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Proposal

The numerical values given in this section are the published CODATA 2017 values.

The BIPM's Consultative Committee for Units (CCU) proposed that, in addition to the speed of light, four further constants of nature should be defined to have exact values. Using the CODATA 2017 values, these would be:

• The Planck constant h is exactly 6966662607015000000♠6.62607015×10⁻³⁴ joule-second (J⋅s).
• The elementary charge e is exactly 6981160217663400000♠1.602176634×10⁻¹⁹ coulomb (C).
• The Boltzmann constant k is exactly 6977138064900000000♠1.380649×10⁻²³ joule per kelvin (J⋅K⁻¹).
• The Avogadro constant N_A is exactly 7023602214076000000♠6.02214076×10²³ reciprocal mole (mol⁻¹).

These constants were described in the 2006 version of the SI manual, but in that version the latter three were defined as "constants to be obtained by experiment" rather than as "defining constants".

The CCU also proposed that the numerical values associated with the following constants of nature be retained unchanged:

• The speed of light c is exactly 7008299792458000000♠299792458 metres per second (m⋅s⁻¹).
• The ground state hyperfine splitting frequency of the caesium-133 atom Δν(¹³³Cs)_hfs is exactly 7009919263177000000♠9192631770 hertz (Hz).
• The luminous efficacy K_cd of monochromatic radiation of frequency 7014540000000000000♠540×10¹² Hz is exactly 7002683000000000000♠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), according to the updated draft of the 9th edition of the SI Brochure (2016). In the list that follows, the symbol sr stands for the dimensionless unit steradian.

• Δν_Cs = Δν(¹³³Cs)_hfs = 7009919263177000000♠9192631770 s⁻¹
• c = 7008299792458000000♠299792458 m⋅s⁻¹
• h = 6966662607015000000♠6.62607015×10⁻³⁴ kg⋅m²⋅s⁻¹
• e = 6981160217663400000♠1.602176634×10⁻¹⁹ A⋅s
• k = 6977138064900000000♠1.380649×10⁻²³ kg⋅m²⋅K⁻¹⋅s⁻²
• N_A = 7023602214076000000♠6.02214076×10²³ mol⁻¹
• K_cd = 7002683000000000000♠683 cd⋅sr⋅s³⋅kg⁻¹⋅m⁻²

In addition the CCU proposed that

• The international prototype kilogram be retired and that the current definition of the kilogram be abrogated,
• The current definition of the ampere be abrogated,
• The current definition of the kelvin be abrogated and
• The current definition of the mole be revised.

These changes will have the effect of redefining the SI base units, though the definitions of the derived SI units in terms of the base units will remain the same.

提案

国际度量衡局提案，除了光速 以外，下列所示的四个自然常数也应被定义为确定的数值：

• 普朗克常数 h = 6966662607015000000♠6.62607015×10⁻³⁴ (J·s)
• 基本电荷 e = 6981160217663400000♠1.602176634×10⁻¹⁹ (C)
• 波兹曼常数 k = 6977138064900000000♠1.380649×10⁻²³ (J⋅K⁻¹)
• 亚佛加厥常数 N_A = 7023602214076000000♠6.02214076×10²³ (mol⁻¹)

这些常数在2006年板的SI指南就已经出现，但在此版本中后三个常数被定义为“由实验所得的常数”，而不是直接的“定义常数”。

国际度量衡局也提案，以下这些自然常数的数值应继续保持不变。

• 光速 c = 7008299792458000000♠299792458 (m·s^-1)
• 铯133原子基态超精细能阶分裂频率 Δν(¹³³Cs)_hfs = 7009919263177000000♠9192631770 (Hz)
• 频率为 7014540000000000000♠540×10¹² Hz 辐射的发光效率 K_cd = 7002683000000000000♠683 (lm·W^-1)

以上七个定义在以下改写为以基本单位表示的形式

• Δν(¹³³Cs)_hfs = 7009919263177000000♠9192631770 (s⁻¹)
• c = 7008299792458000000♠299792458 (m⋅s⁻¹)
• h = 6966662607015000000♠6.62607015×10⁻³⁴ (kg⋅m²⋅s⁻¹)
• e = 6981160217663400000♠1.602176634×10⁻¹⁹ (A⋅s)
• k = 6977138064900000000♠1.380649×10⁻²³ (kg⋅m²⋅K⁻¹⋅s⁻²)
• N_A = 7023602214076000000♠6.02214076×10²³ (mol⁻¹)
• K_cd = 7002683000000000000♠683 (cd⋅sr⋅s³⋅kg⁻¹⋅m⁻²)

此外国际度量衡局也要求：

• 目前公斤的定义应废除并使国际公斤原器退休
• 目前安培的定义应废除
• 目前克耳文的定义应废除
• 目前莫尔的定义应修改

这些改变会影响到基本单位的定义，但对于导出单位的表达形式则不会有所影响。

Impact on base unit definitions

The CCU proposal recommended that the text of the definitions of all the base units be either refined or rewritten changing the emphasis from explicit-unit to explicit-constant type definitions. 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 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 to be exactly 7008299792458000000♠299792458 metres per second and, since the second had been independently defined, the length of the metre could thus be derived.

The current (2008) and proposed (2016) definitions are given below.

Second

The proposed definition of the second is effectively the same as the current definition, the only difference being that the conditions under which the measurements are made are more rigorously defined.

Current definition: The second is the duration of 7009919263177000000♠9192631770 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, 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 7009919263177000000♠9192631770 when expressed in the unit Hz, which is equal to s⁻¹.

Metre

The proposed definition of the metre is effectively the same as the current definition, the only difference being that the additional rigour in 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 1/7008299792458000000♠299792458 of a second.

Proposed 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 7008299792458000000♠299792458 when expressed in the unit m⋅s⁻¹, where the second is defined in terms of the caesium frequency Δν_Cs.

Kilogram

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

The definition of the kilogram is due to change fundamentally—the current definition defines the kilogram as being the mass of the international prototype kilogram, which is an artefact and not a constant of nature, whereas the new definition relates it to the equivalent energy of a photon via the Planck 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, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6966662607015000000♠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 is undergoing a major revision—the current definition, which is difficult to realise with high precision in practice, is being replaced by a definition that is more intuitive and that is easier to realise.

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 6993200000000000000♠2×10⁻⁷ newton per metre of length.

Proposed 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 6981160217663400000♠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.

Since the current definition contains a reference to force, which has the dimensions MLT⁻², it follows that in SI the kilogram, metre and second, the base units representing these dimensions, must be defined before the ampere can be defined. Other consequences of the current definition are that in SI the value of vacuum permeability (μ₀) is fixed at exactly 6993400000000000000♠4π×10⁻⁷ H⋅m⁻¹. Since the speed of light in vacuum (c) is also fixed, it follows from the relationship

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

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

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

that the impedance of free space (Z₀) likewise has a fixed value.

A consequence of the proposed changes to the definition of the ampere is that the definition will no longer depend on the definitions of the kilogram and the metre, but will still depend on the definition of the second. In addition, the vacuum permeability, vacuum permittivity and impedance of free space, which, in the current definition have exact values, will be subject to experimental error.

Kelvin

The definition of the kelvin will undergo a fundamental change if the proposal is accepted. Rather than using the triple point of water 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 1/273.16 of the thermodynamic temperature of the triple point of water.

Proposed 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 6977138064900000000♠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 makes the definition of the kelvin depend 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 Avogadro's number

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 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.

Proposed definition: The mole, symbol mol, is the SI 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. It is defined by taking the fixed numerical value of the Avogadro constant N_A to be 7023602214076000000♠6.02214076×10²³ when 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, the kilogram, and Avogadro's 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 constant

The draft SI brochure assumes the first will remain true, which would mean that the second will no longer be true. The molar mass constant, while still with great accuracy remaining equal to 1 g/mol, will no longer be exactly equal to that.

Candela

The proposed definition of the candela is effectively the same as the current definition, but has been rephrased with the only difference being the additional rigour in the definition of the second and metre will propagate to the candela.

Current definition: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000♠540×10¹² Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Proposed 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 7014540000000000000♠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.

对于基本单位定义的建议与影响

秒

提案中关于秒的定义与目前的定义是等效的，唯一的不同点在于测量的条件被更严格的指定了。

目前的定义：秒，是铯133原子基态两个超精细能阶间跃迁所对应的辐射，其周期 7009919263177000000♠9192631770 倍的时长。

建议的定义：秒，记为 s，是国际单位制中的时间单位。秒是借由指定铯133原子基态超精细能阶间跃迁所对应的辐射频率 Δν_Cs 的数值为 7009919263177000000♠9192631770 (Hz)来做为定义，其中 Hz 为 s^-1。

公尺

提案中关于公尺的定义与目前的定义是等效的，唯一的不同点在于秒被更严格的定义影响了公尺的定义。

目前的定义：公尺，是光在真空中走 1/ 7008299792458000000♠299792458 秒的长度。

建议的定义：公尺，记为 m，是国际单位制中的长度单位。公尺是借由指定真空中光速 c 的数值为 7008299792458000000♠299792458 (m·s^-1)来做为定义，这里秒是借由 Δν_Cs 定义的。

公斤

公斤的定义将会从根本上改变。目前是定义国际公斤原器的质量为一公斤，这是人造的定义而非源于对自然的测量。然而在新的定义中是借由光子的等效能量做为定义，之间是以普朗克常数作为联系。

目前的定义：公斤，是质量的单位。一公斤等于国际公斤原器的质量。

建议的定义：公斤，记为 kg，是国际单位制中的质量单位。公斤是借由指定普朗克常数 h 的数值为 6966662607015000000♠6.62607015×10⁻³⁴ (kg⋅m²⋅s⁻¹)来做为定义，这里秒与公尺分别借由 Δν_Cs 与 c 定义。

这个定义的改变，使得公斤的定义与秒和公尺的定义相关联。

安培

关于安培的定义目前正在做一个巨大的修改。目前的定义在实际上是难以达到高精度的测量，新的定义将会更易于实现。

目前的定义：若将两平行且半径可忽略的无限长直导线，置于真空中并使其相距一公尺。若通以1安培的定电流，他们会对彼此的导线产生作用力，大小将为 6993200000000000000♠2×10⁻⁷牛顿每公尺。

建议的定义：安培，记为 A，是国际单位制中的电流单位。安培是借由指定基本电荷 e 的数值为 6981160217663400000♠1.602176634×10⁻¹⁹ C来做为定义，其中库伦 C 等于 A·s。这里秒是借由 Δν_Cs定义。

由于目前安培的定义是与力相关的，力又与公斤、公尺和秒相关。因此公斤、公尺和秒必须在安培定义之前就预先定义。此外在国际单位制中真空中的磁导率 μ₀ = 6993400000000000000♠4π×10⁻⁷ H·m^-1 为固定的值。因为光速 c 也是固定的值，而且光速 c 满足以下的关系式

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

故真空电导率也会是固定的值，并且

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

因此真空中的阻抗也会是固定的值。

新的定义中关于安培的定义已经不再和公斤与公尺的定义相关，但仍和秒的定义相关。真空中的磁导率、真空中的电导率、真空中的阻抗，在目前的定义中他们都具有确定的数值，这点可能会导致实验上的错误。