A global team of astrophysicists, including Australians, has witnessed a collision between two black holes that was so loud, they were able to use it to test—and prove—Stephen Hawking’s Theory of Black Hole Thermodynamics.

The event, observed by the LIGO, Virgo, and KAGRA collaborations, involved two black holes merging to form a single, larger one, strikingly reminiscent of the historic first detection in 2015. But this time, thanks to a decade of instrumental upgrades and data analysis advances, the signal was captured with three times more clarity, enabling scientists to test two fundamental predictions of black hole physics:

• Black holes obey the laws of thermodynamics — their surface areas always increase; never decrease.
• Disturbed black holes behave exactly as predicted by Einstein’s theory of general relativity

“Excited black holes are known to ‘ring’ like cosmic bells at precise frequencies. This is the strongest and cleanest black hole ‘note’ we’ve ever heard,” said Neil Lu, a lead Australian author from the Australian National University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“For the first time, we can clearly identify more than one of the predicted tones from the final black hole, and they match exactly what Einstein’s theory says they should.”

The discovery also tests a profound idea from Stephen Hawking and Jacob Bekenstein: that a black hole’s surface area encodes entropy, a measure of disorder that can only grow. Using this new observation, scientists measured the surface areas of the two original black holes and compared them to that of the final remnant. The result was unambiguous: the total area increased, confirming that entropy had indeed risen.

“We’ve just witnessed the laws of thermodynamics play out on the grandest scales imaginable,” explained Teagan Clarke, a lead Australian author from Monash University and OzGrav. “The final black hole area is bigger than the sum of the originals, just as Hawking predicted.

“This result represents a new step towards understanding the quantum properties of black holes.”

“This merger shows us that black holes obey both simplicity and chaos,” added Dr Ling Sun from the Australian National University and OzGrav. “They’re described only by mass and spin, yet their horizons grow in a way that encodes the disorder of the universe.”

The result marks the culmination of decades of international effort—perfecting ultra-sensitive instruments, pioneering new analysis techniques, and training a generation of scientists to listen for the faintest ripples in spacetime.

“This is a turning point,” said Dr Sun. “A decade after the first detection, gravitational-wave astronomy has evolved from discovery to precision testing of nature’s deepest laws. And with dozens of signals now being detected each year, we’re no longer hearing isolated notes; we’re beginning to hear the full symphony of spacetime.”

The latest discovery is both a celebration of human ingenuity and a glimpse of the transformative science that lies ahead.