GR describes spacetime as a smooth 4-dimensional surface with weird rules for geometry. The curvature of spacetime near an event (x,y,z,t) is given by a function called the metric tensor. A singularity is a point or region of spacetime where the metric tensor becomes poorly-defined. Physicists distinguish between two types:
Can naked singularities really form in nature, or does the collapse of a massive star always produce an event horizon? Roger Penrose (left in photo above) famously suggested that they cannot form. But later studies showed they do form under physically reasonable conditions, as I discussed in a Scientific American article several years ago. This is one of the most hotly debated frontier issues in modern black hole physics and relativistic astrophysics today.
When a naked singularity is event-like, it looks like an explosion. As the star collapses, it eventually rebounds because of quantum gravitational effects. In this case, observers need to look for an otherwise inexplicable outburst of energy.
When a naked singularity is objectlike, it looks rather like a black hole unclothed by a horizon (hence the name). A swirling disk of matter would form around it, and my colleagues and I recently showed that the disk would be much brighter than the equivalent around a black hole. It would also differentiate itself by its light spectrum, by the higher efficiency of particle collisions in its vicinity, and by the way it bends light.
Those of an adventurous frame of mind might imagine boarding a starship and plunging into a naked singularity. The descent would be rather different than the same trip into a black hole. For the black hole case, as kamikaze astronauts near the event horizon, the light they emit gets dimmer and dimmer, eventually going beyond any observable limit. In the naked singularity case, the light rays could escape from an arbitrary vicinity of the singularity. The astronauts would be able to remain in radio contact all the way down and send back direct pictures of quantum gravitational effects. 2b1af7f3a8