Spin (physics)

In physics, spin is an intrinsic angular momentum associated with microscopic particles. It is a purely quantum mechanical phenomenon without any analogy in classical mechanics. Whereas classical angular momentum arises from the rotation of an extended object, spin is not associated with any rotating internal masses, but is intrinsic to the particle itself. Elementary particles such as the electron can have non-zero spin, even though they are believed to be point particles possessing no internal structure. The concept of spin was introduced in 1925 by Ralph Kronig, and independently by George Uhlenbeck and Samuel Goudsmit.

Properties of spin

As a quantum mechanical property, spin possesses a number of qualities that distinguish it from classical angular momentum. It is quantized, and can only take on discrete values. For instance, the spin angular momentum of an electron, measured along any particular direction given by an external magnetic field, can only take on the values hbar/2 or -hbar/2 (where hbar is Planck's constant divided by 2π). Furthermore, the magnitude of the spin (a direction-independent quantity) is uniquely determined by the type of particle. Electrons are said to be "spin-half" particles, because the magnitude of every electron's spin is one half times hbar. Other spin-half particles include neutrinos, protons, and neutrons. Photons are spin-one particles, and the hypothetical graviton is a spin-two particle. Certain exotic particles, such as pions, possess spin zero. The principles of quantum mechanics indicate that spin is restricted to integer or half-integer values, at least under normal conditions.

Related Topics:
Planck's constant - Neutrino - Proton - Neutron - Photon - Graviton - Pion - Half-integer

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Mathematically, spin is not described by a vector, unlike classical angular momentum. It is described by objects known as spinors, which act differently from vectors under coordinate rotations.

Related Topics:
Vector - Spinor - Coordinate rotation

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It turns out that the spin of a particle is closely related to its properties in statistical mechanics. Particles with half-integer spin obey Fermi-Dirac statistics, and are known as fermions. They are subject to the Pauli exclusion principle, which forbids them from sharing quantum states, and are described in quantum theory by "antisymmetric states" (see the article on identical particles.) Particles with integer spin, on the other hand, obey Bose-Einstein statistics, and are known as bosons. These particles can share quantum states, and are described using "symmetric states". The proof of this is known as the spin-statistics theorem, which relies on both quantum mechanics and the theory of special relativity. In fact, the connection between spin and statistics is one of the most important and remarkable consequences of special relativity.

Related Topics:
Statistical mechanics - Fermi-Dirac statistics - Fermion - Pauli exclusion principle - Quantum state - Identical particles - Bose-Einstein statistics - Boson - Spin-statistics theorem - Special relativity

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Particles with spin possess a magnetic moment, just like a rotating electrically charged body in classical physics. However, this magnetic moment exists even for point particles like the electron, and for electrically neutral particles like the neutron. This magnetic moment can be experimentally observed, by the deflection of particles by inhomogeneous magnetic fields (as in the Stern-Gerlach experiment) or by the magnetic fields generated by the particles themselves. In fact, ferromagnetism arises from the alignment of the spins of the atoms in a solid.

Related Topics:
Magnetic moment - Electrically charged - Magnetic field - Stern-Gerlach experiment - Ferromagnetism

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