Fine-structure constant, about 1/137 — the strength of the electromagnetic interaction, a pure number.The fine-structure constant, denoted α, is a dimensionless fundamental constant that characterizes the strength of the electromagnetic interaction between charged particles. Its value is approximately 1/137.036, and it appears throughout quantum electrodynamics (QED), the quantum theory of light and matter. Because it is a pure number, it is independent of any system of units, making it a candidate for a truly fundamental constant of nature.
The fine-structure constant was introduced by Arnold Sommerfeld in 1916 to explain the fine splitting of hydrogen spectral lines. Its mysterious value, close to 1/137, has intrigued physicists ever since, including Paul Dirac and Richard Feynman, who called it 'one of the greatest damn mysteries of physics.'
In QED, α sets the probability amplitude for a charged particle to emit or absorb a photon. It determines the fine splitting of atomic energy levels (hence the name 'fine-structure'), the strength of the Coulomb force, and the rate of many electromagnetic processes. The constant is not truly constant: it 'runs' with energy scale due to quantum vacuum polarization, increasing slightly at higher energies, though at everyday energies it is effectively fixed at 1/137.
It is essential for precision calculations in atomic physics, quantum chemistry, and the design of high-precision instruments like atomic clocks and quantum sensors.
The fine-structure constant is so precisely measured that it is known to about 10 significant digits, yet its value cannot be predicted from any known theory — it must be measured from experiment.