Author: Yeshe Fenner

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.

In 1916, Albert Einstein published the paper that predicted gravitational waves – ripples in the fabric of space-time resulting from the most violent phenomena in our distant universe, such as supernovae explosions or colliding black holes.

It took a century for Einstein’s theory to be proven when, in September 2015, the newly commissioned Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors observed gravitational waves that resulted from merging black holes approximately 1.3 billion light-years away. This watershed achievement earned three of LIGO’s founding members the 2017 Nobel Prize in Physics.

Today, more than 80 Australian researchers, amongst over 2000 scientists globally, have published data on the whole catalogue of gravitational-wave observations accumulated since September 2015. In total, 218 events have been recorded including three types of binary mergers: binary neutron star; neutron star–black hole; and binary black hole mergers.

Black holes have a gravitational pull so strong that nothing, not even light, can escape it. This makes them difficult to detect with conventional telescopes. They are characterised by their masses, measured in units equivalent to the mass of our Sun, and their spins.

According to lead Australian author, Christian Adamcewicz, from Monash University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), many aspects of these black holes and the stars that form them remain a mystery.

“By observing the rapidly growing population of compact binary mergers through gravitational waves, thanks to our increasingly sensitive detectors, we’re uncovering vital clues about the lives and deaths of stars,” says Dr Ling Sun from the Australian National University and OzGrav. “Taking the 161 of the 218 mergers seen in the last decade, we’ve been able to decipher aspects of their behaviour from their masses,” Adamcewicz added.

“We found that most black holes have masses less than about 40 times that of our Sun. For a while, we’ve had this hypothesis that heavy black hole progenitors – the stars we would normally expect to turn into black holes heavier than 40 Suns – create supernovae so explosive that any evidence of them is annihilated. We’d never seen clear evidence for that previously, but this newly discovered drop off in our observations matches that prediction really well.”

He adds that “it’s not possible to test stuff on this scale in the lab, so, while we wait to collect the data we need, we rely on extrapolating and piecing together our knowledge from other areas. When you’re talking about the most extreme events in the Universe, these assumptions often break down. In this case, what we thought we would see with black holes in that mass range turned out to be accurate.” Sun shares a similar sentiment; “these cosmic collisions serve as natural laboratories, helping us piece together how black holes and neutron stars form, evolve, and interact across the Universe.”