# What is up with spin?

• atom888
In summary, the conversation discusses the concept of spin for subatomic particles. It is mentioned that most particles have spin, except for composite particles like the proton and pion. The definition of spin as an intrinsic degree of freedom is given, and it is clarified that it is not just a mathematical concept but can be measured. The history, derivation, and interaction of spin are also briefly mentioned. It is noted that spin does not necessarily have to be 1/2, and its effect on the energy state of particles can be seen in the Stark and Zeeman effects.

#### atom888

I have a few questions with spin. Actually, quite a alot.

1)What I know is proton and electron have spin. What subatomic particle doesnt' have spin?
2)What is spin?
3)Are they a physical thing for particles?
4)How did scientist measure it?
5)What is up and down spin mean?
6) why is it 1/2 instead of other value?
7) What are the actual interaction between same and different spin?
8) can we speed up or slow down the spin? If so, how?
9) does it take or give energy if spin can be change?

I apologize if the thread already exist. I could have go read wiki but I don't think they have all the answer. Besides... it's solid stuffs in here. lol Thx

1) proton is not an elementary particle, whereas the electron is. But for example the $K_0^*$ has spin zero. The pion is also a composite particle, which belongs to the meson family. You can look on wiki -> "Hadrons" and then "Baryons" and "Mesons". and Scalar mesons and so on. But spin for composite particles is not really the same as the spin concept for elementary particles.

2) Intrinsic spin is an intrinsic degree of freedom that a particle posess.

3) what do you mean by "a physical thing"? It is not just a mathemetical concept, one can measure the spin of particles.

4) Youn search on wiki for 'spin', there is a historical introduction to it. It has to

5) The projection of the spin vector on an reference vector. For each value on S, you can have 2S+1 different projections.

6) spin does not have to be 1/2, it can be 1 or 3/2 etc. The main point is that only integer and half-integer values are allowed. The derivation can be found in almost any QM textbook, e.g Sakurai - modern quantum mechanics.

7) The interaction is due to the coupling of magetic and electic moments and so on. Orbital motion and spinning motion of electric charges in the classical world gives rise to electromagnetic interactions. One can use the same analogy when one introduces orbital and spin- angular momentum in QM.

8) The thing is that spin in QM has nothing to do with particles moving in cirular orbits, as planets moving around the sun. It has nothing to do with that, so one can not "make the electron spin slower around its own axis" since it does not spin around its own axis in the first place.

9) Depends on the physica system. Generally, if you apply a magnetic field on atoms, then the different electrons will occupy an energy state that is most energetically favourble. And since the spin has to do with the magnetic interaction, the electrons with spin up will occupy a different energy state than an electron with spin down. Serach for "stark effect" and "zeeman effect"