Spin

Spin is one of the angular momenta carried by particles. It is considered an intrinsic property along the lines of electric charge and mass.

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As a metaphor, consider the Earth in its motion around the sun. The angular momentum of the Earth due to the orbit is akin to the orbital angular momentum of a particle. The angular momentum of the Earth about its own axis due to its rotation represents the spin.

Every particle can be classified as a boson or fermion by its spin

Bosons are said to have integer spin as a result of their symmetric wave functions. This means they are capable of occupying the same state. This is leveraged in applications such as LASER and superfluids. While the elementary particles classified as bosons are the charge carriers and the Higgs, atoms can also be bosons if they have integer spin.

Fermions are said to be spin-$\frac12$ as a result of their antisymmetric wave functions. Thus they are incapable of occupying the same state and forming condensates. The elementary fermions are quarks and leptons (electrons, muons, taus, and neutrinos) while composite fermions include protons and neutrons. Collectively, fermions are known are as mass.

While helium 4 is a boson, the much less common isotope helium 3 actually has spin-1/2 and thus is classified as a fermion.^[2] The isotope helium-4 has spin-0. Is it classified as a boson or a fermion?

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Only bosons are permitted to have spin-0, so $^4\text{He}$ is classified as a boson. And in fact, at low temperatures, it forms liquid helium which exhibits superfluid properties.

1. Spoon, S. Earth's axis. Retrieved May 4, 2016, from https://commons.wikimedia.org/wiki/File:Earth%27s_Axis.gif
2. W., H. Helium 3 and Helium 4. Retrieved May 4, 2016, from https://commons.wikimedia.org/wiki/File:Helium-3_and_Helium-4.gif