Quantum fields may keep us safe from the weird physics within event horizons NASA/JPL-Caltech

Break out the censor’s black bars for naked singularities. Quantum effects could be obscuring these impossible predictions of general relativity, new calculations show.

Albert Einstein’s classical equations of general relativity do a fairly good job of describing gravity and space-time. But when it comes to the most extreme objects, such as black holes, general relativity runs into problems.

Among those is the prediction of naked singularities: theoretical points in space-time where gravity becomes infinitely large, but without the signature “blackness” of real black holes. Black holes are cloaked by an event horizon, a boundary within which the gravitational force of a singularity is so strong that light cannot escape. Naked singularities have no such cloak.


But naked singularities streaking through space would be a problem for physics as we know it. Physics is founded on the assumption that you can predict the evolution of systems based on some set of initial conditions. But near a naked singularity, which is basically an abrupt puncture in space-time, that predictive power collapses. It produces an area rife with paradoxes, where anything could happen without warning.

That is an issue for the universe as we know it. “We live in a rather classically predictable world and we don’t see infinities everywhere,” says Marc Casals at the Brazilian Center for Research in Physics in Rio de Janeiro.

This same problem is thought to arise in certain regions of regular black holes too, but their event horizons protect us, making sure we can never observe the infinity of a singularity or the implosion of our ability to describe sequences of events.

“Physicists believe that although there are these bizarre solutions to Einstein’s equations, once we add all the complications, these weird properties will disappear,” says Ivan Agullo at Louisiana State University in Baton Rouge. “But actually proving that is very difficult.”

Quantum to the rescue

Now, new calculations by Casals and his colleagues show that quantum effects may force naked singularities to clothe themselves in event horizons just like normal black holes.

“If, by some evil means, somebody created a naked singularity, it would quickly cease to exist,” says Casals. “These quantum fields would very quickly turn the naked singularity into a black hole.” His team’s calculations show that including quantum fields also eliminates the regions in black holes where physics breaks down.

While this still allows singularities to exist within event horizons, it protects the universe outside them from their weird physical effects. “We don’t smooth them out, but at least we shield them,” says Casals.

But these calculations come with a caveat: although we live in a four-dimensional world, they apply to a simpler universe with only three dimensions, two of space and one of time.

“Usually what happens in two-plus-one dimensions is a good guide to what would happen in the real world, so it certainly gives a springboard or a point of reference for explorations in the real world,” says Robert Mann at the University of Waterloo in Ontario, Canada.

It is also a step towards understanding how gravity and general relativity, which operate on huge scales, interact with the quantum nature of matter, which is most evident in small systems. Many physicists hope that the two will eventually be combined into a single theory of quantum gravity.

“There is this belief that quantum gravity will solve these undesirable features of classical space-time,” says Casals. “Our calculations show that this may be true.”

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.118.131102