We no longer just talk about black holes, we listen to them

When researchers first detected gravitational waves in 2015, it fundamentally changed the way we can study black holes. What had been a field dominated by mathematical calculations suddenly became one in which it was possible to observe the signals directly.

“When I started my career, we were essentially deaf. We could study black holes theoretically, but we had almost no observations. Today we can listen to them,” says Vitor Cardoso, Professor of Physics at the Niels Bohr Institute at the University of Copenhagen.

Together with colleagues from Johns Hopkins University and the University of Birmingham, Vitor Cardoso has compiled the most comprehensive overview so far of the research on black holes and their vibrations.

The review, which has just been published in the journal Classical and Quantum Gravity, brings together and compares decades of theoretical work with the past ten years of observations from gravitational-wave astronomy.

From theory to experiment

For decades, researchers had only theory to work with. They knew that Einstein’s theory of relativity, that is, his mathematical equations, predicted gravitational waves and specific black hole vibrations, but they did not have the instruments needed to go out into the universe and investigate them directly.

When two black holes collide and merge, the newborn black hole vibrates before settling down. These vibrations send gravitational waves through the universe, and those are the signals researchers can measure. That is what Vitor Cardoso means when he says that we can now listen to black holes.

According to the professor, the signals can be compared to the sound of a musical instrument.

“Just like you can tell the difference between a guitar and a piano, we can use gravitational waves to identify what is vibrating.”

So black holes themselves do not make any sound?

“Correct. They need to be excited by something. In the same way as a church bell. It needs to be struck by something before you can hear it ring.”

And how does that become a sound?

“Think of it like throwing a stone into a well filled with water. There is an impact, and waves are produced. And they have a sound. Those waves, that sound, are what we can measure,” Cardoso explains.

The field is known as black hole spectroscopy and is about using these vibrations to investigate the properties of black holes and thereby move even closer to understanding our universe.

A field that has grown explosively

According to Cardoso, one of the reasons they set out to create this overview was that research in the field has grown extremely rapidly, with the result that a lot of work has become highly fragmented across topics and across the world. The second motivation was that there had also been considerable disagreements within the community about what the research was actually telling us.

“We realized that we really needed to bring everybody together and we needed to write this and agree. And when we don’t agree, we need to work towards understanding what the source of this disagreement was. And that’s what we did,” he says.

Einstein was right, at least for now

One of the questions physicists are always trying to answer is whether the calculations and observations they make are consistent with Albert Einstein’s general theory of relativity from 1916. The same is true of this monumental review, which Cardoso and 70 other researchers have now published.

“This is what has happened for centuries with all the fundamental laws of nature. And at some point, they all turned out to have limitations. And so, we’re looking for that in relation to Einstein’s theory,” Cardoso explains.

“And so far, we found no signals that nature goes elsewhere.”

The same conclusion is reflected in the new review.

“Einstein so far is correct. Everything we see is consistent with Einstein’s predictions,” says Cardoso.

The search continues

However, this does not mean the matter is settled. The final word is still far from being written when it comes to whether the observations are truly consistent with Einstein’s theory.

“We know that Einstein’s theory is incomplete. Neither Einstein nor any of us know what happens inside black holes. Right now, there is simply nothing. You could say that inside black holes, mathematics breaks down. The question is whether nature will eventually show us where the theory ceases to work,” says Vitor Cardoso.

The review article also points toward the next generation of observatories, including the European space mission LISA, which is expected to provide researchers with even deeper insights into the mysterious black holes of our universe.

“There has to be a better theory than Einstein’s,” says the professor.

“There has to be a better theory than Einstein’s,” says the professor.

Will it happen in your lifetime that you are able to seriously challenge him, you think?

“I certainly hope so! I’m going to live till I’m 200,” Vitor Cardoso concludes with a big laugh.