A black hole is a region of spacetime where gravity is so strong that nothing, including light and other electromagnetic waves, is capable of possessing enough energy to escape it. Einstein 's theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.

General relativity suggests that space and time are a single entity called spacetime, and that gravity arises when objects of mass cause spacetime to curve. Black holes aren't actually objects, but are regions of spacetime created by an infinitely dense and compact mass — a central singularity where the very laws of physics break down.

Einstein's theory of general relativity predicted the existence of black holes and that, no matter what such an object "eats," black holes are characterized only by their mass, spin and electrical charge. Astrophysicists refer to this as the "no-hair" theorem. But there has been a lingering snag to this theorem: magnetic fields.

The Kerr-Newman metric is the unique vacuum solution of the General Relativistic field equations, in which any singularities or spacetime pathologies are hidden behind horizons. They are believed to describe the spacetimes of massive astrophysical objects with no surfaces, which we call black holes.

Astronomers and physicists are working to probe black holes with radio telescopes and gravitational waves, as well as tracking the motions of stars and other matter around black holes to see if they follow the rules laid down by Einstein a century ago. The general theory of relativity has passed every test physicists have devised so far.