Colliding black holes 'ringing' across space and time prove Hawking, Einstein theories right

Once again, it turns out that renowned theoretical physicist Stephen Hawking was right. Oh, and Albert Einstein, too.

Scientists from the U.S. National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational wave, a ripple in space-time that is caused by extremely energetic processes, such as the merger of two black holes or two dense neutron stars colliding.

It turns out that the gravitational wave occurred due to two black holes roughly 1.3 billion light years away from Earth, with masses roughly 30 times that of our sun, colliding and forming another black hole, with the designation GW250114.

This isn't unusual for the LIGO detectors, one located in Hanford, Wash., the other in Livingston, La., which have found close to 300 of these violent interactions. But this time, the researchers were able to learn a lot more than they have before.

The merger of these particular black holes created what scientists refer to as a "ringing," which produced two different tones, allowing researchers to confirm that a black hole can be defined using only two properties: mass and spin, something that was predicted by mathematician Roy Kerr in 1963.

"One of the predicted qualities that's unique to a black hole, is that … sort of like if you hit a tuning fork, it would ring at particular frequencies, and they'd have particular spacing," said Jess McIver, one of the study's co-authors, who is also an associate professor at the University of British Columbia.

"It's sort of like a fingerprint of a black hole. So because this was so loud, such a gorgeous, loud signal, we were able to really crisply resolve those."

Max Isi, an assistant professor at Columbia University, and also a co-author of the study, published in the journal Physical Review Letters, explained it further.

"There are two individual modes of oscillations, two tones: a fundamental tone and an overtone," he said.

Because the tones matched, he says, it proves the Kerr solution. Had the tones been different, it would imply that other properties are necessary to describe a black hole.

This proved both Kerr and Einstein right about their predictions for black holes. Einstein's theory of general relativity predicted the existence of black holes. And in Kerr's case, his calculations specifically centred around spinning black holes.

The merger also confirmed Hawking's area theorem, which states that when two black holes merge, the event horizon — or the area around the black hole from which no light or radiation can escape — can never decrease, only increase.

After the two black holes merged, the scientists were able to use the ringing to determine the final size of the event horizon — also considered the black hole's surface area. Before they merged, each black hole had a surface area of roughly 240,000 square kilometres. After, the new black hole had a surface area of 400,000 square kilometres, proving Hawking correct.

"I imagine that for Hawking, black holes and space and time have been studied as theoretical mathematical abstractions for decades. And finally, being able to see these processes taking place, you know, it's astounding," Isi said.

"I mean, it's astounding to me, and I can't imagine to him, you know, having worked his whole life on this. So it's unfortunate that we couldn't do this while he was alive."

Now, while the surface area increased, the mass of the black hole actually decreased, which is what theory suggests should happen.

"The area of the event horizon, it can only grow," said Janna Levin, a theoretical astrophysicist and professor of physics and astronomy at Columbia University's Barnard College, who was not involved in the research.

"And if I was thinking of it as one original black hole absorbing another, it could only grow, but its mass is not the sum of the two masses. It's actually less. It loses some of that E=MC² energy into the gravitational waves."

Each black hole was roughly 33 times the mass of the sun. After the merger, the black hole had a mass 63 times that of the sun.

A similar analysis of an almost identical black hole merger — the very first one was detected in 2015 — was done in 2021. However, it did not yield the same strong evidence that this new analysis did.

Isi explained that because the signal for that black hole was weaker, scientists were still left guessing, but that the signal for the recent merger was four times stronger.

"You can see that 2021 paper as a demonstration that something like this would be possible, like a proof of principle," he said, adding that this merger was "real, incontrovertible" evidence of it.

The 2015 detection of a gravitational wave earned the researchers the Nobel Prize in Physics in 2017, as part of the LIGO-VIRGO collaboration. Today, there are more observatories detecting gravitational waves, including one in Japan, that make up the LIGO-VIRGO-KAGRA collaboration.

Both McIver and Isi are excited about what future improvements to the detectors will help them discover.

McIver says that no matter how many times scientists "slice and dice this data in different ways" to check the signal against what Einstein, Kerr and Hawking predicted, the answer is that they were right.

"As the detectors continue to improve, our confidence will improve, or potentially we'll uncover something really interesting that we're not expecting."