Fine-structure constant

The fine-structure constant is a dimensionless measure of electromagnetic interaction strength, approximately 1/137 at low energies. It sets the scale of electromagnetic effects in quantum physics.

Elementary charge: The magnitude of the electric charge carried by a proton or oppositely by an electron. The constant’s low-energy value is proportional to the square of this charge.
Perturbation theory: A method for approximating solutions by expanding around a simpler, solvable problem. Small powers of the constant make many electromagnetic calculations tractable.
Quantum Hall effect: The quantization of transverse electrical resistance in a two-dimensional electron system under a strong magnetic field. Its quantized resistance is linked to the conductance quantum and the electromagnetic coupling.
CODATA recommended values: Internationally recommended values of fundamental physical constants, assessed from experimental data. These evaluations combine independent measurements to estimate the constant and its uncertainty.
Vacuum permittivity: A physical constant that relates electric charge to the electric field it produces in vacuum. It appears in the SI expression for electromagnetic coupling strength.
Fine structure: Small splittings in atomic spectral lines caused by relativistic and spin-dependent effects. The constant sets the size of these corrections relative to gross atomic energies.
Josephson effect: The flow of superconducting current between weakly coupled superconductors, including across an insulating barrier. Josephson voltage standards help realize electrical units used in precision measurements.
Running coupling constant: A coupling whose effective value changes with the energy scale in a quantum field theory. The electromagnetic constant is scale-dependent beyond its low-energy value.
Speed of light: The speed at which light propagates in vacuum, denoted by c. It enters the constant’s definition and helps make the combination dimensionless.
Lamb shift: A small energy difference between hydrogen’s 2S and 2P levels, explained by quantum electrodynamic effects. Its theoretical prediction depends on electromagnetic corrections governed by the constant.

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