Another Interpretation of the Optical Scalars

Abstract

In this paper an interpretation of the optical scalars θ and σ (the expansion and shear of an irrotational null congruence) is given in terms of the principal curvatures of a two‐dimensional subspace of the instantaneous rest frame of an arbitrary observer. The two space is defined as the intersection of the observer's rest frame and the particular null hypersurface the observer is intersecting.