A black hole rings like a struck bell, then settles into silence, forgetting almost everything about its history. Why?

by Richard Dyer  BIO

Artist’s impression of GW250114, the celestial collision of two black holes, observed in gravitational waves by the US National Science Foundation LIGO. Courtesy Aurore Simonnet (SSU/EdEon)/LVK/URI/LIGO/CALTECH

is a doctoral researcher at the Institute of Astronomy, University of Cambridge, UK. He specialises in using the gravitational waves from the final part of a black hole merger – called the ringdown – to better understand black holes and gravity. He is also an award-winning science communicator and a writer for the YouTube series PBS Space Time.

Edited byRichard Fisher

It is 9 am on a warm July Sunday in Cambridge, as I head towards a café in the city centre. The bells of Great St Mary’s church play the familiar melody of the Cambridge Quarters. The sound, now often associated with the chimes of Big Ben in London, was first composed here in 1793. The quarters, named for the melody at each quarter-hour, are an immediately recognisable reminder of the passage of time.

Each bell produces a note: a wave with a characteristic frequency that travels through the air and reaches our ears. A hammer strikes the bell, and it oscillates, pushing air molecules back and forth. Those jostling molecules reach our ears and we hear a musical note. Then the bell settles, gives up its energy, and returns to silence.

My research here at the University of Cambridge often brings me back to the ringing of a bell. I don’t study the physics of sound, but I do focus on a remarkably similar process, taking place throughout the Universe. I am interested in another object that also rings: a black hole.

As I sit down with a coffee, I open my laptop to a wavy signal that gradually fades to a flat line. Two black holes have finished merging – a frenzied process of coalescence – leaving an extremely brief wiggle originating from the remnant black hole.

Much like the struck bell, this leftover black hole is ringing in a process called ringdown, and it’s consistent across every merger I investigate. Ringdown allows physicists to test their theories of gravity: did Albert Einstein really have the last word? Just as an off-sounding note from a bell might indicate a design flaw, a discrepancy in the frequencies of a ringing black hole could be the first crack in our understanding of gravity.

That’s the scientific rationale for studying ringdown, but what fascinates me equally is what happens next. After the black hole rings, it settles into silence. In that moment, lasting only a few milliseconds, the power and intensity of the merger are lost. From the exterior, almost all of that history suddenly becomes inaccessible, and what is left is featureless. The black hole settles into a simple, peaceful object of pure spacetime, with no trace of the violence that just happened. In some respects, the black holes forget.

It seems counterintuitive that something so chaotically created can become so simple. Why do black holes seem to forget their violent histories? These strange objects, scattered throughout the Universe, are unlike anything else we know, yet their forgetfulness does not belong to them alone. Just like their ringing, the underlying physics also governs our world. The same ideas connect black holes and our own journey through time, washing away our histories and resulting in loss. The physics is called entropy. And its central lesson is that, when we zoom out, it is not just black holes that forget, but the world around us too.

Much of today’s excitement around black holes concerns their perplexing quantum behaviour. However, I want to tell you about an older, classical understanding of black holes that is strange enough. It traces back to the First World War, when Karl Schwarzschild – a German astronomer serving on the Russian front – uncovered a solution to Einstein’s equations for gravity. His solution described the simplest type of black hole that can exist. Its defining feature is a spherical one-way surface called the event horizon. Things can pass beyond it into the black hole, but they can never leave again, no matter how fast they move or how forcefully they try. That means we can never see what happens beyond this point.

However, sometimes objects approaching this boundary can perturb the black hole enough for us to detect it. In 1957, Tullio Regge and John Wheeler wondered what would happen if Schwarzschild’s perfect black hole was disturbed: struck, like a bell, by something falling in. This was 10 years before Wheeler even popularised the term black hole, and almost 60 years before we heard a real one ring.

Regge and Wheeler wanted to know whether such an object could survive in a Universe full of stuff.

Of course, we now know it can. Regge and Wheeler reached the same conclusion: black holes are stable against small disturbances. But, like a bell, these disturbances set them oscillating for a brief time at predictable frequencies. This is the ringdown. In this way, black holes follow the same rules that govern our seemingly more ordinary world, with its clanging church bells.

Yet, whereas a bell sends sound waves through the air, a black hole sends ripples through spacetime itself, called gravitational waves. Some of these........

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