Extrinsic curvature of a spatial hypersurface
= Extrinsic curvature of a spatial hypersurface
{title2=$K_{ij}$}
With future unit normal $n^\mu$, the convention $K_{\mu\nu}=-\perp^\rho{}_\mu\nabla_\rho n_\nu$ measures how the hypersurface is embedded in spacetime.