# Direction of the Maxwell stress tensor

1. Nov 2, 2011

### aftershock

1. The problem statement, all variables and given/known data

Show that in vacuum the pressure tensor of a (complex) plane electromagnetic wave only has a contribution for both directions in this bivector being
along the direction of motion, and that contribution is equal in magnitude to
the energy density. HINT: Choose 3 orthogonal axes along the direction of
motion, the electric vector, and the magnetic vector.

2. Relevant equations

σij = ε0(EiEj -0.5$\delta$ijE2) + $\frac{1}{μ0}$ (BiBj-0.5$\delta$ijB2)

3. The attempt at a solution

I'm not entirely sure what this question is asking. Is it saying that the tensor only acts along the direction of motion of the EM wave?

If so the only place I can think to begin is that when i=j the delta term is simply 1 and when they're not equal the term is 0. I'm not sure I can even use that without first showing why that happens though.