Caesium-133 defining frequency
ΔνCs denotes the unperturbed ground-state hyperfine transition frequency of caesium-133. Its numerical value is fixed exactly in SI units.
Key knowledge
ΔνCs = 9 192 631 770 Hz, exactly by the SI definition.
Reference values
ΔνCs9192631770Hz
Exact value · SI definitionValues and equations
ΔνCs = 9 192 631 770 Hz
- ΔνCs
- ΔνCs denotes the unperturbed ground-state hyperfine transition frequency of caesium-133. Hz
Conditions and scope
- Refers to the specified unperturbed atomic transition used to define the second.
Exact definitions do not remove uncertainty from practical measurements.
Sources · 5
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Bureau International des Poids et Mesures (BIPM)3 sources
- SI defining constantsBureau International des Poids et Mesures (BIPM)Reference guide
- The International System of UnitsBureau International des Poids et Mesures (BIPM)Reference guide
- The International System of Units, 9th edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guide
- Meet the ConstantsNational Institute of Standards and Technology (NIST)Reference guide
- 国際単位系(SI)第9版 · Japanese translation, V3.01National Metrology Institute of Japan, AISTReference guide · PDF
Evidence and review
Exact SI value, symbol, unit and defining scope.
2026-10-09 · English value and scope checked against BIPM; value compared with NIST. This review covers the definition only.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Agreement within this scope
The unperturbed caesium133 ground-state hyperfine transition defines the second through exactly 9192631770 Hz.
The unperturbed transition is the reference; actual clocks need corrections and carry uncertainty.
National Metrology Institute of Japan, AIST
- 国際単位系(SI)第9版 · Japanese translation, V3.01 JapanJapaneseShared standard
2.2 Table1; 2.3.1 Second, printed pp14–15
- 国際単位系(SI)第9版 · Japanese translation, V3.01
Entry details
Permanent link to this entry ↗- Record identity
- si-caesium-frequency
- Revision
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- Structured reference record
- SI