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Spin is a quantum-mechanical property of some particles. It has characteristics similar to those of angular momentum (a measure of the particle’s rotation speed), even in particles which do not rotate because they are point-like. It’s as if these particles had an inherent rotation built into them: that’s why spin is sometimes called “intrinsic angular momentum”. In fact, the modern definition of angular momentum accommodates spin.

Spin is measured in multiples of the reduced Planck’s constant ℏ. Some particles have half-integer spin: they are called fermions. Examples of fermions are electrons, quarks or muons. Some others (mostly force carriers) have integer spin, for example the photon or the graviton.

Spin may be positive or negative: particles with half-integer spin like electrons may have spin ½ or -1/2, depending on the direction of the rotation. Spin ½ is usually called “spin up” and -1/2 is usually called “spin down”.

Spin is also related to the statistical properties of ensembles of particles through the spin-statistics theorem.

For more information, visit Wikipedia.

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