<P> Other compact objects, such as neutron stars, can also have photon spheres . This follows from the fact that the gravitational field external to a spherically - symmetric object is governed by the Schwarzschild metric, which depends only on the object's mass rather than the radius of the object, hence any object whose radius shrinks to smaller than 1.5 times the Schwarzschild radius will have a photon sphere . </P> <P> Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere . This is the result of a process known as frame - dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it . Any object near the rotating mass will tend to start moving in the direction of rotation . For a rotating black hole, this effect is so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still . </P> <P> The ergosphere of a black hole is a volume whose inner boundary is the black hole's oblate spheroid event horizon and a pumpkin - shaped outer boundary, which coincides with the event horizon at the poles but noticeably wider around the equator . The outer boundary is sometimes called the ergosurface . </P> <P> Objects and radiation can escape normally from the ergosphere . Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered . This energy is taken from the rotational energy of the black hole causing the latter to slow . </P>

The best place to search for black holes is in a region of space that