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See Markus Ehrenfried's easy to understand description of spin here: (external link)

Fermions and Bosons
Those particles with half-integer spins?, such as 1/2, 3/2, 5/2, are known as fermions, while those particles with integer spins, such as 0, 1, 2, are known as Bosons. The two families of particles obey different rules and broadly have different roles in the world around us. A key distinction between the two families is that fermions obey the Pauli exclusion principle; that is, there cannot be two identical fermions simultaneously having the same quantum numbers (meaning roughly, being in the same place with the same velocity). In contrast, Bosons obey the rules of Bose Einstein statistics? and have no such restriction, so they may "bunch" together even if in identical states. Also, composite particles can have spins different from the particles which comprise them. For example, a helium atom can have spin 0 and therefore can behave like a Bosons even though the protons, neutrons, and electrons which make it up are all fermions.

This has profound practical applications:

Fermions: Quarks and leptons (including electrons and neutrinos), which make up what is classically known as matter, are all fermions with spin-1/2. The common idea that "matter takes up space" actually comes from the Pauli exclusion principle acting on these particles to prevent the fermions that make up matter from being in the same quantum state. Further compaction would require electrons to occupy the same energy states, and therefore a kind of pressure (sometimes known as degeneracy pressure of electrons) acts to resist the fermions being overly close. [see resistance] It is also this pressure which prevents stars collapsing inwardly, and which, when it finally gives way under immense gravitational pressure in a dying massive star, triggers inward collapse and the dramatic explosion into a supernova?. [see overtones?, Law of Cycles]

Elementary fermions with other spins (3/2, 5/2 etc.) are not known to exist, as of 2013.

Bosons: Elementary particles which are thought of as carrying forces are all bosons with spin-1. They include the photon which carries the electromagnetic force, the gluon (strong force), and the W and Z Bosons (weak force). The ability of Bosons to occupy the same quantum state is used in the laser?, which aligns many photons having the same quantum number (the same direction and frequency), superfluid liquid helium resulting from helium-4 atoms being Bosons, and superconductivity where pairs of electrons (which individually are fermions) act as single composite Bosons.

Elementary Bosons with other spins (0, 2, 3 etc.) were not historically known to exist, although they have received considerable theoretical treatment and are well established within their respective mainstream theories. In particular theoreticians have proposed the graviton? (predicted to exist by some quantum gravity theories) with spin 2, and the Higgs Boson (explaining electroweak symmetry breaking) with spin 0.

As of 2013 the Higgs boson with spin-0 is considered proven to exist. It is the first scalar particle (spin-0) known to exist in nature.

Theoretical and experimental studies have shown that the spin possessed by elementary particles cannot be explained by postulating that they are made up of even smaller particles rotating about a common center of mass (see classical electron radius?); as far as can be presently determined, these elementary particles have no inner structure. The spin of an elementary particle is therefore seen as a truly intrinsic physical property, akin to the particle's electric charge and rest mass?. Wikipedia, Spin (external link)

In quantum mechanics and particle physics?, spin is a fundamental characteristic property of elementary particles, composite particles (hadrons), and atomic nuclei?.

All elementary particles of a given kind have the same spin quantum number, an important part of a particle's quantum state. When combined with the spin-statistics theorem?, the spin of electrons results in the Pauli exclusion principle, which in turn underlies the periodic table of chemical elements. The spin direction (also called spin for short) of a particle is an important intrinsic degree of freedom?.

Wolfgang Pauli? was the first to propose the concept of spin, but he did not name it. In 1925, Ralph Kronig?, George Uhlenbeck?, and Samuel Goudsmit? suggested a physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927. When Paul Dirac? derived his relativistic quantum mechanics in 1928, electron spin was an essential part thereof.

Spin is a type of angular momentum, where angular momentum is defined in the modern way (as the "generator of rotations", see Noether's theorem?). This modern definition of angular momentum is not the same as the historical classical mechanics definition, L=r×p. (The latter definition, which does not include spin, is more specifically called "orbital angular momentum".)

Since spin is a type of angular momentum, it has the same dimensions: J·s in SI units. In practice, however, SI units are almost never used to describe spin: Instead, it is written as a multiple of the reduced Planck's constant h. In natural units, the h is omitted, so the units of spin are implied. However, by definition the "spin quantum number?" is always dimensionless. (wikipedia)

See Also

angular momentum
half integer spin
integer spin
Modes of Vibration
Laws of Being
orbital angular momentum
spin angular momentum
spin-orbit coupling

Page last modified on Wednesday 18 of September, 2013 04:10:16 MDT