Fine-structure constant

The fine-structure constant or Sommerfeld fine-structure constant, usually denoted lpha , is the fundamental physical constant characterizing the strength of the electromagnetic interaction. It was originally introduced into physics in 1916 by Arnold Sommerfeld, as a measure of the relativistic deviations in atomic spectral lines from the predictions of the Bohr model.

Related Topics:
Fundamental physical constant - Electromagnetic interaction - 1916 - Arnold Sommerfeld - Relativistic - Atom - Spectral line - Bohr model

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The fine-structure constant is a dimensionless quantity, and its numerical value is independent of the system of units used. The value recommended by 2002 CODATA is

Related Topics:
Dimensionless quantity - CODATA

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: lpha = 7.297 352 568(24) imes 10^{-3} = rac{1}{137.035 999 11(46)} .

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It can be defined as

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:lpha = rac{e^2}{hbar c 4 pi epsilon_0} = rac{e^2}{2 epsilon_0 h c}

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where e is the elementary charge, hbar = h/(2 pi) is the reduced Planck's constant, c is the speed of light in a vacuum, and epsilon_0 is the permittivity of free space.

Related Topics:
Elementary charge - Planck's constant - Speed of light - Permittivity

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In electrostatic cgs units, the unit of electric charge (the Statcoulomb or esu of charge) is defined in such a way that the permittivity factor, 4 pi epsilon_0 , is the dimensionless one. Then the fine-structure constant becomes

Related Topics:
Cgs - Electric charge - Statcoulomb

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:lpha = rac{e^2}{hbar c} .

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The fine-structure constant can also be thought of as the square of the ratio of the elementary charge to the Planck charge.

Related Topics:
Elementary charge - Planck charge

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:lpha = left( rac{e}{q_P} ight)^2 .

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