Black Hole Vision: An Interactive iOS App to Visualize Warped Spacetime.

How warped is the spacetime around a black hole? A key signature of a black hole’s strong gravity is the "photon ring”: a bright, thin ring of light encircling the event horizon that is produced in images of a black hole by photons that traverse many orbits around it before reaching a distant observer. The Black Hole Explorer (BHEX), an orbiting sub-millimeter radio telescope, will take the sharpest images in the history of astronomy and resolve the photon ring of two supermassive black holes, thus reading off their relativistic imprint.

To help visualize the strong lensing effects that BHEX will probe, we conduct a live demo of the iOS app Black Hole Vision. This application takes in real-time video feeds from the front and rear facing cameras of an iPhone, evolves the corresponding light trajectories through the Kerr geometry via the exact analytic solutions to the null geodesic equation, and outputs the resulting strongly lensed image to the user's screen. The application also provides controls for manipulating physical parameters such as the black hole's spin and inclination, allowing the user to develop intuition for how these parameters affect the bending of light by a black hole.

We hope that Black Hole Vision will help build physical intuition for the seasoned relativist and also function as a useful pedagogical resource for the curious novice.