Four different black holes, with spin S, and their "tendex lines" (upper two panels) or "vortex lines" (lower two panels). The horizons are colored by their "tendicity" or "vorticity," ranging from most negative (red) to most positive (blue).

From the article:

Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime
Robert Owen, Jeandrew Brink, Yanbei Chen, Jeffrey D. Kaplan, Geoffrey Lovelace, Keith D. Matthews, David A. Nichols, Mark A. Scheel, Fan Zhang, Aaron Zimmerman, and Kip S. Thorne
Phys. Rev. Lett. 106, 151101 (2011)

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