We’ve learned a lot about black holes in recent years, from imaging them directly to seeing them emit waves of pure gravity.

But the more we learn, the more we realize how mysterious they are. And there’s one mystery about them that’s been bugging physicists for half a century.

That mystery is that black holes seem to be breaking basic rules of physics by destroying information itself, in what’s called the black hole information paradox. Because, y’know, black holes being places where gravity is so strong even light can’t escape isn’t enough of a paradox on its own! While we’ve covered the black hole information paradox on SciShow before, there have been some huge breakthroughs in efforts to tackle the problem since then, including the idea that black holes might have quantum hair.

So let’s talk about what the paradox is, how quantum mechanics and Einstein’s relativity connect to it, and how this hairy idea might solve it. If you fell into a black hole, there’d be no escape, you’d inevitably fall to the center and be crushed. But the paradox arises when you ask what happens to the matter you’re made of after the crushing.

In the ‘70s, Stephen Hawking showed that the matter would escape… eventually, as a stream of particles. He predicted that there should be very faint particle emission, Hawking radiation, from the edge of a black hole, called the “event horizon”. Around the same time Hawking proposed this, scientists had also shown that black holes aren’t as unique as they seem.

Regardless of how they’re formed, every black hole can be described by just three numbers: their total mass, spin, and charge. If two black holes weigh the same, spin as fast, and have the same charge they should look exactly the same from the outside, because they don’t have other features you could use to tell them apart. Without any characteristic lumps and bumps like planets and stars have, black holes should be perfectly smooth, so scientists called this idea the “no-hair theorem”.

Ah, scientists, they have such a way of naming things. It meant that the Hawking radiation can’t have anything to do with the stuff that goes into the black hole. On the outside, everything, even the radiation, should be smooth and homogeneous no matter what they gobble up.

And if at any point something escapes, you should never be able to tell what it was originally. Otherwise, that’d count as a unique characteristic of the black hole, also known as “hair,” because it goes against the no-hair theorem. But the fact that you can’t tell what the black hole had for dinner seems to violate a fundamental physical principle.

Based on our current understanding of physics, you should always be able to work that out, at least in theory. Scientists hypothesize that, based on the math behind the laws of physics, those laws should work just as well to describe the past and the future. So if you have all the information about the present, you should be able to tell what the future will look like, and what the past looked like before equally well.

That may sound weird, because to us, knowing the past is way easier than predicting the future, but that’s just a consequence of the world we humans live in, it’s not fundamental. I mean sure, if you burn a notebook, good luck reassembling it to read what it said, but it’s at least in principle doable, all the parts are out there. But even that’s not possible with black holes, looking from the outside, you’re missing puzzle pieces.

Some of the information is stuck within the inaccessible center. That’s why it’s a paradox. But after almost 50 years, there have been some big breakthroughs on how to potentially tap into that once-thought-lost information.

Some researchers think the answer might lie with the no-hair theorem, which they say isn’t entirely correct. If there are some holes in that theorem, it means that black holes can have “hair.” So it’s possible, in principle, that what a black hole gobbles up affects the outside, which means that some of that information can exit the black hole. Recently a few different research groups have tackled this hypothesis from different angles.

In 2021, three US researchers used very elaborate simulations to show that extreme black holes that spin as fast as possible can emit signals or “hair” that change based on how they formed. Those signals distort spacetime, so they should, in principle, be detectable by gravitational wave detectors. And in 2022, researchers from the UK, US, and Italy worked together to show that the laws of quantum mechanics might give all black holes some kind of “quantum hair”.

See, quantum mechanics is our best theory of reality, but it works very differently from regular, everyday laws of physics. And it breaks down completely with extreme gravity. So although they complement each other to better understand our world, they’re like oil and water, they really don’t mix.

And so quantum ideas can only be used sparingly when it comes to black hole physics due to their gravity. What the new research shows is workarounds in the math to discover new quantum effects around black holes, and those effects leave imprints on the surface that depend on the interior. It’s nowhere near a complete quantum picture of a black hole, but combining the two theories in any way is hard.

And it’ll probably take more than math workarounds to fully solve this problem, but working on it could also offer hints and clues as to how to fully explain quantum theory in extreme gravity. Like with black holes, we believe the information we need is out there, and we just need to find it. Thanks for watching this episode of SciShow Space!

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