PHYSICAL CONSTANTS / SI-2019
物理常数Physical Constants
2019 年 5 月 20 日起，国际单位制（SI）由七个自然常数重新定义——单位不再依赖实物基准，而是锚定在不变的物理常数上。我为每个常数做了一个科普网站：点击数字即可访问。
Since 20 May 2019, the International System of Units (SI) has been redefined by seven constants of nature — units no longer depend on physical artefacts, but are anchored to unchanging constants. I built one science-popularization site for each constant: click the number to visit.
c
真空中的光速Speed of light in vacuum
299792458 m/s
定义"米"：光在真空中于 1/299792458 秒内行进的距离Defines the metre: the distance light travels in vacuum in 1/299792458 of a second
h
普朗克常数Planck constant
6.62607015 × 10-34 J·s
定义"千克"：取代国际千克原器，质量从此锚定量子常数Defines the kilogram: replacing the physical artefact, mass is now anchored to a quantum constant
ΔνCs
铯-133 超精细跃迁频率Caesium-133 hyperfine transition frequency
9192631770 Hz
定义"秒"：铯-133 基态两个超精细能级间跃迁辐射的周期数Defines the second: the number of periods of radiation from the hyperfine transition of caesium-133
e
基本电荷Elementary charge
1.602176634 × 10-19 C
定义"安培"：电流单位由单个电子的电荷量锚定Defines the ampere: the unit of current anchored to the charge of a single electron
NA
阿伏伽德罗常数Avogadro constant
6.02214076 × 1023 mol⁻¹
定义"摩尔"：1 摩尔即恰好包含 6.02214076×10²³ 个基本实体Defines the mole: one mole contains exactly 6.02214076×10²³ elementary entities
k
玻尔兹曼常数Boltzmann constant
1.380649 × 10-23 J/K
定义"开尔文"：温度单位由能量与温度的换算关系锚定Defines the kelvin: the unit of temperature anchored to the energy–temperature relation
Kcd
发光效能常数Luminous efficacy constant
683 lm/W
定义"坎德拉"：频率 540×10¹² Hz 单色辐射的发光效能Defines the candela: the luminous efficacy of monochromatic radiation at frequency 540×10¹² Hz
#为什么是这七个数Why These Seven Numbers
在 2019 年之前，千克的定义依赖保存在巴黎的一个铂铱合金圆柱体（国际千克原器），它会随时间发生极其微小的质量变化。新 SI 把全部七个基本单位都建立在恒定不变的自然常数之上：秒（ΔνCs）、米（c）、千克（h）、安培（e）、开尔文（k）、摩尔（NA）与坎德拉（Kcd）。数值一旦固定，单位制就与任何实物解绑——这就是"用宇宙本身做砝码"。
Before 2019, the kilogram was defined by a platinum–iridium cylinder kept in Paris (the International Prototype of the Kilogram), whose mass drifts ever so slightly over time. The new SI grounds all seven base units on invariant constants of nature: the second (ΔνCs), metre (c), kilogram (h), ampere (e), kelvin (k), mole (NA) and candela (Kcd). Once the numbers are fixed, the unit system is decoupled from any artefact — using the universe itself as the reference weight.