ConstantID

Thomson cross section

Atomic & nuclearMeasured

σe = 6.652 458 7051(62) × 10−29 m2

Value
6.652 458 7051 × 10−29 m2
Standard uncertainty
6.2 × 10−38 m2
Relative uncertainty
9.3 × 10−10 (932 parts per trillion)
Status
Measured
Symbol
σe
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# Thomson cross section, CODATA 2022 (NIST)
thomson_cross_section = 6.6524587051e-29  # m^2, standard uncertainty 6.2e-38

# SciPy: scipy.constants.physical_constants['Thomson cross section']

What it is

The effective target area a free electron presents to low-energy electromagnetic radiation. It governs how light is scattered in plasmas, stellar interiors and the early universe.

Equations

  • σe=8π3re2definition

In other units

UnitValue
CGS (cm2)6.652 458 7051 × 10−25
b barn0.665 245 870 51

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. The relative uncertainty is unchanged by conversion to an exactly defined unit.

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).

Thomson cross section: CODATA values 1998–2022 relative to the 2022 value, in parts per billion, with one-standard-uncertainty bars−40−200+20+40parts per billion1998200220062010201420182022

Relative uncertainty

Thomson cross section: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻¹⁰10⁻⁹10⁻⁸10⁻⁷1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20226.652 458 7051(62) × 10−299.3 × 10−10 −4.5 σ of 2018
20186.652 458 7321(60) × 10−299.0 × 10−10 +1.8 σ of 2014
20140.665 245 871 58(91) × 10−281.4 × 10−9 −1.4 σ of 2010
20100.665 245 8734(13) × 10−282.0 × 10−9 +6.5 σ of 2006
20060.665 245 8558(27) × 10−284.1 × 10−9 −1.3 σ of 2002
20020.665 245 873(13) × 10−282.0 × 10−8 +1.3 σ of 1998
19980.665 245 854(15) × 10−282.3 × 10−8

“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).

Related constants

Atomic & nuclear re Classical electron radius 2.817 940 3205(13) × 10−15 m Atomic & nuclear a0 Bohr radius 5.291 772 105 44(82) × 10−11 m Atomic & nuclear λC Compton wavelength 2.426 310 235 38(76) × 10−12 m Atomic & nuclear α Fine-structure constant 7.297 352 5643(11) × 10−3 Atomic & nuclear Eh Hartree energy 4.359 744 722 2060(48) × 10−18 J Atomic & nuclear Eh Hartree energy in eV 27.211 386 245 981(30) eV Atomic & nuclear α−1 1/α 137.035 999 177(21) Atomic & nuclear πℏ/me Quantum of circulation 3.636 947 5467(11) × 10−4 m2 s−1