Despite the fact that astronomers cannot peer at what goes on inside the black hole, a black hole's gravitational effects on its neighborhood allow for a number of indirect observations. Swirls of infalling gas heat up and give off radiation to illuminate a black hole's vicinity, and the orbits of stars around a black hole allow astronomers to estimate its mass. Now researchers have proposed a new optical technique to observe and study black holes by measuring the imprint they should leave on the light that passes near an event horizon.

• Report in Scientific American
• Nature Physics Article
• Commentary by Martin Bojowald