When black holes crash into each other, they ring like cosmic bells — and now scientists are learning how to listen.
A massive team of more than 70 experts from countries across the world has published a comprehensive review showing that "listening" to these black hole vibrations could unlock some of the biggest mysteries in physics. The research was led by scientists at the University of Birmingham in the U.K., Johns Hopkins University in the U.S., and Instituto Superior Técnico in Lisbon, Portugal, and appeared in the journal Classical and Quantum Gravity.
Here's the idea: when two black holes collide and merge, the new black hole they form "rings" as it settles into shape. This ringing sends out invisible ripples through space called gravitational waves. Scientists at the LIGO-Virgo-KAGRA collaboration have already caught hundreds of these collisions since first detecting gravitational waves in 2015. By studying the specific frequencies of the ringing — like identifying the unique tone of a bell — researchers can figure out the black hole's mass, how fast it's spinning, and whether Einstein's famous theory of general relativity holds up in these extreme conditions.
So far, every ringdown they've measured agrees with Einstein's predictions. But that might just be because today's detectors aren't sensitive enough to catch the finer details.
Dr. Gregorio Carullo from the University of Birmingham, one of the review's lead authors, said this work could change everything. "By listening to the ringing of newly formed black holes, we are turning gravitational waves into a tool for exploring some of the deepest questions in physics, from the nature of gravity itself to the possibility of discovering entirely new forms of matter and energy."
The review was sparked by the largest international workshop ever held on the topic, hosted in Copenhagen in 2024. Researchers say they've already spotted multiple "overtones" in the ringing — similar to how a musical instrument produces harmonics. They've also found strange behaviors where vibration modes merge and interact in unexpected ways.
Looking ahead, new detectors like the European-led Einstein Telescope, the U.S. Cosmic Explorer, and the space-based LISA mission should be powerful enough to catch many more collisions and measure their ringing in much greater detail. These instruments could reveal how black holes actually form, test Einstein's theory with far higher precision, and perhaps even find evidence of particles or forces we've never seen before.
"Black hole spectroscopy promises to transform black holes from mysterious objects into precision laboratories," Carullo said.