An international collaboration of astrophysicists has recorded one of the most detailed observations ever of two black holes colliding—yielding the strongest confirmation yet of Stephen Hawking’s 1971 prediction about black hole thermodynamics. Researchers from the Australian National University (ANU) and partners report that this cosmic event supports the idea that black holes can only grow, never shrink.
A Violent Union
On 14 January 2025, detectors picked up the gravitational waves from two black holes spiralling inward and merging into a single, more massive black hole. The source is estimated to lie about 1.3 billion light-years away.
Each of the progenitor black holes had a mass between 30 and 40 times that of the Sun.
Hawking’s Prediction Under Test
Hawking’s 1971 theory of black hole thermodynamics postulated that when two black holes merge, the surface area of the resulting horizon must be at least as large as the sum of the areas of the original horizons. This is sometimes thought of as a “second law” for black holes, analogous to thermodynamic laws in more familiar physical systems.
By measuring the event, the team determined:
- The two original black holes had a combined event horizon surface area of roughly 240,000 square kilometres (slightly larger than the Australian state of Victoria).
- The merged black hole has an area around 400,000 square kilometres—far exceeding the sum of the originals.
This growth in area is precisely what Hawking’s hypothesis demands. It is, the researchers say, the cleanest and strongest evidence yet for the prediction.
Listening to the Ringing Black Hole
As two black holes merge, their final form vibrates—“rings”—like a struck bell, producing gravitational wave tones called quasinormal modes. The new analysis is the first clear detection not just of one, but several of these tones. These modes match the predictions of Einstein’s theory of General Relativity with remarkable precision.
Dr. Ling Sun of ANU, who led key aspects of the analysis, emphasized that black holes may be characterised only by their mass and spin—yet those horizons encode the disorder, or entropy, of the universe.
Why This Matters
- Historical Context: This detection comes ten years after the first gravitational wave discovery in 2015, itself a century after Einstein predicted such waves.
- Instrumental Advances: Improved sensitivity of LIGO and its partner observatories, Virgo and KAGRA, has permitted much clearer signal capture—roughly three times greater clarity than the 2015 landmark event.
- Volume of Data: Today, the gravitational-wave network captures dozens of black hole mergers per year, opening the possibility to test general relativity and related predictions across many different astrophysical regimes.
Looking Ahead
With this event, we now have the most precise evidence yet that black holes conform to Hawking’s area law. Beyond its theoretical significance, the result strengthens confidence in our ability to use gravitational waves as a tool for probing fundamental physics. It shines a light not only on how these most extreme objects behave, but also on how the universe’s geometric and thermodynamic laws are intertwined.
