[Physics] Tensors, indices and matrix notation – is there a common convention

notationtensor-calculus

For a tensor named T with two indices, there are four possibilities: $T_{ij}$ , $T_i^{\ j}$, $T^i{\ _j}$ and $T^{ij}$. Is there a common convention as to how these tensors would be represented as matrices, i.e. where the entries would go? Is it the left-right order of the indices that determines which matrix entry is meant, or some other convention? What if the the order of the indices in a mixed tensor is not indicated at all (as in $T_i^j$)? Is it true that, for instance, the component with i=2 and j=3 would go on the second row and the third column in all of the above cases? The books will just say "$F_{μν}$ = [some matrix]", and you don't know which is which.

Below is an example that is in itself contradictory. To convey the idea that F is antisymmetric, they use two different conventions in the very same line – here it is the order of the Greek subscripts that determines the order.

$$
F_{\mu \nu} = \left( \begin{array}{cccc}
0 & -E_1 & -E_2 & -E_3 \\
E_1 & 0 & B_3 & -B_2 \\
E_2 & -B_3 & 0 & B_1 \\
E_3 & B_2 & -B_1 & 0
\end{array}
\right)
= -F_{\nu \mu}
$$

Best Answer

In my experience, reading the indices left to right and top to bottom, the first index is the row and the second is the column.

Your screenshot from Carroll doesn't have to be contradictory (although it's definitely confusing/doesn't make rigorous sense). You can just imagine he omits a little "$_{\mu \nu}$" on the matrix:

$$F_{\mu \nu}=\Bigg( \cdots \Bigg)_{\mu \nu}=-F_{\nu \mu}$$

now it's a true real number equation.