In general relativity, strong gravity can warp the geometry of spacetime so much that black holes are formed—regions from which not even light can escape. The interior of a black hole, where curvature becomes infinite, is an extremely complex configuration that defies our current theories. But according to general relativity, these complexities might be hidden to observers outside the black hole’s horizon. The so-called “no-hair theorem” implies that isolated black holes in equilibrium are, in fact, extraordinarily simple  and can be fully characterized by just two numbers, the mass (M) and the angular momentum (J). As physicist John Wheeler put it, “black holes have no hair”—a statement that uses hair as a metaphor for all complicated details.
However, this simplicity emerges only if the black hole is isolated from everything else—an assumption not met in most astrophysical scenarios. Norman Gürlebeck of the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen, Germany, has now unraveled a new aspect of black hole simplicity. He has shown that, under certain assumptions, the no-hair theorem is still valid when the black hole is not isolated . The extended theorem would, for instance, apply when a black hole is surrounded by a matter disk (see Fig. 1).