>[!abstract] >In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by $\alpha$, is a fundamental physical constant that quantifies the strength of the electromagnetic interaction between elementary charged particles. > >It is a dimensionless quantity independent of the system of units used, which is related to the strength of the coupling of an elementary charge e with the electromagnetic field, by the formula $\alpha = \dfrac{e^2}{4 \pi {\epsilon}{_0} \hbar c}$ where: >- $e$ is the elementary charge; >- ${\epsilon}{_0}$ is the electric constant; >- $\hbar$ is the reduced Planck constant; >- $c$ is the speed of light. > >(Wikipedia, 2025). >[!quote] >There is a most profound and beautiful question associated with the observed coupling constant, $e$ – the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) > >Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by humans. You might say the "hand of God" wrote that number, and "we don't know how He pushed His pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out – without putting it in secretly! (Richard Feynman) >[!related] >- **North** (upstream): — >- **West** (similar): — >- **East** (different): — >- **South** (downstream): —