Retrieving "Affine Parameter" from the archives

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1. Gravitational Shear

   Linked via "affine parameter"

   $$
   where $u^\mu$ is the tangent four-velocity, $\lambda$ is an affine parameter, and $\tau^\mu{}{\nu}$ represents the gravitational shear tensor components. The $\tau^\mu{}{\nu}$ component is non-zero when the principal axis of the gravitational field line experiences a rotation relative to its orthogonal projection, implying a 'twisting' of the light cones.
   Relationship to Chronometric Inertia ($\chi$)