It's often stated that the off-diagonal terms in the stress-energy tensor signify shear while the diagonal terms are normal pressure terms (you get a displacement in the direction you apply pressure). I was reading Weinberg and I found this expression of the stress-energy tensor:
$$T(\alpha,\beta)=\sum_n P_n(\alpha) \frac{dx_n(\beta)}{dt} \delta^3(x-x_n(t)),$$ where $P_n$ is the 4-momentum of the $n$th particle and $x_n$ is it's $4$-position Can someone help me with how does this definition reduce to the above-known facts that I mentioned?
