Some physicists suggest that we need to seriously change how we think of black holes. That's exactly what a theory suggesting they resemble huge "fuzzballs" would do.

Stephen Hawking discovered something impossibly wrong with black holes in the 1970s: they could mathematically shrink or even disappear. If a black hole vanished, it meant that everything it had sucked in vanished with it. And in physics, things aren't supposed to just vanish — they can change their shape, but their subatomic information must still exist in the universe. In other words, black holes are capable of destroying information about subatomic particles that should not be able to be destroyed (according to quantum physics). For decades, physics has struggled with a problem known as the black hole information paradox. However, theoretical physicist Samir Mathur has provided a solution: see black holes as "fuzzballs" rather than black holes.

Based on the classic view handed down by the likes of Einstein, “quantum mechanics is broken when you have a black hole,” says Mathur, a professor at The Ohio State University who specialises in black hole physics. "When you have a circumstance like that, you don't have a physics theory." It’s the task of physicists, he argues, to reconcile general relativity, which explains the behaviour of massive things, and quantum mechanics, which works for the incredibly tiny. The fuzzball idea, according to Mathur, has the potential to achieve precisely that.

The Traditional Picture of Black Holes

The information paradox — and the fuzzballs that might resolve it — hinges upon the structure and behaviour of black holes themselves. The attracting force of gravity is responsible for the existence of black holes. "If there's a lot of mass somewhere, everything in that mass attracts everything else," Mathur explains. "Every point in a star attracts every other point in the star if it's a star." As a result, it seeks to contract."

That shrinking results in a super-dense region of space-time that gobbles up everything that encounters it (even light) called a black hole. Though don’t let dread of getting pulled into a black hole keep you up at night, adds Lia Medeiros, a National Science Foundation postdoctoral scholar at the Institute for Advanced Study; you’d have to get extremely near for that to happen. From far away, a black hole behaves like any other object with a lot of gravity, like a star. But if you were to drift too close to a black hole, you’d reach a point of no return called the event horizon, “the distance from the black hole where even if you’re moving at the speed of light, you still can’t escape it,” explains Medeiros.

And beyond that event horizon, the classic theory goes, there’s a large chunk of empty space, with all the things that the black hole has absorbed compressed into one tiny spot in the centre. The singularity is what we call that moment of compression.

Problems and Paradoxes

Two complications arise from this typical idea of a black hole, says Fabio Pacucci, an astrophysicist at Harvard University. One is that the concept of a singularity — a place of infinite density and infinite gravity — clashes with reality. Pacucci recalls a high school physics teacher who informed him, “If you find an infinite, this is not physics, its mathematics.” He claims that nature does not work in infinitesimals. “You will never find a tree that is indefinitely tall, or a planet that is infinitely massive.” While black holes may be the only thing in nature with endless qualities, it's reasonable to remain dubious.

As a result, the singularity poses a problem. And, as it turns out, so is some of the activity we see surrounding a black hole’s event horizon. Black holes emit energy, as Stephen Hawking and his colleagues demonstrated. That in and of itself isn’t an issue, but it carries a lot of mathematical baggage. The process, called Hawking radiation, involves the instance of a paired particle and antiparticle sprouting up adjacent to the event horizon of a black hole.

It’s possible that one of them (let’s assume the particle, but it works both ways) would interact with the event horizon and then go spinning back off into space, without taking up any of the information within the black hole to take with it. However, its antiparticle counterpart may be swept into the black hole. Once within the black hole, the lone anti-particle could collide with a single particle, causing the two to "annihilate one other," as Pacucci puts it.

"To an outside observer, it appears that the black hole is losing mass one particle at a time," Pacucci says. That would result in the black hole losing a tiny amount of the information connected with that particle, although information is meant to always be conserved. Therein lies the paradox. It’s also worth noting that this scenario could only happen on a very small scale, because there aren’t enough free-floating anti-particles to happen en masse. However, even a minor loss of data is a significant issue. With enough time, this extremely slow process could eventually result in the complete disappearance of a black hole.

At its heart, the information paradox occurs because black holes straddle two worlds: They can be huge, and hence should follow the rules of general relativity, yet they also shrink to an infinitesimally tiny point, making them quantum territory. For decades, physicists have struggled to reconcile their understanding of black holes with quantum mechanics. The fuzzball solution, developed by theoretical physicist Mathur, requires constructing a totally new picture.

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