But what if some black holes are naked — completely lacking such frontiers? As far as we can tell, singularities are always wrapped in event horizons, but a more detailed look at the math of general relativity suggests that doesn't have to be the case.
If such naked black holes dot the universe, new research reveals how we might be able to detect one: by looking at the ring of light surrounding it.
Black holes are a consequence of the mathematics of Einstein's theory of general relativity. Those equations tell us that if a clump of matter collapses on itself into too small of a volume, the gravity of that matter will just keep shrinking it ever smaller until it crushes into an infinitely tiny point. That point is called a singularity, and it's a signal that the math we're using to describe spacetime is completely breaking down.
The gravitational pull of a singularity is infinitely strong. Objects can be pulled toward the singularity faster than the speed of light. Near a singularity, the physics of general relativity can no longer predict the future trajectory of particles — which is one of the main points of physics. Without the power to make predictions, physics falls apart.