See Markus Ehrenfried's easy to understand description of spin here: http://markusehrenfried.de/science/physics/hermes/whatisspin.html

In Late Professor Stephen Hawking's book that spin 2 means if you rotate a particle by 180 (360 ÷ 2) degrees it will look same. For spin 1 (360 ÷1) particle rotation is 360 degrees, for 0 spin any rotation will make it look same but for spin half (1/2) particle to make it look same rotation must be of 720 degrees (360 ÷ 1/2) [source unknown]

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

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)

"When our senses tell us that a speeding bullet is not a direction of motion it is difficult to think of it as stillness which is centering fast moving motion. It might help to remold your concept by imagining a motionless, floating cloud which changes its position because of a wind. The direction of motion is not in the cloud.

It is in the spiraling of the wind. The cloud is still in the same position in relation to gravity even though its position has been changed by motion, which is outside of itself, for it is still floating at rest. Even the slightest change in its position of stillness causes a spiral motion to encircle it and spin around it at an angle of 90 degrees from its gravity shaft, which extends from the earth. It may be quite an effort to comprehend that stillness cannot change, but can seem to." [Atomic Suicide, page 285-286]

"Their particles are similar to speeding bullets, or equators of planets and suns which always spin in spiral rings around their gravity shafts, and never in the direction shown in their explanatory drawings." [Atomic Suicide, page 286]

"If you should cause an explosion in the very center of a perfectly spherical room you would form spherical layers of increasingly dense pressures with maximum density at the surface of the sphere. The center of the sphere would be maximum in vacuity. The explosion would be symmetrically radial. The reaction to that explosion would also be its reverse. The reflections which would return by radar from the spherical walls of that room would collide at its very center. Compression would then be exerted from the outside and density would increase in the direction of the center. Nature does not work that way, however. Nature causes her explosions to take place as though they were confined within the flat walls of a room of four or many walls of such shapes as we see in crystals. If you caused such an explosion in your six sided room the outward expansion would no longer be even. It would not even be spherical because of the four corners, which would have to be filled. The outward explosion could no longer produce straight radial lines, which would reflect back in straight radial lines. Every radial line would have to curve in the direction of its corners, and as they approached those corners their curvature would twist and increase in speed as they approached the corners. In a sphere all radial lines are equal, but in a cube the diagonals are longer than the diameters. This fact accounts for the curvature, the spin and the shaft. It also accounts for the disappearance of all curvature." [Atomic Suicide, page 289]

"Spinning rings then move around a center, instead of an extended shaft of gravity, however, caused by the motion of the propellers. The plane itself would then occupy a position of stillness. Can you not see that it would always occupy a position of stillness, no matter where it seemingly moved? And can you not see that the spinning rings are the cause of that seeming motion? And can you not further see that the direction of motion is not in the projectile, or the plane, or of gravity, but solely in the divided pressures which cause that spinning motion?" [Atomic Suicide, page 292]

See Also

angular momentum
half integer spin
integer spin
Laws of Being
Modes of Vibration
orbital angular momentum
spin angular momentum
spin-orbit coupling
spinning effect
Spinning rings
spiral motion
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