Fine-structure constant

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In physics, the fine-structure 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 fine-structure 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, ke, or the permittivity factor, 4πε0, is 1 and dimensionless. Then the expression of the fine-structure 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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