Wolfgang Pauli’s life is as wonderful as his scientific legacy—but we’ll just talk about *one* of his many contributions to quantum mechanics here in this post—*not *about his life.

This post should be fairly straightforward. We just want to review some of the math. Indeed, we got the ‘Grand Result’ already in our previous post, as we found the Hamiltonian coefficients for a spin one-half particle—read: *all* matter-particles, practically speaking—in a *magnetic* field—but then we can just replace the *magnetic *dipole moment by an *electric* dipole moment, if needed, and we’ll find the same formulas, so we’ve basically covered everything you can possible think of.

[…] Well… Sort of… 🙂

OK. Jokes aside, we have a magnetic field **B**, which we describe in terms of its components: **B** = (B_{x}, B_{y}, B_{z}), and we’ve defined two mutually exclusive states – call them ‘up’ or ‘down’, or 1 or 2, or + or −, whatever − along *some *direction, which we call the *z*-direction. Why? Convention. Historical accident. The *z*-direction is the direction in regard to which we *measure *stuff. What stuff? Well… Stuff like the *spin *of an electron: quantum-mechanical stuff. 🙂 In any case, the *Hamiltonian *that comes with this system is:

Now, because this matrix doesn’t look impressive enough, we’re going to re-write it as:

* Huh? *Yes. It looks good, doesn’t it? And the σ

_{x}, σ

_{y}and σ

_{z}matrices are given below, so you can check it’s actually

*true*. […] I mean: you can check that the two notations are equivalent, from a math point of view, that is. 🙂

As Feynman puts it: “This is what the professionals use all of the time.” So… Well… Yes. We had better learn them by heart. 🙂

The identity matrix is actually *not *one of the so-called Pauli spin matrices, but we need it when we’d decide to *not *equate the average energy of our system to zero, i.e. when we’d decide to shift the zero point of our energy scale so as to include the equivalent energy of the rest mass. In that case, we re-write the Hamiltonian as:

In fact, as most academics want to hide their knowledge from us by confusing us deliberately, they’ll often omit the Kronecker delta, and simply write:

It’s OK, as long as you know what it is that you’re trying to do. 🙂 The point is, we’ve got *four *‘elementary’ matrices now which allow us to write *any *matrix – literally, *any *matrix – as a linear combination of them. In Feynman‘s words:

Now, the Pauli matrices have lots of interesting properties. Their *products*, for example, taken two at a time, are rather special:

The most interesting property, however, is that, when *chosing some other represenation*, i.e. when *changing to another coordinate system*, **the three Pauli matrices behave like the components of a vector**. That vector is written as **σ**, and so it’s a matrix you can use in different coordinate systems, as though it’s a vector. It allows us to re-write the Hamiltonian we started out with in a particularly nice way:

You should compare this to the classical formula for the energy of a little magnet with the magnetic moment **μ** in the same magnetic field:

There are several differences, of course. First, note that the *quantum-mechanical *magnetic moment is like the quantum-mechanical angular momentum: there’s only a limited set of *discrete *values, given by the following relation:

That’s why we write it as a *scalar *in the quantum-mechanical equation, and as a vector, i.e. in boldface (**μ**), in the second equation. The two equations differ more fundamentally, however: the first one is a *matrix *equation, while the second one is… Well… Just a simple vector dot product.

The point is: the classical energy becomes the Hamiltonian *matrix*, and the classical **μ **vector becomes the μ**σ** *matrix*. As Feynman puts it: “It is sometimes said that *to each quantity in classical physics there corresponds a matrix in quantum mechanics*, but it is really more correct to say that the Hamiltonian matrix corresponds to the energy, and any quantity that can be defined via energy has a corresponding matrix.”

[…]

What does he mean by a quantity that can be *defined via* *energy*? It’s simple: the magnetic moment, for example, can be *defined* via energy by saying that the energy, in an external field **B**, is equal to −**μ**·**B**.

* Huh?* Wasn’t it the other way around? Didn’t we

*define*the energy by saying it’s equal to −

**μ**·

**B**?

We did. In our posts on electromagnetism. That was classical theory. However, in quantum mechanics, it’s the *energy *that’s the ‘currency’ we need to be dealing in. So it makes sense to look at things the other way around: we’ll first think about the energy, and *then *we try to find a *matrix *that corresponds to it.

So… Yes. Many classical quantities have their quantum-mechanical counterparts, and those quantum-mechanical counterparts are often some *matrices*. But not all of them. Sometimes there’s just no comparison, because the two worlds *are *actually different. Let me quote Feynman on what he thinks of how these two worlds relate, as he wraps up his discussion of the two equations above:

Well… That says it all, doesn’t it? 🙂 We’ll talk more tomorrow. 🙂