# Rethinking black holes

## Delving deep into the workings of these high-gravity objects could turn our theories on their head

Delving deep into the workings of these high-gravity objects could turn our theories on their head

Almost every diagram of a black hole highlights two key parts of these behemoth beasts: the singularity (the centre) and event horizon (the perimeter). But there’s a chance some black holes don’t actually look like this. Some black holes may not have an event horizon at all. If they exist, these naked singularities wouldn’t just call every black hole illustration into question, but many of our established notions of the universe, too.

A standard black hole is thought to be the consequence of a large star collapsing in on itself. Once compressed into a point of infinite density, the forces of gravity are so extreme that nothing can escape its pull, not even light. But for something to get in, it still has to get close enough, past the event horizon. “The simplest way to describe them is as a point of no return,” says Maximiliano Isi, a postdoctoral researcher at the Laser Interferometer Gravitational-Wave Observatory (LIGO) Lab at the Massachusetts Institute of Technology (MIT). “It’s sort of like the frontier between countries. It’s not a geographical thing. If you try to get out, once you’ve gone past that point, you will find that you won’t be able to.”

If you were unfortunate enough to be caught in this fatalistic force, you would be pulled right into the heart of the black hole with no means of escape. This engulfing process would even occur at different rates for different parts of your body. Limbs closest to the black hole would be yanked the hardest, stretching and tearing your body in a process known as spaghettification.

After this gruesome event, your spaghettified body would be sucked into the singularity. It’s here where your remains would be compressed down into an infinitely tiny point, along with any other matter that had befallen the black hole. But here’s where things get confusing. Up until now, every part of the process of falling into a black hole, including the black hole itself, can be explained by Einstein’s theory of general relativity. But within this singularity, it’s thought that the curvature of space-time itself would split from equations, rendering this universal theory somewhat less universal. It’s partly why Einstein and many of his contemporaries doubted that such a physical black hole was possible. Einstein’s collaborator in proving general relativity, Arthur Eddington, even went as far as insisting “there should be a law of nature to prevent a star from behaving in this absurd way”. For decades, an open belief in black holes would get you laughed out of a physics lecture hall.

Attitudes changed as time went on, in large part because of the calculations and theories of two men: Stephen Hawking and Roger Penrose. While Hawking may be the more well known of the two, it was Penrose who first salvaged black holes from the rubbish heap of physics. Realising in 1969 that Einstein’s theory and black holes could happily coexist if singularities were somehow hidden from the rest of space-time, Penrose postulated the existence of event horizons, great cloaks that would surround black holes, hide them from view and act as a perimeter for their gravitational effects.

“Einstein was trying to get rid of singularities in black holes, because

he thought it was just a horrible thing,” says Tomas Andrade, a postdoctoral theoretical physicist at the University of Barcelona. “Penrose and Hawking realised that these singularities are there. If general relativity is correct, singularities are going to happen. Penrose realised that you don’t come across singularities very often, and he conjectured that’s the case because the universe protects the singularities by covering them with a black hole horizon.”

Penrose called his idea the weak cosmic censorship conjecture. From then on, physicists largely acted like event horizons were a compulsory part of black holes – the mandatory packaging that came with a singularity. But are they? After all, even with inclusion of event horizons, the theory of general relativity still seems to crumble apart when it reaches the singularity.

It’s partly why physicists are trying to set out a more complete theory of quantum gravity, which could account for such inconsistencies. And if a new theory is ever devised, there’s a chance it could happily incorporate the naked singularities Penrose’s conjecture prevents. But the theoretical physicists working on this quantum theory had better get a move on, because the evidence for naked singularities is already piling up.

“A huge amount of these counter examples come from looking at higher dimensional gravity,” says Andrade. “If you look at general relativity, not in four dimensions, like us – we live in three space dimensions plus time – if you increase the number of space-time dimensions, you can form the

[naked] singularities in a much more natural way.”

A universe with more than four dimensions might sound a little science fiction, even for physics, but the concept is a crucial part of string theory, another rival for general relativity’s top spot in the league of theories to describe the universe. As the theory goes, all matter and forces in the universe are actually tiny vibrating strings that twist and turn in all kinds of directions, including into other dimensions. And within such 5D, 6D or 7D universes, the rules for black holes may be very different to how we currently understand them – different enough that naked singularities could freely exist. This is what Andrade and his colleagues at the University of Barcelona tried to determine in a recent computer simulation.

“[We have] a toy model that allows you to look at collisions of black holes in a higher number of dimensions,” he says. “This means you can simplify Einstein’s equations, and something that can take months to do in a supercomputer, you can simulate a simplified version of this in less than a minute on your laptop. It’s quite cool.”

After running the simulation, Andrade and his colleagues found that if two rotating black holes were to collide and form a new black hole in a 7D universe, the force would be enough to elongate the new black hole’s event horizon, eventually making it untenable. “We saw that you can form a tube of horizon that then breaks,” he explains. “And when you get really close to the breaking point is when you form a naked singularity. Then the description of general relativity ceases to be valid.”

While Andrade’s study is a significant step towards describing and understanding naked singularities, it’s only part of the puzzle. After all, we can’t say for certain the universe really does have six or seven dimensions. To study horizonless black holes in the standard four dimensions, we might just have to find them.

“Our role in LIGO is to try to bring some more observational information to this discussion,” Isi tells All About Space. From his position at LIGO,

Isi and his colleagues have been able to analyse and test the most potent proof of black holes there is: gravitational waves. Back in 2015 the observatory provided the first tangible evidence that these kinds of waves existed when it detected the gravitational waves of two colliding black holes. More recently, Isi used the data to confirm a concept Stephen Hawking described in 1971 – the Hawking area theorem.

According to the eminent physicist, the event horizon area of the new black hole shouldn’t be any smaller than the total horizon area of its parent black holes. Indeed, 50 years after the theorem was made and six years after the gravitational

“Einstein was trying to get rid of singularities in black holes, because he thought it was just a horrible thing” Tomas Andrade

waves were first detected, Isi and his team proved Hawking right when they found that the new black hole’s event horizon was no smaller than the total area of its parents’ horizons. “So far everything we’ve thrown at the data has come back and told us it’s in agreement with the expectation from the vanilla picture that we have from Einstein’s theory, which includes these theorems about the area from Hawking,” he says.

But as impressive as LIGO is, Isi says its accuracy is a bit duller than ideal. To really test gravitational waves and the black holes that generate them, the team at MIT are going to need to upgrade their kit. Work is already underway to improve the sensitivity of LIGO and Virgo, a laser observatory in Italy, but the real advance will come with the Laser Interferometer Space Antenna (LISA), a network of three gravitational-wave detectors that will span vast distances in space. With a colossal range and the quiet stillness of space, it’s hoped the NASA and European Space Agency project, due to launch in 2034, will be able to detect the movements of black holes to an unprecedented degree. “Those instruments will see every single black hole collision in the universe… every single one,” Isi says. “They’re so sensitive that they can reach to the point where there are no longer any black holes, so far away that they haven’t had time to form.”

As soon as LISA launches to its spot among the stars, there’ll be no place for black holes to hide. When this day comes, maybe physicists like Isi will detect a singularity that seems bereft of its event horizon. And that’s when things will get really interesting. “Maybe some portion of the objects that we see [will be] more like the objects we expect from Einstein’s theory, with the look more like a normal black hole with a normal horizon,” Isi says. “And maybe there’s some fraction of them that are more exotic, that were formed by a different process and have different properties. Finding these rare objects will require analysing a very large number of signals, which will be facilitated by the improving instruments.”