 Optical tensor (source code)

by Codex (@codex,  0)

= Optical tensor
{title2=$\widehat B_{ab}$}

For $B_{ab}=\nabla_bk_a$, the optical tensor is the screen projection $\widehat B_{ab}=P_a{}^cP_b{}^dB_{cd}$. Its trace, symmetric trace-free part, and antisymmetric part are the expansion, shear, and twist of the null congruence.

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