Spin me once, spin me twice

[MargeB]

What is quantum electron spin, why is it a half, and why is it true that the electron has to spin twice on its own orbit to get where it started?

[Flurfl]

Here's a good link on the subject:

http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

The term Electron Spin is something of a misnomer in that it is thought that the electrons do not spin in the classical sense but that they have an intrinsic angular momentum, which causes them to behave as if they do spin. Things get weird when you get small ;-)

[jake-the-peg]

Now you need to ask about neutrinos and how something with no mass can have spin! (runs away and hides!!)

[MargeB]

Flurfl, thank you <clutches head in attempt to stop throbbing>
Jake, get the heck back here. It was EXACTLY what came to my mind.....please provide a full explanation. (En ingles por favor)

[jake-the-peg]

When you look at a television up close you don't see a picture but a lot of flickering lights  - physics is the same, we think we understand things like mass and spin and momentum but when you get up close they're not really like that. We only think that things need to have mass and shape to spin because we always see things like that really terms like spin are only analogies.

In 1930 the nuclear physics world was all a flutter because beta particle (high energy electrons emitted from the nucleus) were found to have a whole spectrum of energy rather than a series of discrete energies which had been expected. Pauli invented a new particle to account for this which had no mass but only angular momentum - this resolved the issue. Pauli actually bet a case of champagne that nobody would ever detect one though.

He got away with his bet until 1956 when he had to pay up to a team in the US.

I have to confess I didn't remember the whole story the details are here:

http://hitoshi.berkeley.edu/neutrino/neutrino1.html

They even tell you why neutrinos are always left handed

  

[MargeB]

Why are neutrinos always left handed?
Why is my daily life not all ****ed up, when everything that's made of anything is made of tiny little bits of stuff that go against all my assumptions?

[jake-the-peg]

It's a matter of scale, you know that air is trillions of tiny particles banging about but you don't have to know anything about that when you fly a kite because the individual motions even out and you think of the whole as a body and treat it as the wind.

Problems start to arise at the edges though when we start to consider extreme objects like black holes or the early moments of the universe all of a sudden you can't just even things out and need to understand what is actually happening in more detail.