The Meanings of Frame-Drag Vortex Lines and Tidal Tendex Lines

Frame-drag vortex lines and tidal tendex lines provide a tensorial visualization of gravitational waves and their generation that contains more information than traditional scalar quantities such as the Newman-Penrose $\Psi_4$. By examining the example of a head-on merger of identical black holes with transverse spins, I will present some relationships between the vortex and tendex viewpoint and other formalisms in wave zone. I will also discuss analogies to electromagnetism, with the aim of demonstrating how one might build up intuition into the information embodied in vortex and tendex lines, and their vorticities and tendicities.