What is the spin of a particle?


What is the spin of a particle?

In: Physics

“Spin” is the name given to a specific kind of inherent angular momentum in a particle that can take on discreet directions depending on the particle.

It doesn’t make lots of sense because there isn’t really any macroscopic counterpart.

As a note, you can take Quantum Mechanics classes, know all the properties of spin, use it to solve the wavefunction of the particle, and still not know “what” spin is.

Spin is a type of intrinsic angular momentum which some fundamental particles have. Angular momentum is basically the rotational equivalent of momentum, in classical physics the heavier an object and the faster it is spinning, the greater its angular momentum.

Electrons and other fundamental particles have spin, this acts exactly like angular momentum but it can’t be taken away from the particle, it is intrinsic to that particle. This means that mathematically it is very similar to the particle spinning around, for example in electrically charged particles like electrons the spin generates a magnetic field. This is just an analogy though, nothing is physically spinning.

There were some new properties identified in looking at subatomic particles, and so they needed names. The names are: spin, charge, and mass. Charge and mass mean the same thing they mean for ordinary particles, except charge has values like ±1/3 and ±2/3 unlike their normal integer values. Spin is related to angular momentum in a way that’s not very intuitive, so let’s just consider it another generic parameter.

Naming things is hard, when they are invisible, leading to generations as a term that has nothing to do with biology, not to mention charm and strange. I suppose it could have been better if they just made up words, rather than recycling words, but then we’d also be spelling them wrong.

Imagine you have a collection of balls, and they differ in three ways: some are red and some are green; some are big, some medium, some small; some are heavy and some are light.

To uniquely identify a type of ball you clearly need three pieces of information: colour, size and weight. But you don’t need the exact value on a continuous scale; just knowing that a ball is “big” is sufficient. These properties are “quantised” which means that they can only take certain values.

Well it turns out that subatomic particles can also be sorted into types by specifying the value of a number of quantised parameters, called “quantum numbers”. So for example an electron needs 4 numbers to describe it. One number corresponds to how much energy the electron has, and two correspond to the pattern it makes as it goes round the nucleus. The fourth number doesn’t really correspond to anything we can easily describe, but it got called “spin”.