Black holes are mysterious objects, and the most mysterious thing about them is the singularity: the point of infinite density and an infinite gravitational field — not to mention infinite entropy — that lies at the center of a black hole. Since no further change can take place, no physical processes can occur, so time can be said to have stopped, at least from our limited perspective.
The universe appears to have a built-in “cosmic censorship” principle at work that shields a black hole’s singularity from direct observation.
In fact, we detect black holes themselves through indirect evidence, namely, their gravitational effects on local spacetime, and bursts of radiation resulting from bits of matter falling onto the accretion disk. Anything that passes beyond the event horizon — not so much a physical surface as the theoretical point of no return — gets caught in the black hole’s powerful gravitational field, never to re-emerge.
Ah, but there’s been some debate over the years as to whether there might be rare instances when the mysterious singularity could be stripped bare, revealing its secrets to prying scientific eyes. This would be known as a “naked singularity,” and it has as many naysayers as proponents.
Proponents of the existence of naked singularities include Ted Jacobson and Thomas Sotiriou, two physicists at the University of Maryland, College Park. They published an intriguing paper last October speculating that under just the right circumstances, a naked singularity could occur.
The key is that pesky event horizon: all we need to do, the physicists said, is destroy it, or at least temporarily overcome it — say, by sending a rapidly spinning object on a collision course with a black hole, with the object spinning in the same direction as the black hole. That combined momentum might be sufficient to overcome the gravitational power of the event horizon. And voila! We could finally get a peek inside a black hole.
Chief among the naysayers is Stephen Hawking, who famously made a bet with two Caltech physicists, Kip Thorne and John Preskill, in 1991 that naked singularities could not exist, although the stakes weren’t especially high: 100 pounds sterling, plus an article of clothing “embroidered with a suitable concessionary message.”
Hawking was forced to backpedal a bit on his stance in 1997, after new calculations revealed a limited set of conditions that could give rise to naked singularities. Hawking showed that black holes radiate energy and decay over time (the aforementioned Hawking radiation). Assuming that the protective event horizon evaporates along with the black hole, the singularity at the center could be exposed at the moment the black hole winks out of existence.
It’s highly improbable, but even Hawking admits it is theoretically (i.e., mathematically) possible.
He may have lost the bet on a technicality, but Hawking still adheres to a general cosmic censorship principle, and the message on the T-shirt he presented was far from concessionary: “Nature abhors a naked singularity.” That Nature is such a prude.
Team Hawking just got another boost from a new paper by Enrico Barausse, Vitor Cardoso and Guarav Khanna, who spent a year refining the original mathematical model devised by Jacobson and Sotiriou. See, it’s standard practice in physics when doing these sorts of calculations to make approximations to get a rough answer. Then physicists can refine the model bit by bit, according to how given physical systems are known to behave.
That’s known as a perturbative approach. Usually these factors are very minor, and unlikely to have a significant impact on the results. Jacobson and Sotiriou essentially ignored the perturbations in their original simulation.
But Barausse et al. added some of those back into the equation, and got a very different result. They found that the object itself would be kicked away from the black hole as a result of its own gravity, and the event horizon would remain intact.
Score another point in the “con” column for naked singularities. We eagerly await the next volley in this fascinating scientific debate.