Quantum Physics For Dummies, Revised Edition

Chapter 6

Getting Dizzy with Spin

In This Chapter

• Discovering spin with the Stern-Gerlach experiment
• Looking at eigenstates and spin notation
• Understanding fermions and bosons
• Comparing the spin operators with angular momentum operators
• Working with spin ½ and Pauli matrices

Physicists have suggested that orbital angular momentum is not the only kind of angular momentum present in an atom — electrons could also have intrinsic built-in angular momentum. This kind of built-in angular momentum is called spin. Whether or not electrons actually spin will never be known — they're as close to point-like particles as you can come, without any apparent internal structure. Yet the fact remains that they have intrinsic angular momentum. And that's what this chapter is about — the intrinsic, built-in quantum mechanical spin of subatomic particles.

The Stern-Gerlach Experiment and the Case of the Missing Spot

The Stern-Gerlach experiment unexpectedly revealed the existence of spin back in 1922. Physicists Otto Stern and Walther Gerlach sent a beam of silver atoms through the poles of a magnet — whose magnetic field was in the z direction — as you can see in Figure 6-1.

Figure 6-1: The Stern-Gerlach experiment.

Because 46 of silver's 47 electrons are arranged in a symmetrical cloud, they contribute nothing to the orbital angular momentum of the atom. The 47th electron can be in

• The 5s