Last year, novelist Sergio De La Pava compared the American criminal justice system to the strange physics concept of naked singularities. That inspired me to ask the author of Sci Am‘s article on the concept, theoretical physicist Pankaj Joshi of the Tata Institute of Fundamental Research in Mumbai (right in photo), for an update. Watch his lecture on YouTube, too.

Imagine a black hole without the “black” part. That is, imagine a black hole that you could fall into and, with effort, escape back out, as long as you didn’t hit the very center of the hole–its so-called spacetime singularity–and get crushed into oblivion. In other words, imagine a black hole without its outer perimeter (its so-called event horizon) but with its central singularity. That’s what physicists and cosmologists call a naked singularity.

It would be a strange beast, like a star so tiny you could hold it in your hand–although you would definitely not want to do that. The thing can have the mass of an entire star (or more) compressed into something approaching a mathematical point. Close to it, the energy densities, spacetime curvatures, and all other physical quantities would become arbitrarily large. Extreme physics would govern the happenings there.

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.

Because Einstein’s theory of gravity allows for both black holes and naked singularities, it’s up to observers to settle the question. But how would a naked singularity distinguish itself from a clothed one? Singularities generally come in two types: those which are like events and others which are like objects. The big bang is an event-like singularity; a black hole is like an object. Both types can be produced as end state of gravitational collapse, depending on variables such as how fast or slow the collapse is, how the shells of matter fall in, and what the internal structure of the matter cloud is. A catastrophic collapse, such as a massive star imploding in matter of seconds, will produce an event-like naked singularity. A slower collapse could produce an object-like one.

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.

If you point a camera straight a black hole, you will see only inky blackness. If you point one at a naked singularity, you will see a small but bright object, albeit one with an unusual spectrum of light. Technically, you do not see the singularity itself, but its immediate environs. The astronauts would be able to say, “Ah, that’s how physics is unified,” and inform their friends back home of their achievement. They might even live to tell the tale.

Photo of Roger Penrose (left) and Pankaj Joshi (right) courtesy of H. Chauhan