New SI definitions
Encyclopedia
A committee of the International Committee for Weights and Measures (CIPM) has proposed revised formal definitions of the SI base units, which are being examined by the CIPM and which may be considered by the 25th CGPM, in 2014.
The last major overhaul of the metric system was in 1960 when the International System of Units (SI)
was formally published as a coherent set of units of measure. SI is structured around seven base units that have apparently "arbitrary" definitions and another twenty units that are derived from these base units. Although the units themselves form a coherent system, the definitions do not. The proposals before the CIPM seek to remedy this by using the fundamental quantities of nature as the basis for deriving the base units. This will mean, amongst other things, archiving the prototype kilogram. The second
and the metre
are already defined in such a manner.
There have been numerous criticisms of the revised definitions since their initial proposal, and it has been argued that the proposal for the new SI requires frank and open discussion before decisions are made.
Since 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype metre, the only unit of measure that has been dependent on a particular artifact has been the kilogram. Over the years, small drifts which could be as high as kilograms per annum in the mass of the international prototype kilogram have been detected. At the 21st meeting of the GCPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artifact.
A report published in 2007 by the Consultative Committee for Thermometry 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 was of the view that the Boltzmann constant provided a better basis for temperature measurement than did the triple point
of water, as it overcame these difficulties.
At its 23rd meeting (2007), the GCPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artifacts that were then in use. The following year this was endorsed by the International Union of Pure and Applied Physics
(IUPAP). At a meeting of the CCU held in Reading, United Kingdom
in September 2010, a resolution 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. The CIPM meeting of October 2010 found that "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. For this reason the CIPM does not propose a revision of the SI at the present time"; however the CIPM presented a resolution for consideration at the 24th CGPM (17 - 21 October 2011) to agree the new definitions in principle, but not to implement them until the details have been finalised. This resolution was accepted by the conference and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014.
The CCU has proposed that in addition to the speed of light, four constants of nature be defined to have exact values:
These constants were described in the 2006 version of the SI manual; the latter three were defined as "constants to be obtained by experiment".
It is proposed that the following constants of nature be retained unchanged:
The seven definitions above are rewritten below after converting the derived units (joule, coulomb, hertz, lumen and watt) into the seven base units (second, metre, kilogram, ampere, kelvin, mole and candela). In the list that follows, the symbol sr stands for the dimensionless unit steradian.
In addition the CCU has proposed that:
These changes will have the effect of redefining the SI base units, though the definitions of the derived SI units will remain the same.
via Planck's constant.
One consequence of this change is that the new definition makes the definition of the kilogram dependent on the definitions of the second and the metre.
One consequence of this change is that the new definition of the Ampere will not be dependent on the definitions of the kilogram and the metre. In addition, by fixing the elementary charge to an exact value, the vacuum permeability
, vacuum permittivity and impedance of free space, which are currently exact along with the speed of light
, will all consequently carry experimental error.
One consequence of this change is that the new definition makes the definition of the kelvin depend on the definitions of the second, the metre, and the kilogram.
One consequence of this change is that the current defined relationship between the mass of the 12C atom, the dalton
, the kilogram, and Avogadro's number will no longer be valid. One of the following must change:
The following table catalogues the improvements:
The relative uncertainty in measuring the second will remain at and that for the metre will remain at .
Leonard has argued that "the fundamental concept of the mole requires the number of entities comprising one mole, i.e. Avogadro's number, to be exactly equal to the gram-to-dalton mass ratio" and that the proposal breaks this compatibility condition by defining the kilogram, dalton and mole independently.
Pavese has argued that a number of issues need to be better understood before the definitions are changed. The issues include the count nature and the value of the Avogadro number; the loss of the concept of base unit; the possibility of checking future changes in the 'fundamental constants', and the shift to the unit of the experimental uncertainty.
The last major overhaul of the metric system was in 1960 when the International System of Units (SI)
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
was formally published as a coherent set of units of measure. SI is structured around seven base units that have apparently "arbitrary" definitions and another twenty units that are derived from these base units. Although the units themselves form a coherent system, the definitions do not. The proposals before the CIPM seek to remedy this by using the fundamental quantities of nature as the basis for deriving the base units. This will mean, amongst other things, archiving the prototype kilogram. The second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
and the metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
are already defined in such a manner.
There have been numerous criticisms of the revised definitions since their initial proposal, and it has been argued that the proposal for the new SI requires frank and open discussion before decisions are made.
Background to the proposal of new SI definitions
In 1875, twenty of the most industrially developed nations of the world met for the Convention of the Metre. The result was the signing of the Treaty of the Metre under which three bodies were set up to regulate units of measure that were to be used internationally. 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 representatives of the nations who 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 metrologicalMetrologyMetrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
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 to advise the CGPM. The CIPM has set up a number of sub-committees, each charged with a particular area of interest. One of these, the CCU (Consultative Committee for Units), amongst other things, advises the CIPM on matters concerning units of measurement.
- BIPM (International Bureau for Weights and Measures/Bureau international des poids et mesures) – The Bureau provides laboratory facilities and in the secretariat for the CIPM and the CGPM.
Since 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype metre, the only unit of measure that has been dependent on a particular artifact has been the kilogram. Over the years, small drifts which could be as high as kilograms per annum in the mass of the international prototype kilogram have been detected. At the 21st meeting of the GCPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artifact.
A report published in 2007 by the Consultative Committee for Thermometry 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 was of the view that the Boltzmann constant provided a better basis for temperature measurement than did the triple point
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...
of water, as it overcame these difficulties.
At its 23rd meeting (2007), the GCPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artifacts that were then in use. The following year this was endorsed by the International Union of Pure and Applied Physics
International Union of Pure and Applied Physics
The International Union of Pure and Applied Physics is an international non-governmental organization devoted to the advancement of physics...
(IUPAP). At a meeting of the CCU held in Reading, United Kingdom
Reading, Berkshire
Reading is a large town and unitary authority area in England. It is located in the Thames Valley at the confluence of the River Thames and River Kennet, and on both the Great Western Main Line railway and the M4 motorway, some west of London....
in September 2010, a resolution 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. The CIPM meeting of October 2010 found that "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. For this reason the CIPM does not propose a revision of the SI at the present time"; however the CIPM presented a resolution for consideration at the 24th CGPM (17 - 21 October 2011) to agree the new definitions in principle, but not to implement them until the details have been finalised. This resolution was accepted by the conference and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014.
The proposals
- In this section, an "X" at the end of a number means that the final digit is yet to be agreed upon.
The CCU has proposed that in addition to the speed of light, four constants of nature be defined to have exact values:
- Planck's constant h is exactly .
- An elementary chargeElementary chargeThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
e is exactly . - Boltzmann constant k is exactly .
- Avogadro constant NA is exactly .
These constants were described in the 2006 version of the SI manual; the latter three were defined as "constants to be obtained by experiment".
It is proposed that the following constants of nature be retained unchanged:
- The speed of lightSpeed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
c is exactly . - The ground state hyperfine splitting frequencyFrequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the caesiumCaesiumCaesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
-133 atom Δν(133Cs)hfs is exactly hertz (Hz). - The luminous efficacyLuminous efficacyLuminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total electric power consumed by the source.Which sense of the term is...
Kcd of monochromatic radiation of frequency is exactly 683 lumen per watt (lm·W−1).
The seven definitions above are rewritten below after converting the derived units (joule, coulomb, hertz, lumen and watt) into the seven base units (second, metre, kilogram, ampere, kelvin, mole and candela). In the list that follows, the symbol sr stands for the dimensionless unit steradian.
- Δν(133Cs)hfs =
- c =
- h =
- e =
- k =
- NA =
- Kcd = 683 s3·m−2·kg−1·cd·sr
In addition the CCU has proposed that:
- The international prototype kilogram is retired and that the current definition of the kilogram is abrogated.
- The current definition of the ampereAmpereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
is abrogated. - The current definition of the kelvinKelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
is abrogated. - The current definition of the moleMole (unit)The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
is revised.
These changes will have the effect of redefining the SI base units, though the definitions of the derived SI units will remain the same.
Proposed changes to the base units
It is proposed that the text of the definitions of all the base units be either refined or rewritten. The current (2008) and proposed (2011) definitions are given below. In many cases the final digit of any constant is yet to be agreed, so it has been represented by an "X"Second
The proposed definition is effectively the same as the current definition, the only difference being that the conditions under which the measurements are made more rigorous.- Current definition: The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
- Proposed definition: The second, s, is the unit of time; its magnitude is set by fixing the numerical value of the ground state hyperfine splitting frequency of the caesium-133 atom, at rest and at a temperature of 0 K, to be equal to exactly when it is expressed in the unit s−1, which is equal to HzHZHz is the International Standard symbol for Hertz, the unit of frequencyHZ may also stand for:* Habitable zone, the distance from a star where a planet can maintain Earth-like life* Hamilton Zoo, in New Zealand...
.
Metre
The proposed definition is effectively the same as the current definition, the only difference being that the tightening up of the definition of the second will propagate to the metre- Current definition: The metre is the length of the path travelled by light in vacuum during a time interval of of a second.
- Proposed definition: The metre, m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly when it is expressed in the unit m·s−1.
Kilogram
The definition of the kilogram is undergoing a fundamental change - the current definition defines the kilogram as being the mass of the international prototype kilogram, the new definition relates it to the equivalent energy of a photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
via Planck's constant.
- Current definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
- Proposed definition: The kilogram, kg, is the unit of mass; its magnitude is set by fixing the numerical value of the Planck constant to be equal to exactly when it is expressed in the unit s−1·m2·kg, which is equal to J·s.
One consequence of this change is that the new definition makes the definition of the kilogram dependent on the definitions of the second and the metre.
Ampere
The definition of the ampere is undergoing a major overhaul—the current definition, which is difficult to realise with high precision in practice, is being replaced by a definition which is more intuitive and which is easier to realise in practice.- Current 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 newton per metre of length.
- Proposed definition: The ampere, A, is the unit of electric current; its magnitude is set by fixing the numerical value of the elementary chargeElementary chargeThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
to be equal to exactly when it is expressed in the unit A·s, which is equal to C.
One consequence of this change is that the new definition of the Ampere will not be dependent on the definitions of the kilogram and the metre. In addition, by fixing the elementary charge to an exact value, the vacuum permeability
Vacuum permeability
The physical constant μ0, commonly called the vacuum permeability, permeability of free space, or magnetic constant is an ideal, physical constant, which is the value of magnetic permeability in a classical vacuum...
, vacuum permittivity and impedance of free space, which are currently exact along with the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, will all consequently carry experimental error.
Kelvin
The definition of the kelvin will undergo a fundamental change if the proposals are accepted. Rather than using points where water changes state to fix the temperature scale the proposal recommends that the energy equivalent as given by Boltzmann's equation be used.- Current definition: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
- Proposed definition: The kelvin, K, is the unit of thermodynamic temperature; its magnitude is set by fixing the numerical value of the Boltzmann constant to be equal to exactly when it is expressed in the unit s−2·m2·kg K−1, which is equal to J·K−1.
One consequence of this change is that the new definition makes the definition of the kelvin depend on the definitions of the second, the metre, and the kilogram.
Mole
The current definition of the mole links it to the kilogram. The proposed definition will break that link by making a mole a specific number of entities of the substance in question.- Current definition: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12Carbon-12Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
. 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.
- Proposed definition: The mole, mol, is the unit of amount of substance of a specified elementary entity, which may be an atom, molecule, ion, electron, any other particle or a specified group of such particles; its magnitude is set by fixing the numerical value of the Avogadro constant to be equal to exactly when it is expressed in the unit mol−1.
One consequence of this change is that the current defined relationship between the mass of the 12C atom, the dalton
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
, the kilogram, and Avogadro's number will no longer be valid. One of the following must change:
- the mass of 12C, which would no longer be exactly 12 dalton
- the mass of the dalton itself. This would change the numerical masses of all atoms except 12C
- the number of 12C atoms in 12 grams or 0.012 kilogram, which is currently NA by definition.
Candela
The proposed definition is effectively the same as the current definition, but rephrased.- Current definition: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency and that has a radiant intensity in that direction of 1/683 watt per steradianSteradianThe steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...
.
- Proposed definition: The candela, cd, is the unit of luminous intensity in a given direction; its magnitude is set by fixing the numerical value of the luminous efficacy of monochromatic radiation of frequency to be equal to exactly 683 when it is expressed in the unit s3·m−2·kg−1·cd·sr, or cd·sr·W−1, which is equal to lm·W−1.
Impact on reproducibility
Overall the changes in the definitions will give an improvement in the uncertainty of the reproduction of the base units using the standard mise en pratique (practical technique).The following table catalogues the improvements:
| Relative uncertainty of various physical measurements | ||||
|---|---|---|---|---|
| Unit | Constant used as reference | Symbol | Current definitions | Proposed definitions |
| kg | Mass of International prototype kilogram | m(K) | exact | 5.0 × 10−8 |
| Planck constant | h | 5.0 × 10−8 | exact | |
| A | magnetic constant | μ0 | exact | 6.9 × 10−10 |
| Elementary charge | e | 2.5 × 10−8 | exact | |
| K | Temperature of triple point of water | TTPW | exact | 1.7 × 10−6 |
| Boltzmann constant | k | 1.7 × 10−6 | exact | |
| mol | Molar mass 12C | M(12C) | exact | 1.4 × 10−9 |
| Avogadro constant | NA | 1.4 × 10−9 | exact | |
The relative uncertainty in measuring the second will remain at and that for the metre will remain at .
Criticisms of the proposed new SI definitions
Price has argued that the new proposal will:- cause confusion because the new explicit-constant definitions do not relate a unit to an example of its quantity
- risk damage to the enterprise of science because the circular definition of units will render it impossible to detect any future change in fundamental constants
- cause economic damage due to increased transaction costs and barriers to international trade
Leonard has argued that "the fundamental concept of the mole requires the number of entities comprising one mole, i.e. Avogadro's number, to be exactly equal to the gram-to-dalton mass ratio" and that the proposal breaks this compatibility condition by defining the kilogram, dalton and mole independently.
Pavese has argued that a number of issues need to be better understood before the definitions are changed. The issues include the count nature and the value of the Avogadro number; the loss of the concept of base unit; the possibility of checking future changes in the 'fundamental constants', and the shift to the unit of the experimental uncertainty.
See also
- International Vocabulary of MetrologyInternational vocabulary of metrologyThe International vocabulary of metrology is an attempt to find a common language and terminology in metrology, e.g. the science of measurements, across different fields of science, legislature and commerce...
- International System of UnitsInternational System of UnitsThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
- Physical constantPhysical constantA physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...
- SI base unitSI base unitThe International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...