Black holes are regions of infinite density, known as a singularity. And according to mainstream physics, each of these cosmic matter munchers is fringed by an event horizon –- a boundary where once you fall in, you never come out.
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.
Related: What’s inside a black hole?
‘The Emperor Has No Clothes’
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.
Related: 8 ways you can see Einstein’s theory of relativity in real life
Thankfully, as far as we know, all singularities are wrapped in an event horizon. The event horizon is the distance away from the singularity where the gravitational attraction is strong enough to pull in anything –- the point where you would have to travel faster than the speed of light to escape. That’s what makes a black hole black — even light can’t escape them.
Ever since we first discovered the existence of black holes, we’ve wondered if it’s possible to form a singularity without the associated event horizon — a so-called “naked” singularity. This would be a very dangerous place indeed, because it would be a location where the laws of physics break down that is fully accessible to the rest of the universe. At least with a traditional black hole, the singularity is safely wrapped beneath an event horizon, so even though it’s a place of extreme and unknown physics, at least whatever happens there is locked away from the rest of the cosmos.
Twisting a point
If naked singularities exist, they certainly aren’t common. We know of only one confirmed way of forming singularities, and that’s when a giant star runs out of fuel and collapses in on itself. When that happens, the singularity naturally gets an event horizon.
The presence of a naked singularity