What Is Spin?

Origin of magnetism

Particles have a very interesting and complicated characteristic called spin. It is mostly true to think of this as an intrinsic angular momentum, however the name “spin” is misleading. The particles do not actually spin. It is a fundamental property of particles that gives charged particles, like electrons, a small magnetic field called the magnetic moment. This magnetic moment, along with the orbital motion of electrons, causes magnetism.

There is a problem with the magnetic fields, though. For the charge and size of the electrons, they would have to be spinning faster than the speed of light in order to produce the fields they give off. Obviously, this is impossible. The classical picture of a small, definite particle spinning about its axis needs to be thrown away in order to accurately describe particles. Particles are more like smears of probability of their location and momentum as described by Heisenberg’s uncertainty principle.

Quanta and the Pauli Exclusion Principle

Spin is a quantized property. This means that for spin, there exists discrete values describing it. A particle’s spin cannot be sped up nor slowed down, but exists only in incremental values of ħ, where ħ is the reduced Planck constant. Spin can have values increasing or decreasing from 0 in increments of  ħ/2. The direction of spin is only either up or down and is able to change. Research is being done in the field of spintronics to take advantage of an electron’s ability to change spin direction. Powerful computers can be made using this method.

A rather odd property of spin is that for a particle of spin 1/2, it would have to be rotated twice for it to look the same. If you imagine a basketball, you would only need to rotate it once for it to look the same. For spin 1 particles, they obey intuition and will look the same when rotated once. Two oddballs include the Higgs Boson (spin 0) and the graviton (spin 2, if it exists). The spin number of a particle gives way to the Pauli Exclusion Principle.

The Pauli Exclusion Principle (PEP) states that particles with the same spin cannot exist in the same place as each other. Spin 1 particles are called bosons and they do not obey the PEP. These particles mediate the four fundamental forces of nature such as the weak, strong, and electromagnetic forces. The theoretical particle which would mediate gravity, the graviton, has yet to be discovered if it exists. All other particles obey the PEP and are called fermions. These fermions are the reason why your body stays together! If it were made of bosons, all of the particles would essentially phase together. For more on particles, click here.

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