Spin
In quantum mechanics, spin is a magnitude, or quantum number, associated with the particles, which contributes to defining its quantum state. Spin is a form of angular momentum, having such a physical dimension in size and, although there is no corresponding magnitude in classical mechanics, by analogy recalls the rotation of the particle around its axis. However, unlike macroscopic objects, for which the angular momentum is associated with the mass, for spin it is not required: for example, photons, which have mass at zero rest, or elementary particles such as electrons, which are considered point-like, they have a spin. In addition, contrary to classical rotation, in the case of semi-integral value, the spin is described by a two-component object instead of a vector, with respect to which it is transformed by rotating the coordinates by a different process. Spin is not provided by non-relativistic quantum mechanics, where it is introduced as ad hoc size; is instead provided by the relativistic version through the Dirac equation.