I started trying to write about this...oh, maybe two months ago. I finally cobbled something together and posted it but I swept something under the rug that is very embarassing. It turns out I don't know how to count spin states. I thought I did, but I didn't. I'm going to try and explain the problem.

If you have one electron, there are two spin states. People say the spin must be either up or down, but that's nonsense. The spin can be in any direction, but those directions are

*expressible*as the superposition of an "up" state and a "down" state. The total spin is one-half.

If you have two electrons, there are now four spin states. Both up, both down, and one-up/one-down. Wait...that's only three spin states. Not exactly...it turns out that depending on the relative phase of the up-down combination, they can form a spin-zero state. They don't

*have*to...they can form a spin-one state in a superposition of left- and right-spin. That's not spin-zero. There is a unique spin-zero state that has no spin anywhere, called the "singlet state". I've written about it here.

So for two electrons, you can describe the total spin by giving the complex amplitudes for the up- and down- spings of both electrons...that's four "basis" states. Or you can be more "elegant", as they say, and describe the total spin as the three amplitudes for the triplet states...that is, (+1, 0, or -1) along the z-axis, plus the amplitude for the singlet states. Either way, it's four basis states.

With three electrons, it's the same. You have now six amplitudes to describe the individual electrons. Or, you can group them in states...they can form either a spin-3/2 state with four z-axis levels, or a spin-1/2 state with two z-axis levels. Six states.

With four electrons...now bear with me...it's the same.; You now have eight amplitudes to describe the individual electrons. Or, you can group them into spin-2 states (five z-axis levels), spin-1 states (3 z-axis levels), and a spin-zero state. Eight states.

No...wait. There are

*nine*states if you group them into total-spin states, and only

*eight*if you just list the individual electron states.

And it gets worse. For five electrons, you have ten numbers to describe the individual spins. Or you can group them by total spin: spin-5/2 (six states), spin-3/2 (four states), and spin-1/2 (two states).

I don't think you can have more spin states than you get by listing the states of each individual electron. But at the same time, I don't see why there is any redundancy in describing a box of 21 electrons as having such-and-such amplitude to be l=11, m=6, such-and-such amplitude to be l=9, m=7, plus whatever. Because it works perfectly for a two-electron system. You have such-and-such amplitude to be in each of the triplet states, plus an amplitude to be in the singlet state. Why does this system break down when we add the fourth electron?

So there it is. I don't know how to count spin states. But at least I

*know*that I don't know how to count them. That counts for something, doesn't it?

## 2 comments:

Start thinking like Erwin Schrodinger

I hope this a call for you: http://wavewatching.net/2013/05/25/the-meaning-of-wave-mechanics-and-the-mongol-physics-project/

Thanks for the tip, UFO. Yes, that's definitely my territory. I went to that website and also to the "Mongol Physics Project" and posted some links to my articles.

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