What exactly is the 'spin' of subatomic particles such as electrons and protons?

Question

Does it have any physical significance, analogous to the spin of a planet?

Answer 1

Yes. And no. See an electron is a point and can't really rotate around a center of mass. However, it acts like it does in certain situations only it can only *rotate* at one *speed* in a *clockwise* or *counterclockwise* direction, where the *'s around things mean you shouldn't take this physical analogy very far. The spin of the electron only becomes apparent in a magnetic field, where the presence of the field differentiates the clockwise/counterclockwise rotation into two different energy levels. I am not even going to attempt explaining why this happens, google "electron spin resonance" for details.

The wikipedia entry on this is pretty good, in my opinion. But then I am a complete chowderhead so your mileage may vary.

http://en.wikipedia.org/wiki/Spin_(physics)

Early on in QM people realized they needed spin to explain not only the Stern-Gerlach experiment, but also how two electrons could occupy the same orbital in an atom (electrons are fermions and two fermions can't have the same quantum number). So they added a "spin" term and it explained a lot of other things as well, at least to other physicists. For the rest of us, well, we muddle along.