Hyperfine transition frequency of Cs-133
= 9 192 631 770 Hz
- Value
- 9 192 631 770 Hz
- Standard uncertainty
- exact
- Relative uncertainty
- exact
- Status
- Exact by definition (the second, 1967)
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# hyperfine transition frequency of Cs-133, CODATA 2022 (NIST)
dnu_Cs = 9.19263177e9 # Hz, exact
# SciPy: scipy.constants.physical_constants['hyperfine transition frequency of Cs-133']What it is
The frequency of the microwave radiation absorbed or emitted when the outermost electron of an undisturbed caesium-133 atom in its ground state flips between the two hyperfine energy levels. The second is defined as exactly 9 192 631 770 periods of this radiation, and caesium atomic clocks realise it.
The second has been defined from this frequency since 1967; the 2019 revision restated the definition as a fixed value of the constant.
Equations
- the SI second
In other units
| Unit | Value | |
|---|---|---|
| kHz kilohertz | 9 192 631.77 | exact |
| MHz megahertz | 9192.631 77 | exact |
| GHz gigahertz | 9.192 631 77 | exact |
| THz terahertz | 0.009 192 631 77 | exact |
Converted with ConstantID's unit table; digits are rounded to the precision of the value, and a long exact value is rounded only for display.
How the value changed, 1998–2022
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 9 192 631 770 | exact | exact |
| 2018 | 9 192 631 770 | exact |
“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).