Fine-structure constant
Atomic & nuclearMeasured
= 7.297 352 5643(11) × 10−3
- Value
- 7.297 352 5643 × 10−3
- Standard uncertainty
- 1.1 × 10−12
- Relative uncertainty
- 1.5 × 10−10 (151 parts per trillion)
- Status
- Measured
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# fine-structure constant, CODATA 2022 (NIST)
alpha = 7.2973525643e-3 # standard uncertainty 1.1e-12
# SciPy: scipy.constants.physical_constants['fine-structure constant']What it is
A pure number, close to 1/137, that measures how strongly charged particles interact with light. Having no unit, it has the same value in every system of units, which is why physicists treat it as one of the most fundamental numbers in nature. Its best values come from the electron's magnetic moment and from atom-recoil measurements.
Equations
- definition
- Rydberg constant
- vacuum permeability since 2019
How the value changed, 1998–2022
Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).
Relative uncertainty
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 7.297 352 5643(11) × 10−3 | 1.5 × 10−10 | −4.5 σ of 2018 |
| 2018 | 7.297 352 5693(11) × 10−3 | 1.5 × 10−10 | +1.7 σ of 2014 |
| 2014 | 7.297 352 5664(17) × 10−3 | 2.3 × 10−10 | −1.4 σ of 2010 |
| 2010 | 7.297 352 5698(24) × 10−3 | 3.3 × 10−10 | +6.4 σ of 2006 |
| 2006 | 7.297 352 5376(50) × 10−3 | 6.9 × 10−10 | −1.3 σ of 2002 |
| 2002 | 7.297 352 568(24) × 10−3 | 3.3 × 10−9 | +1.3 σ of 1998 |
| 1998 | 7.297 352 533(27) × 10−3 | 3.7 × 10−9 |
“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).