electric charge and the QED coupling

Why does a magnet grip steel firmly but barely tug on a copper coin, and why does light pass through glass without shoving the atoms aside very hard? The answer is that the electromagnetic interaction has a definite strength, and that strength is set by electric charge. In quantum electrodynamics, an object's electric charge is also its 'coupling' — the number that decides how eagerly it emits and absorbs photons. More charge means stronger coupling means more vigorous interaction with light and with other charges.

Concretely, every time an electron emits or absorbs a photon, the calculation picks up a factor proportional to the electron's charge, written e. Because a typical process involves two such events (one charge emits, another absorbs), the probability comes out proportional to e times e, that is e squared. This combination, suitably scaled, is the fine-structure constant alpha, roughly 1/137. The smallness of that number is precisely why electromagnetism, though it runs our everyday world, is a fairly gentle force at the level of single particles, and why QED's step-by-step calculations converge.

The coupling is the dial that connects the abstract theory to measurable reality. A particle with twice the charge couples twice as strongly to the photon and shows up with four times the interaction probability. This is why quarks, with fractional charges of plus-or-minus one-third or two-thirds, interact electromagnetically more weakly than electrons, and why a neutral particle like the neutrino, with zero charge and zero coupling, ignores photons altogether and is so fiendishly hard to detect.