spin (as a particle label)

A spinning top has a built-in twirl that gives it a kind of stubborn steadiness and a direction it points. Particles carry something that behaves a lot like that twirl, called spin — except that, unlike a top, a particle is not actually a little ball turning around. Spin is a fixed, built-in property each particle simply has, as much a part of its identity as its charge or mass.

Spin is intrinsic angular momentum: a quantity of rotation-like behaviour that a particle possesses without anything physically rotating. Its key feature is that it comes only in fixed amounts, measured in units of a basic constant: matter particles all have spin one-half (a half-integer value), while force carriers have whole-integer spin (one, and for the Higgs, zero). This single number splits all particles into two camps. Particles with half-integer spin are called fermions and they refuse to share a state (which is why matter is solid); particles with whole-integer spin are called bosons and they happily pile together (which is why a laser beam can be so intense). The link between spin and this behaviour, the spin-statistics connection, is one of the deepest results in physics.

Spin matters because, together with charge and mass, it is one of the three labels that fully identify what kind of particle you are looking at. It governs magnetism (a particle's spin makes it a tiny magnet), it underlies the structure of the periodic table through the exclusion principle, and it is essential to reading the products of a collision. The common misconception to avoid is taking the spinning-top picture literally: an electron is a structureless point, so there is nothing inside it to physically spin — spin is real and measurable, but it is a quantum property, not a tiny rotation.