What's up with spin?

Question

We know that everything revolves, right? We know there is a deeper meaning behind these revolutions. But what do we really know about spin? Why spin? WHat does spin cause? How does spin affect our being? And since we know Spin, how come we can't figure out more about Spin?

Answer 1

People often confuse mechanical spin with particle spin. We know of practical examples of spin, such as the spin of the Earth and the spin of an ice skater, which all have angular momentum. If something has no angular momentum, then it's not spinning. That's not the case with particle spin. A particle may have no angular momentum and still possess spin! Particle spin was first theorized to explain Zeeman splitting of atomic spectral lines, introducing the concept of electron self-rotation. However, this kind of spin has quantized angular momentum, so for example, electrons have spin of 1/2, and other multiples of 1/2, which defines fermions, while photons have spin 0, bosons being defined as particles with integer spin numbers. The existence of particle spin can have profound consequences in the physics of particle interactions and properties of atoms. For example, the very reason why you cannot walk through a wall like a ghost is because you are mostly composed of fermions with 1/2 integral spins! The Pauli exclusion principle bars any two particles with 1/2 integral spins in occupying the same place, while particles with integral spins (like photons) may.

It's still a mystery to physicists as to the cause of particle spin, but it was first expected mathematically as a consequence of Dirac's equation fo the electron, which is the first relativistic quantum treatment of it. Apparently, it's a relativistic effect.

Addendum: In response to Epidavros' answer, if particle spin was not a mystery, physicists wouldn't still be trying to offer a conceptual explanation for it. Dirac's equation that first pointed to the existence of spin begs the question, "what's a spinor, physically speaking?". Here's a link to a typical aritcleon this subject. It's far from a closed subject.

Answer 2

Quantum spin is definitely not a mystery - it is a relativistic effect on the wavepacket (the relativistic form of the Schreodinger equation is the Dirac equation). It got misnamed because it is seen as a magnetic moment to the particle that makes it respnd to a magnetic field, and the only classical way to get magentism out of charge is to spin it. However, spin does not require charge and does not involve rotation.

Answer 3

All electrons have the same identical charge and weight. We can't "see" them to determine how two electrons may be different (color, shape, density, etc.). But the electrons orbiting an atom behave differently including how many of each kind can occupy an orbital path. Because an electron can hold energy either by rotating or moving in a straight (or circular) path, spin is an obvious "property" to assign to the different energy states of the electrons.

Answer 4

The statement that "a particle can have spin and no angular momentum" isn't really true. Spin contributes to angular momentum, it has units of angular momentum - I imagine that's why it's called spin.

Answer 5

Because it makes my head spin.


P.S. I hate it when the liberal TV media puts their SPIN on the facts.