# Otimes notation and tau matrices used in definition of gamma matrices?

1. Jun 19, 2011

### Peeter

In Zee's Quantum Field theory book he writes

\begin{align}\gamma^0 &= \begin{bmatrix}I & 0 \\ 0 & -I\end{bmatrix}=I \otimes \tau_3 \\ \gamma^i &= \begin{bmatrix}0 & \sigma^i \\ \sigma^i & 0\end{bmatrix}=\sigma^i \otimes \tau_2 \\ \gamma^5 &=\begin{bmatrix}0 & I \\ I & 0\end{bmatrix}=I \otimes \tau_1 \end{align}

The Pauli matrices $\sigma^i$ I've seen. However, I have two questions

1) What are these $\tau_i$ matrices?
2) I'm not familiar with this $\otimes$ notation.

2. Jun 20, 2011

### strangerep

3. Jun 20, 2011

### Peeter

I find that the tau matrices are just the sigma matrices. Odd that two different notations would be used. One notation when defining the gamma matrices in terms sigmas directly, and an entirely different notation when using the tensor product?

4. Jun 21, 2011

### strangerep

... which is what Zee says beneath his eq(4) on p90.

I don't use tau's myself.

BTW, your earlier expression for gamma^i doesn't match Zee's.