# Spin angular momentum operator queries

Gold Member
Hello,

For the spin angular momentum operator, the eigenvalue problem can be formed into matrix form. I will use ##S_{z}## as my example

$$S_{z} | \uparrow \rangle = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \begin{pmatrix} 1 \\ 0 \end{pmatrix} = \frac {\hbar}{2} \begin{pmatrix} 1 \\ 0 \end{pmatrix}$$

$$S_{z} | \downarrow \rangle = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \begin{pmatrix} 0 \\ 1 \end{pmatrix}= \frac {-\hbar}{2} \begin{pmatrix} 0 \\ 1 \end{pmatrix}$$

Why is it that the matrix on the left hand side is assumed to be a 2x2 matrix? Also, why is it when solving for a,b,c, and d the second equation is used. I don't understand how I can just say

$$\begin{pmatrix} a \\ c \end{pmatrix} \begin{pmatrix} 1 \\ 0 \end{pmatrix} = \frac {\hbar}{2} \begin{pmatrix} 1 \\ 0 \end{pmatrix}$$
$$\begin{pmatrix} b \\ d \end{pmatrix} \begin{pmatrix} 0 \\ 1 \end{pmatrix}= \frac {-\hbar}{2} \begin{pmatrix} 0 \\ 1 \end{pmatrix}$$

I don't understand how I can just isolate the second column of the 2x2 matrix for the spin down case, and isolate the first column for spin up.

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blue_leaf77
Homework Helper
Why is it that the matrix on the left hand side is assumed to be a 2x2 matrix?
Those two equations are only for particles with spin 1/2 such as electron. Following the commutation relations for angular momenta, which is satisfied by any angular momentum operator, the z component of an angular momentum L can only be between -L and L with unit increment. That's why spin operator matrix for spin 1/2 particle must be of size 2x2.
I don't understand how I can just isolate the second column of the 2x2 matrix for the spin down case, and isolate the first column for spin up.
To be honest this is also my first time seeing such expressions. But I think you can understand it by viewing the matrix multiplication using coordinate-by-column rule. If you had known this, you can see that such way of writing can only be justified because either for spin up or down vector, one of the element is zero.

Gold Member
Could you explain the part about the matrix size in more detail? I still don't understand how one follows from the other

blue_leaf77