THE FINE-STRUCTURE CONSTANT

Almost every constant in physics depends on the units you measure it in. The speed of light is 299,792,458 in metres per second and something else in furlongs per fortnight. The fine-structure constant is not like that: it is a combination of the elementary charge, the reduced Planck constant, the speed of light and the vacuum permittivity in which every unit cancels. What is left is a bare number, close to 1/137, that any civilisation anywhere would measure identically.

It sets the strength of the electromagnetic interaction — how readily charged particles emit and absorb photons — and through that it fixes the size of atoms, the fine splitting of spectral lines that gave it its name, and the chemistry that depends on both. Arnold Sommerfeld introduced it in 1916 while extending the Bohr model to elliptical orbits.

Its value has attracted more numerology than any other quantity in physics, mostly because 137 is close to an integer and to a prime. Arthur Eddington argued in the 1920s that the reciprocal had to be exactly 136, then exactly 137 when measurement moved, an episode physicists still cite as a caution. Modern measurement settles the matter: the reciprocal is 137.035999, known to about a part in a billion, and it is not an integer. Richard Feynman called it one of the greatest damn mysteries of physics — a magic number that comes to us with no understanding of where it comes from.

This entry is RESERVED. The idea is in the archive; whether it becomes an object depends on whether the number reads as identity rather than as trivia — the hardest of the three tests this collection applies.