Editing Physical constant (section)

=== International System of Units ===
{{Main | SI base unit}}

Since [[2019 revision of the SI|2019 revision]], all of the units in the [[International System of Units]] have been defined in terms of fixed natural phenomena, including three fundamental constants: the speed of light in vacuum, ''c''; the Planck constant, ''h''; and the [[elementary charge]], ''e''.<ref name="SI9th">{{SIbrochure9th}}.</ref>{{rp|128}}

As a result of the new definitions, an SI unit like the [[kilogram]] can be written in terms of fundamental constants and one experimentally measured constant, Δ''ν''<sub>Cs</sub>:<ref name=SI9th/>{{rp|131}}
: 1&nbsp;kg = {{math|{{sfrac|({{val|299792458}}){{sup|2}}|({{val|6.62607015|e=-34}})({{val|9192631770}})}}{{sfrac|{{gaps|''h''|Δ''ν''<sub>Cs</sub>}}|''c''{{sup|2}}}}}}.