Black hole collision supports Hawking's landmark theory

A global team of astrophysicists 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, with their findings now published in the journal Physical Review Letters.

The event, observed by the LIGO-Virgo-KAGRA (LVK) Collaboration on 14 January 2025, involved two black holes around 1.3 billion light-years away merging to form a single, larger one, strikingly reminiscent of the first such 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:

1. Black holes obey the laws of thermodynamics, meaning their surface areas always increase and never decrease.
2. Disturbed black holes behave exactly as predicted by Albert Einstein’s theory of general relativity.

The scientists were also able to test 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.

The team measured the surface areas of the two original black holes — each one with masses between 30 to 40 times that of the Sun — and compared them to that of the final remnant. They determined that the two original black holes had a total surface area of 240,000 km², while the newly formed black hole covered about 400,000 km². The total area had indeed increased, confirming that entropy had risen.

Lead researcher Neil Lu, from the Australian National University (ANU) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), said the results provide the best evidence yet to support both Hawking and Einstein’s hypotheses.

“As black holes spiral together and violently collide into one another, they create ripples in the fabric of space-time known as gravitational waves,” Lu said.

“When two black holes merge to become one, it causes the final, larger black hole to vibrate like a struck bell, ringing out into the cosmos.

“By analysing the frequencies of these gravitational waves created by the black hole merger, we have detected the strongest and cleanest black hole ‘note’ we’ve ever heard. 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.”

Study co-author Distinguished Professor Susan Scott, leader of gravitational wave data analysis at ANU, added, “We now know that the universe is full of black holes. And black holes are not stars — they are geometric objects in space. This analysis sheds light on two of their key properties: black holes can only get bigger, and black holes resonate or ‘ring’ when perturbed.”

The black hole discovery coincides with the 10-year anniversary of the first ever detection of gravitational waves, which were predicted by Einstein 100 years prior to their discovery in 2015. Since then, the gravitational-wave-hunting LVK Collaboration has captured a total of about 300 black hole collisions — as well as a handful of binary neutron star and black hole-neutron star mergers — with scientists now detecting a black hole merger roughly once every three days.

“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 space-time,” Sun said.

Image caption: An artist’s impression of the final, larger black hole, ringing out into the cosmos like a cosmic bell. Image credit: LIGO Scientific Collaboration/Sonoma State University/A Simmonet.