Stephen Hawking was wrong. Black holes are bald.

Back in 2017, a gravitational wave rang across Earth like the clear tone of a bell. It stretched and squished every person, ant and scientific instrument on the planet as it passed through our region of space. Now, researchers have gone back and studied that wave, and found hidden data in it — data that help confirm a decades-old astrophysics idea.

That 2017 wave was a big deal: For the first time, astronomers had a tool that could detect and record it as it passed, known as the Laser Interferometer Gravitational-Wave Observatory (LIGO). That first wave was the result, they found, of two black holes crashing together far away in space. And now, a team of astrophysicists has taken another look at the recording and found something others thought would take decades to uncover: precise confirmation of the "no-hair theorem." This essential aspect of black hole theory dates back at least to the 1970s — a theorem that Stephen Hawking famously doubted.

When physicists say black holes don't have "hair," said Maximiliano Isi, a physicist at MIT and lead author of the paper, they mean that astrophysical objects are very simple. Black holes only differ from each other in three ways: rate of spin, mass and electric charge. And in the real world, black holes probably don't differ much in electrical charge, so they really only differ in terms of mass and spin. Physicists call these bald objects "Kerr black holes."

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That hairlessness makes black holes very different from just about every other object in the universe, Isi told Live Science. When a real bell rings, for example, it emits sound waves and some undetectable, incredibly faint gravitational waves. But it's a much more complicated object. A bell is made of a material, for example (bronze maybe, or cast iron), while according to the no-hair model, black holes are all uniform singularities. Each bell also has a somewhat unique shape, whereas black holes are all infinitesimal, dimensionless points in space surrounded by spherical event horizons. All those features of a bell can be detected in the sound that a bell makes — at least if you know something about bells and sound waves. If you could somehow sense a bell's gravitational waves, you'd detect those differences in bell composition and shape in them as well, Isi said.

"The secret to this whole business is that the waveform — the pattern of this stretching and squeezing — encodes information on the source, the thing that made this gravitational wave," he told Live Science.

And astronomers studying the 2017 wave learned a great deal about the black hole collision that spawned it, Isi said.

But the recording was faint, and not very detailed. LIGO, the best gravitational wave detector in the world , used a laser to measure the distances between mirrors arranged 2.5 miles (4 kilometers) apart in an L-pattern in Washington state. (Virgo, a similar detector, also picked up the wave in Italy.) As the wave rolled over LIGO, it warped space-time itself and ever so slightly changed that distance. But the details of that graviational wave were  not intense enough for the detectors to record, Isi said.

"But it's like we're listening from very far away," Isi said.

At the time, that wave offered a lot of information. The black hole behaved as expected. There was no obvious evidence that it lacked an event horizon (the region beyond which no light can escape) and it didn't dramatically deviate from the no-hair theorem, Isi said.

But researchers couldn't be very certain of many of those points, particularly the no-hair theorem. The simplest part of the waveform to study, Isi said, came after the two black holes merged into one larger black hole. It kept ringing for a while, very much like a struck bell, sending its excess energy into space as gravitational waves — what astrophysicists call the "ringdown" process.

At the time, researchers looking at LIGO data spotted just one waveform in the ringdown. Researchers thought it would take decades to develop instruments sensitive enough to pick up any quieter overtones in the ringdown. But one of Isi's colleagues, Matt Giesler, a physicist at the California Institute of Technology, figured out that there was a brief period right after the collision where the ringdown was intense enough that LIGO recorded more detail than usual. And in those moments the wave was loud enough that LIGO picked up an overtone — a second wave at a different frequency, very much like the faint secondary notes that are carried in the sound of a struck bell.

In musical instruments, overtones carry most of the information that give instruments their distinctive sounds. The same is true of the overtones of a gravitational wave, he said. And this newly uncovered overtone clarified the data on the ringing black hole a great deal, Isi said.

It showed, he said, that the black hole was at least very close to a Kerr black hole. The no-hair theorem can be used to predict what the overtone will look like; Isi and his team showed that the overtone pretty much matched that prediction. However, the recording of the overtone wasn't very clear, so it's still possible that the tone was somewhat different— by about 10% — from what theorem would predict. .

To get beyond that level of precision, he said, you'd need to extract a clearer overtone from the waveform of a black hole collision, or build a more sensitive instrument than LIGO, Isi said.

"Physics is about getting closer and closer," Isi said. "But you can never be sure."

It's even possible that the signal from the overtone isn't real, but occurred by mere chance due to random fluctuations of the data. They reported a "3.6σ confidence" in the overtone's existence. That means there's about a 1-in-6,300 chance that the overtone isn't a true signal from the black hole.

As instruments improve and more gravitational waves are detected all of these numbers should become more confident and precise, Isi said. LIGO has already been through upgrades that have made detecting black hole collisions fairly routine. Another upgrade, planned for mid-2020, should increase its sensitivity tenfold, according to Physics World. Once the space-based Laser Interferometer Space Antenna (LISA) is launched in the mid-2030s, astronomers should be able to confirm the hairlessness of black holes to degrees of certainty impossible today.

However, Isi said, it's always possible that black holes aren't completely bald — they may have some quantum peach fuzz that's simple too soft and short for our instruments to pick up.

Originally published on Live Science.