of all of the base units were either refined or rewritten, changing the emphasis from explicit-unit to explicit-constant-type definitions. ^[38] Explicit-unit-type definitions define a unit in terms of a specific example of that unit; for example, in 1324 Edward II defined the inch as being the length of three barleycorns ^[39] and since 1889 the kilogram has been defined as being the mass of the International Prototype Kilogram. In explicit-constant definitions, a constant of nature is given a specified value and the definition of the unit emerges as a consequence; for example, in 1983, the speed of light was defined as exactly 299792458 metres per second. The length of the metre could be derived because the second had been independently defined. The previous ^[19] (as of 2018 ^[update] ) and 2019 ^{[4] [37]} definitions are given below.

Second

The new definition of the second is effectively the same as the previous one, the only difference being that the conditions under which the definition applies are more rigorously defined.

• Previous definition: The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
• 2019 definition: The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹.

Metre

The new definition of the metre is effectively the same as the previous one, the only difference being that the additional rigour in the definition of the second propagated to the metre.

• Previous definition: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.
• 2019 definition: The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s⁻¹, where the second is defined in terms of the caesium frequency Δν_Cs.

Kilogram

A Kibble balance, which is being used to measure the Planck constant in terms of the international prototype kilogram.

The definition of the kilogram changed fundamentally; the previous definition defined the kilogram as the mass of the international prototype kilogram, which is an artefact rather than a constant of nature. ^[41] The new definition relates the kilogram to the equivalent mass of the energy of a photon given its frequency, via the Planck constant.

• Previous definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
• 2019 definition: The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10⁻³⁴ when expressed in the unit J⋅s, which is equal to kg⋅m²⋅s⁻¹, where the metre and the second are defined in terms of c and Δν_Cs.

A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the metre.

Ampere

The definition of the ampere underwent a major revision. The previous definition, which is difficult to realise with high precision in practice, was replaced by a definition that is more intuitive and easier to realise.

• Previous definition: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10⁻⁷ newton per metre of length.
• 2019 definition: The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10⁻¹⁹ when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of Δν_Cs.

Because the previous definition contains a reference to force, which has the dimensions MLT⁻², it follows that in the previous SI the kilogram, metre, and second –the base units representing these dimensions –had to be defined before the ampere could be defined. Other consequences of the previous definition were that in SI the value of vacuum permeability (μ₀) was fixed at exactly 4π×10⁻⁷ H⋅m⁻¹. ^[42] Because the speed of light in vacuum (c) is also fixed, it followed from the relationship

${\displaystyle c^{2}={\frac {1}{\mu _{0}\varepsilon _{0}}}}$

that the vacuum permittivity (ε₀) had a fixed value, and from

${\displaystyle Z_{0}={\sqrt {\frac {\mu _{0}}{\varepsilon _{0}}}},}$

that the impedance of free space (Z₀) likewise had a fixed value. ^[43]

A consequence of the revised definition is that the ampere no longer depends on the definitions of the kilogram and the metre; it does, however, still depend on the definition of the second. In addition, the numerical values of the vacuum permeability, vacuum permittivity, and impedance of free space, which were exact before the redefinition, will be subject to experimental error after the redefinition. ^[44] For example, the numerical value of the vacuum permeability will have a relative uncertainty equal to that of the experimental value of the fine-structure constant ${\displaystyle \alpha }$. ^[45] The CODATA 2018 value for the relative standard uncertainty of ${\displaystyle \alpha }$ is 1.5×10⁻¹⁰. ^[46]

Kelvin

The definition of the kelvin underwent a fundamental change. Rather than using the triple point of water to fix the temperature scale, the new definition uses the energy equivalent as given by Boltzmann's equation.

• Previous definition: The kelvin, unit of thermodynamic temperature, is 1/273.16 of the thermodynamic temperature of the triple point of water.
• 2019 definition: The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10⁻²³ when expressed in the unit J⋅K⁻¹, which is equal to kg⋅m²⋅s⁻²⋅K⁻¹, where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.

One consequence of this change is that the new definition of the kelvin depends on the definitions of the second, the metre, and the kilogram.

Mole

A near-perfect sphere of ultra-pure silicon – part of the Avogadro project, an International Avogadro Coordination project to determine the Avogadro number

The previous definition of the mole linked it to the kilogram. The revised definition breaks that link by making a mole a specific number of entities of the substance in question.

• Previous definition: The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
• 2019 definition: ^{[7] :22} The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076×10²³ elementary entities. This number is the fixed numerical value of the Avogadro constant, N_A, when expressed in the unit mol⁻¹ and is called the Avogadro number. ^{[7] [47]} The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.

One consequence of this change is that the current defined relationship between the mass of the ¹²C atom, the dalton, the kilogram, and the Avogadro number will no longer be valid. One of the following must change:

• The mass of a ¹²C atom is exactly 12 dalton.
• The number of dalton in a gram is exactly the numerical value of the Avogadro number: (i.e., 1 g/Da=1 mol ⋅ N_A).

The wording of the ninth SI Brochure ^{[4] [Note 5]} implies that the first statement remains valid, which means the second is no longer true. The molar mass constant, while still with great accuracy remaining 1 g/mol, is no longer exactly equal to that. Draft Resolution A, which was voted on at the 26th CGPM, only stated that "the molar mass of carbon 12, M(¹²C), is equal to 0.012 kg⋅mol⁻¹ within a relative standard uncertainty equal to that of the recommended value of N_Ah at the time this Resolution was adopted, namely 4.5×10⁻¹⁰, and that in the future its value will be determined experimentally", which makes no reference to the dalton and is consistent with either statement.

Candela

The new definition of the candela is effectively the same as the previous definition, the only difference being that the additional rigour in the definition of the second and metre will propagate to the candela.

• Previous definition: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10¹² Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.
• 2019 definition: The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz, K_cd, to be 683 when expressed in the unit lm⋅W⁻¹, which is equal to cd⋅sr⋅W⁻¹, or cd⋅sr⋅kg⁻¹⋅m⁻²⋅s³, where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.

Impact on reproducibility

All seven of the SI base units will be defined in terms of defined constants ^{[Note 6]} and universal physical constants. ^{[Note 7] [48]} Seven constants are needed to define the seven base units but there is not a direct correspondence between each specific base unit and a specific constant; except the second and the mole, more than one of the seven constants contributes to the definition of any given base unit.

When the New SI was first designed, there were more than six suitable physical constants from which the designers could choose. For example, once length and time had been established, the universal gravitational constant G could, from a dimensional point of view, be used to define mass. ^{[Note 8]} In practice, G can only be measured with a relative uncertainty of the order of 10⁻⁵, ^{[Note 9]} which would have resulted in the upper limit of the kilogram's reproducibility being around 10⁻⁵ whereas the current international prototype kilogram can be measured with a reproducibility of 1.2 × 10⁻⁸. ^[44] The physical constants were chosen on the basis of minimal uncertainty associated with measuring the constant and the degree of independence of the constant in respect of other constants that were being used. Although the BIPM has developed a standard mise en pratique (practical technique) ^[49] for each type of measurement, the mise en pratique used to make the measurement is not part of the measurement's definition –it is merely an assurance that the measurement can be done without exceeding the specified maximum uncertainty.

Acceptance

See also: CODATA 2018

Much of the work done by the CIPM is delegated to consultative committees. The CIPM Consultative Committee for Units (CCU) has made the proposed changes while other committees have examined the proposal in detail and have made recommendations regarding their acceptance by the CGPM in 2014. The consultative committees have laid down a number of criteria that must be met before they will support the CCU's proposal, including:

• For the redefinition of the kilogram, at least three separate experiments yielding values for the Planck constant having a relative expanded (95%) uncertainty of no more than 5×10⁻⁸ must be carried out and at least one of these values should be better than 2×10⁻⁸. Both the Kibble balance and the Avogadro project should be included in the experiments and any differences between these must be reconciled. ^{[50] [51]}
• For the redefinition of the kelvin, the relative uncertainty of the Boltzmann constant derived from two fundamentally different methods such as acoustic gas thermometry and dielectric constant gas thermometry must be better than 10⁻⁶, and these values must be corroborated by other measurements. ^[52]

As of March 2011, the International Avogadro Coordination (IAC) group had obtained an uncertainty of 3.0×10⁻⁸ and NIST had obtained an uncertainty of 3.6×10⁻⁸ in their measurements. ^[24] On 1 September 2012 the European Association of National Metrology Institutes (EURAMET) launched a formal project to reduce the relative difference between the Kibble balance and the silicon sphere approach to measuring the kilogram from (17±5)×10⁻⁸ to within 2×10⁻⁸. ^[53] As of March 2013 ^[update] the proposed redefinition is known as the "New SI" ^[3] but Mohr, in a paper following the CGPM proposal but predating the formal CCU proposal, suggested that because the proposed system makes use of atomic scale phenomena rather than macroscopic phenomena, it should be called the "Quantum SI System". ^[54]

As of the 2014 CODATA-recommended values of the fundamental physical constants published in 2016 using data collected until the end of 2014, all measurements met the CGPM's requirements, and the redefinition and the next CGPM quadrennial meeting in late 2018 could now proceed. ^{[55] [56]}

On 20 October 2017, the 106th meeting of the International Committee for Weights and Measures (CIPM) formally accepted a revised Draft Resolution A, calling for the redefinition of the SI, to be voted on at the 26th CGPM, ^{[7] :17–23} The same day, in response to the CIPM's endorsement of the final values, ^{[7] :22} the CODATA Task Group on Fundamental Constants published its 2017 recommended values for the four constants with uncertainties and proposed numerical values for the redefinition without uncertainty. ^[37] The vote, which was held on 16 November 2018 at the 26th GCPM, was unanimous; all attending national representatives voted in favour of the revised proposal. The new definitions will become effective on 20 May 2019. ^[57]

Concerns

In 2010, Marcus Foster of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) published a wide-ranging critique of SI; he raised numerous issues ranging from basic issues such as the absence of the symbol "Ω" (Omega) from most Western computer keyboards to the abstract issues such as inadequate formalism in the metrological concepts on which SI is based. The changes proposed in the New SI only addressed problems with the definition of the base units, including new definitions of the candela and the mole  –units Foster argued are not true base units. Other issues raised by Foster fell outside the scope of the proposal. ^[58]

Explicit-unit and explicit-constant definitions

Concerns that the use of explicit-constant definitions of the unit being defined that are not related to an example of its quantity will have many adverse effects have been expressed. ^[59] Although this criticism applies to the proposed linking of the kilogram to the Planck constant h via a route that requires a knowledge of both special relativity and quantum mechanics, ^[60] it does not apply to the proposed definition of the ampere, which is closer to an example of its quantity than is the current definition. ^[61] Some observers have welcomed the proposal to base the definition of electric current on the charge of the electron rather than the current definition of a force between two parallel, current-carrying wires; because the nature of the electromagnetic interaction between two bodies is somewhat different at the quantum electrodynamics level than at classical electrodynamic levels, it is considered inappropriate to use classical electrodynamics to define quantities that exist at quantum electrodynamic levels. ^[44]

Mass and the Avogadro constant

When the scale of the divergence between the IPK and national kilogram prototypes was reported in 2005, a debate about whether the kilogram should be defined in terms of the mass of the silicon-28 atom or by using the Kibble balance began. The mass of a silicon atom could be determined using the Avogadro project and using the Avogadro number, it could be linked directly to the kilogram. ^[62] Concerns that the authors of the proposal had failed to address the impact of breaking the link between the mole, kilogram, dalton, and the Avogadro constant (N_A) have also been expressed. ^{[Note 10]} This direct link has caused many to argue that the mole is not a true physical unit but, according to the Swedish philosopher Johansson, a "scaling factor". ^{[58] [63]}

The SI Brochure (8th edition) defines the dalton in terms of the mass of an atom of ¹²C. ^[64] It defines the Avogadro constant in terms of this mass and the kilogram, making it determined by experiment. The proposal fixes the Avogadro constant and the draft of the Ninth SI Brochure ^[4] retains the definition of dalton in terms of ¹²C, with the effect that the link between the dalton and the kilogram will be broken. ^{[65] [66]}

In 1993, the International Union of Pure and Applied Chemistry (IUPAC) approved the use of the dalton as an alternative to the unified atomic mass unit with the qualification that the CGPM had not given its approval. ^[67] This approval has since been given. ^[68] Following the proposal to redefine the mole by fixing the value of the Avogadro constant, Brian Leonard of the University of Akron, writing in Metrologia , proposed that the dalton (Da) be redefined such that N_A=(g/Da) mol⁻¹, but that the unified atomic mass unit (m_u) retain its current definition based on the mass of ¹²C, ceasing to exactly equal to the dalton. This would result in the dalton and the atomic mass unit potentially differing from each other with a relative uncertainty of the order of 10⁻¹⁰. ^[69] The draft of the ninth SI Brochure, however, defines both the dalton (Da) and the unified atomic mass unit (u) as exactly 1/12 of the mass of a free carbon-12 atom and not in relation to the kilogram, ^[4] with the effect that the above equation will be inexact.

Temperature

Temperature is somewhat of an enigma; room temperature can be measured by means of expansion and contraction of a liquid in a thermometer but high temperatures are often associated with colour. Wojciech T. Chyla, approaching the structure of SI from a philosophical point of view in the Journal of the Polish Physical Society, argued that temperature is not a real base unit but is an average of the thermal energies of the individual particles that comprise the body concerned. ^[44] He noted that in many theoretical papers, temperature is represented by the quantities Θ or β where

${\displaystyle \Theta =kT;\ \ \ \beta ={1 \over kT}}$

and k is the Boltzmann constant. Chyla acknowledged, however, that in the macroscopic world, temperature plays the role of a base unit because much of the theory of thermodynamics is based on temperature. ^[44]

The Consultative Committee for Thermometry, part of the International Committee for Weights and Measures, publishes a mise en pratique (practical technique), last updated in 1990, for measuring temperature which, at very low and at very high temperatures, often links energy to temperature via the Boltzmann constant. ^{[70] [71]}

Luminous intensity

Foster argued that "luminous intensity [the candela] is not a physical quantity, but a photobiological quantity that exists in human perception", questioning whether the candela should be a base unit. ^[58]

See also

• International System of Units  – a system of units of measurement for base and derived physical quantities
• International Vocabulary of Metrology
• Physical constant  – Universal and unchanging physical quantity
• SI base unit
• 2005-2019 definitions of the SI base units

Notes

1. The metre was redefined again in 1983 by fixing the value of the speed of light. That definition will remain in effect after 2019.
2. The dalton is not defined in the formal proposal to be voted upon by the CGPM, only in the (draft) Ninth SI Brochure.
3. Prototype No. 8(41) was accidentally stamped with the number 41, but its accessories carry the proper number 8. Since there is no prototype marked 8, this prototype is referred to as 8(41). 
4. In particular the CIPM was to prepare a detailed mise en pratique for each of the new definitions of the kilogram, ampere, kelvin and mole set by the 23rd CGPM ^[29]
5. A footnote in Table 8 on non-SI units states: "The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state."
6. Though the three quantities temperature, luminous intensity and amount of substance may be regarded from a fundamental physical perspective as derived quantities, these are perceptually independent quantities and have conversion constants defined that relate the historically defined units to the underlying physics.
7. The definition of the candela is atypical within the base units; translating physical measurements of spectral intensity into units of candela also requires a model of the response of the human eye to different wavelengths of light known as the luminosity function and denoted by V(λ), a function that is determined by the International Commission on Illumination (CIE).
8. The dimensions of G are L³M⁻¹T⁻² so once standards have been established for length and for time, mass can, in theory, be deduced from G. When fundamental constants as relations between these three units are set, the units can be deduced from a combination of these constants; for example, as a linear combination of Planck units.
9. The following terms are defined in International vocabulary of metrology – Basic and general concepts and associated terms Archived 17 March 2017 at the Wayback Machine :
   • measurement reproducibility – definition 2.25
   • standard measurement uncertainty – definition 2.30
   • relative standard measurement uncertainty – definition 2.32
10. The two quantities of the Avogadro constant N_A and the Avogadro number N_N are numerically identical but while N_A has the unit mol⁻¹, N_N is a pure number.

References

1. "BIPM statement: Information for users about the proposed revision of the SI" (PDF). Archived (PDF) from the original on 21 January 2018. Retrieved 5 May 2018.
2. "Decision CIPM/105-13 (October 2016)". Archived from the original on 24 August 2017. Retrieved 31 August 2017.
3. Kühne, Michael (22 March 2012). "Redefinition of the SI". Keynote address, ITS⁹ (Ninth International Temperature Symposium). Los Angeles: NIST. Archived from the original on 18 June 2013. Retrieved 1 March 2012.
4. "Draft of the ninth SI Brochure" (PDF). BIPM. 28 December 2018. Archived (PDF) from the original on 21 March 2019. Retrieved 20 March 2019.
5. "Historic Vote Ties Kilogram and Other Units to Natural Constants". NIST. 16 November 2018. Archived from the original on 18 November 2018. Retrieved 16 November 2018.
6. Milton, Martin (14 November 2016). Highlights in the work of the BIPM in 2016 (PDF). SIM XXII General Assembly. Montevideo, Uruguay. p. 10. Archived (PDF) from the original on 1 September 2017. Retrieved 13 January 2017. The conference ran from 13–16 November and the vote on the redefinition was scheduled for the last day. Kazakhstan was absent and did not vote.
7. Proceedings of the 106th meeting (PDF). International Committee for Weights and Measures. Sèvres. 16–20 October 2017. Archived (PDF) from the original on 27 January 2018. Retrieved 27 January 2018.
8. Crease, Robert P. (2011). "France: "Realities of Life and Labor"". World in the Balance. New York: W. W. Norton & Company, Inc. pp. 83–84. ISBN   978-0-393-07298-3.
9. Alder, Ken (2002). The Measure of all Things – The Seven-Year-Odyssey that Transformed the World. London: Abacus. p. 1. ISBN   978-0-349-11507-8.
10. "Metric Convention of 1875 [English translation]". Washington, D.C.: Office of the President of the United States. 1876. Archived from the original on 1 March 2005.
11. "The Metre Convention". Sèvres, France: International Bureau of Weights and Measures. Archived from the original on 26 September 2012. Retrieved 21 June 2013.
12. "CIPM: International Committee for Weights and Measures". Sèvres, France: BIPM. Archived from the original on 24 September 2012. Retrieved 3 October 2010.
13. "Resolution of the 1st meeting of the CGPM (1889)". Sèvres, France: International Bureau of Weights and Measures. Archived from the original on 21 May 2013. Retrieved 21 June 2013.
14. Jabbour, Z.J.; Yaniv, S.L. (2001). "The Kilogram and Measurements of Mass and Force" (PDF). Journal of Research of the National Institute of Standards and Technology. 106 (1): 25–46. doi:10.6028/jres.106.003. PMC   4865288 . PMID   27500016. Archived from the original (PDF) on 4 June 2011. Retrieved 28 March 2011.
15. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 95, 97, 138–140, ISBN   92-822-2213-6, archived (PDF) from the original on 14 August 2017
16. "Resolution 6 of the 9th meeting of the CGPM (1948): Proposal for establishing a practical system of units of measurement". Archived from the original on 14 May 2013. Retrieved 23 March 2011.
17. "Resolution 12 of the 11th meeting of the CGPM (1960): Système International d'Unités". Sèvres, France. Archived from the original on 14 April 2013. Retrieved 23 March 2011.
18. Stephenson, F. R.; Morrison, L. V.; Hohenkerk, C. Y. (December 2016). "Measurement of the Earth's rotation: 720 BC to AD 2015". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 472 (2196). §4(a). doi:10.1098/rspa.2016.0404. PMC   5247521 . PMID   28119545.
19. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 112–116, ISBN   92-822-2213-6, archived (PDF) from the original on 14 August 2017
20. Girard, G. (1994). "The Third Periodic Verification of National Prototypes of the Kilogram (1988–1992)". Metrologia. 31 (4): 317–336. Bibcode:1994Metro..31..317G. doi:10.1088/0026-1394/31/4/007.
21. Peter, Mohr (6 December 2010). "Recent progress in fundamental constants and the International System of Units" (PDF). Third Workshop on Precision Physics and Fundamental Physical Constant. Archived (PDF) from the original on 24 August 2011. Retrieved 2 January 2011.
22. Whipple, Tom (7 January 2013). "The dirty secret of why you are not quite as heavy as you think". The Times . London. p. 15. Archived from the original on 17 January 2013. Retrieved 23 March 2011.
23. Ghose, Tia (6 January 2013). "The Kilogram Has Gained Weight". LiveScience. Archived from the original on 26 March 2013. Retrieved 23 March 2011.
24. Crease, Robert P. (22 March 2011). "Metrology in the balance". Physics World. Institute of Physics . Retrieved 28 June 2012.
25. Fischer, J.; et al. (2 May 2007). "Report to the CIPM on the implications of changing the definition of the base unit kelvin" (PDF). Archived (PDF) from the original on 23 November 2008. Retrieved 2 January 2011.
26. "Resolution proposal submitted to the IUPAP Assembly by Commission C2 (SUNAMCO)" (PDF). International Union of Pure and Applied Physics. 2008. Archived (PDF) from the original on 5 March 2016. Retrieved 6 September 2015.
27. Mills, Ian (29 September 2010). "On the possible future revision of the International System of Units, the SI" (PDF). CCU. Archived (PDF) from the original on 13 January 2012. Retrieved 1 January 2011.
28. Mills, Ian (29 September 2010). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Archived (PDF) from the original on 23 June 2013. Retrieved 1 January 2011.
29. "Resolution 12 of the 23rd meeting of the CGPM (2007)". Sèvres, France: General Conference on Weights and Measures. Archived from the original on 21 April 2013. Retrieved 21 June 2013.
30. "Towards the "new SI"". International Bureau of Weights and Measures (BIPM). Archived from the original on 14 May 2011. Retrieved 20 February 2011.
31. "On the possible future revision of the International System of Units, the SI – Draft Resolution A" (PDF). International Committee for Weights and Measures (CIPM). Archived (PDF) from the original on 6 August 2011. Retrieved 14 July 2011.
32. "Resolution 1: On the possible future revision of the International System of Units, the SI" (PDF). 24th meeting of the General Conference on Weights and Measures. Sèvres, France: International Bureau for Weights and Measures. 21 October 2011. It was not expected to be adopted until some prerequisite conditions are met, and in any case not before 2014. See"Possible changes to the international system of units". IUPAC Wire. 34 (1). January – February 2012.
33. "General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram" (PDF) (Press release). Sèvres, France: General Conference on Weights and Measures. 23 October 2011. Archived (PDF) from the original on 9 February 2012. Retrieved 25 October 2011.
34. Mohr, Peter (2 November 2011). "Redefining the SI base units". NIST Newsletter. NIST. Archived from the original on 12 August 2016. Retrieved 1 March 2012.
35. "Resolutions adopted by the CGPM at its 25th meeting (18–20 November 2014)" (PDF). Sèvres, France: International Bureau for Weights and Measures. 21 November 2014. Archived (PDF) from the original on 25 March 2015. Retrieved 1 December 2014.
36. "Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)" (PDF). Archived (PDF) from the original on 29 April 2018. Retrieved 5 May 2018.
37. Newell, David B.; Cabiati, F.; Fischer, J.; Fujii, K.; Karshenboim, S.G.; Margolis, H.S.; de Mirandés, E.; Mohr, P.J.; Nez, F.; Pachucki, K.; Quinn, T.J.; Taylor, B.N.; Wang, M.; Wood, B.M.; Zhang, Z.; et al. (Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants (TGFC)) (20 October 2017). "The CODATA 2017 Values of h, e, k, and N_A for the Revision of the SI". Metrologia. 55 (1): L13. Bibcode:2018Metro..55L..13N. doi: 10.1088/1681-7575/aa950a . Archived from the original on 14 February 2018. Retrieved 27 January 2018.
38. Mills, Ian (September – October 2011). "Part II – Explicit-Constant Definitions for the Kilogram and for the Mole". Chemistry International. 33 (5): 12–15. ISSN   0193-6484. Archived from the original on 9 July 2017. Retrieved 28 June 2013.
39. Travenor, Robert (2007). Smoot's Ear – The Measure of Humanity. Yale University Press. pp. 35–36. ISBN   978-0-300-14334-8.
40. "The BIPM watt balance". International Bureau of Weights and Measures. 2012. Archived from the original on 21 April 2013. Retrieved 28 March 2013.
41. Taylor, Barry N (November – December 2011). "The Current SI Seen From the Perspective of the Proposed New SI". Journal of Research of the National Institute of Standards and Technology. 116 (6): 797–80. doi:10.6028/jres.116.022.
42. "Unit of electric current (ampere)". Historical context of the SI. NIST. Archived from the original on 27 June 2013. Retrieved 7 September 2015.
43. Orfanidis, Sophocles J. (31 August 2010). Electromagnetic Waves and Antennas (PDF). ECE Department, Rutgers University. 1.3 Constitutive Relations. Archived (PDF) from the original on 15 September 2013. Retrieved 24 June 2013.
44. Chyla, W.T. (December 2011). "Evolution of the International Metric System of Units SI" (PDF). Acta Physica Polonica A. 120 (6): 998–1011. Archived (PDF) from the original on 20 March 2017. Retrieved 22 June 2013.
45. Davis, Richard S. (2017). "Determining the value of the fine-structure constant from a current balance: getting acquainted with some upcoming changes to the SI". American Journal of Physics . 85 (5): 364–368. arXiv: 1610.02910 . doi:10.1119/1.4976701.
46. "CODATA Value: fine-structure constant". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. 20 May 2019. Retrieved 20 May 2019. 2018 CODATA recommended values
47. "Redefining the Mole". NIST. NIST. 23 October 2018. Archived from the original on 24 October 2018. Retrieved 24 October 2018.
48. Wyszecki, G.; Blevin, W.R.; Kessler, K.G.; Mielenz, K.D. (1983). Principles covering Photometry (PDF). Sevres: Conférence général des poids et mesures (CGPM). Archived (PDF) from the original on 11 October 2008. Retrieved 23 April 2012.
49. "What is a mise en pratique?". BIPM. 2011. Archived from the original on 22 September 2015. Retrieved 6 September 2015. is a set of instructions that allows the definition to be realised in practice at the highest level.
50. "Recommendations of the Consultative Committee for Mass and Related Quantities to the International Committee for Weights and Measures" (PDF). 12th Meeting of the CCM. Sèvres: Bureau International des Poids et Mesures. 26 March 2010. Archived from the original (PDF) on 14 May 2013. Retrieved 27 June 2012.
51. "Recommendations of the Consultative Committee for Amount of Substance: Metrology in Chemistry to the International Committee for Weights and Measures" (PDF). 16th Meeting of the CCQM. Sèvres: Bureau International des Poids et Mesures. 15–16 April 2010. Archived from the original (PDF) on 14 May 2013. Retrieved 27 June 2012.
52. "Recommendations of the Consultative Committee for Thermometry to the International Committee for Weights and Measures" (PDF). 25th Meeting of the Consultative Committee for Thermometry. Sèvres: Bureau International des Poids et Mesures. 6–7 May 2010. Archived from the original (PDF) on 14 May 2013. Retrieved 27 June 2012.
53. "kilogram NOW – Realization of the awaited definition of the kilogram". European Association of National Metrology Institutes. Archived from the original on 4 March 2016. Retrieved 8 October 2012.
54. Mohr, Peter J. (2008). The Quantum SI: A Possible New International System of Units. Advances in Quantum Chemistry. 53. Academic Press. p. 34. Bibcode:2008AdQC...53...27M. doi:10.1016/s0065-3276(07)53003-0. ISBN   978-0-12-373925-4 . Retrieved 2 April 2012.
55. "Universe's Constants Now Known with Sufficient Certainty to Completely Redefine the International System of Units" (Press release). NIST. 22 November 2016. Archived from the original on 1 January 2017. Retrieved 31 December 2016.
56. Mohr, Peter J.; Newell, David B.; Taylor, Barry N. (26 September 2016). "CODATA recommended values of the fundamental physical constants: 2014". Reviews of Modern Physics . 88 (3): 035009–1–73. arXiv: 1507.07956 . Bibcode:2016RvMP...88c5009M. doi:10.1103/RevModPhys.88.035009. This is a truly major development, because these uncertainties are now sufficiently small that the adoption of the new SI by the 26th CGPM is expected.
57. Conover, Emily (16 November 2018). "It's official: We're redefining the kilogram". Science News . Archived from the original on 16 November 2018. Retrieved 16 November 2018.
58. Foster, Marcus P (5 October 2010). "The next 50 years of the SI: a review of the opportunities for the e-Science age". Metrologia. 47 (6): R41–R51. Bibcode:2010Metro..47R..41F. doi:10.1088/0026-1394/47/6/R01. Archived from the original on 6 March 2016. Retrieved 24 June 2013.
59. Price, Gary (2011). "A sceptic's review of the New SI". Accreditation and Quality Assurance. 16 (3): 121–132. doi:10.1007/s00769-010-0738-x.
60. Censullo, Albert C. (September – October 2011). "Part I – From the Current "Kilogram Problem" to a Proposed Definition". Chemistry International. 33 (5): 9–12. ISSN   0193-6484. Archived from the original on 9 July 2017. Retrieved 28 June 2013.
61. Burns, D Thorburn; Korte, EH (2013). "The Background and Implications of the "New SI" for Analytical Chemists" (PDF). Journal of the Association of Public Analysts (Online) (41 2): 28–44. Archived (PDF) from the original on 6 March 2016. Retrieved 25 June 2013.
62. Davis, Richard (October 2011). "Proposed change to the definition of the kilogram: Consequences for legal metrology" (PDF). OIML Bulletin. LII (4). Archived (PDF) from the original on 27 March 2015. Retrieved 28 June 2013.
63. Johansson, Ingvar (2011). "The Mole is Not an Ordinary Measurement Unit". Accreditation and Quality Assurance. 16 (16): 467–470. doi:10.1007/s00769-011-0804-z.
64. http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf SI Brochure (8th edition)
65. Leonard, B.P. (2010). "Comments on recent proposals for redefining the mole and kilogram". Metrologia . 47 (3): L5–L8. Bibcode:2010Metro..47L...5L. doi:10.1088/0026-1394/47/3/L01 . Retrieved 15 September 2013.
66. Pavese, Franco (2011). "Some reflections on the proposed redefinition of the unit for the amount of substance and of other SI units". Accreditation and Quality Assurance. 16 (3): 161–165. doi:10.1007/s00769-010-0700-y.
67. Mills, Ian; Cvitaš, Tomislav; Homann, Klaus; Kallay, Nikola; Kuchitsu, Kozo (1993). Quantities, Units and Symbols in Physical Chemistry International Union of Pure and Applied Chemistry; Physical Chemistry Division (2nd ed.). International Union of Pure and Applied Chemistry, Blackwell Science Ltd. ISBN   978-0-632-03583-0.
68. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 114, 115, ISBN   92-822-2213-6, archived (PDF) from the original on 14 August 2017
69. Leonard, Brian Phillip (May 2012). "Why the dalton should be redefined exactly in terms of the kilogram". Metrologia. 49 (4): 487–491. Bibcode:2012Metro..49..487L. doi:10.1088/0026-1394/49/4/487.
70. "Mise en pratique for the definition of the kelvin" (PDF). Sèvres, France: Consultative Committee for Thermometry (CCT), International Committee for Weights and Measures (CIPM). 2011. Archived (PDF) from the original on 8 May 2013. Retrieved 25 June 2013.
71. Consultative Committee for Thermometry (CCT) (1989). "The International Temperature Scale of 1990 (ITS-90)" (PDF). Procès-verbaux du Comité International des Poids et Mesures, 78th Meeting. Archived (PDF) from the original on 23 June 2013. Retrieved 25 June 2013.

Further reading

• International Bureau of Weights and Measures (5 February 2018), SI Brochure: The International System of Units (SI) (PDF) (Draft) (9th ed.)
• International Bureau of Weights and Measures (BIPM) (10 August 2017). "Input data for the special CODATA-2017 adjustment". Metrologia (Updated ed.). Retrieved 14 August 2017.

External links

• BIPM website on the New SI, including a FAQ page.