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In physics, the finestructure constant (usually denoted α, the Greek letter alpha) is a fundamental physical constant, namely the coupling constant characterizing the strength of the electromagnetic interaction. The numerical value of α is the same in all systems of units, because α is a dimensionless quantity. Arnold Sommerfeld introduced the finestructure constant in 1916.
Contents
Definition
Three equivalent definitions of α in terms of other fundamental physical constants are:
where:
In electrostatic cgs units, the unit of electric charge, the statcoulomb, is defined so that the Coulomb constant, k_{e}, or the permittivity factor, 4πε_{0}, is 1 and dimensionless. Then the expression of the finestructure constant becomes the abbreviated
an expression commonly appearing in physics literature.
Measurement
According to 2006 CODATA, the defining expression and recommended value for α are:^{[1]}
However, after the 2006 CODATA adjustment was completed, an error was discovered in one of the main data inputs.^{[2]} Nevertheless, the 2006 CODATA recommended value was republished in 2008.^{[3]} A revised standard value, taking recent research and adjustments to SI units into account, is expected to be published in 2010 or early in 2011.
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