More than five decades after Stephen Hawking proposed his controversial black hole area theorem, physicists have finally confirmed it with the most detailed observation to date of a black hole merger. The breakthrough, published in Physical Review Letters and supported by researchers from Caltech, MIT, Columbia University, and the international LIGO-Virgo-KAGRA (LVK) collaboration, validates a key law in black hole thermodynamics—one that links Einstein’s theory of general relativity with the entropy of the universe.
A Cosmic Collision Heard with Unprecedented Clarity
On 14 January 2025, LIGO’s detectors in the US picked up the gravitational wave signal GW250114, caused by the collision of two black holes roughly 1.3 billion light-years away. This wasn’t the first time LIGO had detected such an event, but it was by far the clearest—recorded with a signal-to-noise ratio of 80, significantly sharper than the groundbreaking 2015 detection (GW150914), which had a ratio of just 20.
The two black holes, weighing 33 and 32 solar masses respectively, spiralled into each other and merged into a new black hole of 62 solar masses. The remaining mass—roughly three Suns’ worth—was converted into gravitational energy, rippling across space-time and reaching Earth as a low, fading “chirp”.
This clarity enabled physicists to test Hawking’s area theorem with unprecedented precision. The theory, introduced in 1971, posits that the surface area of a black hole’s event horizon can never decrease—a principle akin to the second law of thermodynamics. In simpler terms: black holes, once formed, can only grow.
A Theory Once Unprovable, Now Unshakeable
Until now, confirming that idea had proven maddeningly difficult. Gravitational waves from black hole mergers had been detected before, but the signals were often too faint or noisy to measure surface areas reliably. Not this time.
Researchers calculated that the combined area of the two original black holes was about 243,000 square kilometres—comparable to the size of Oregon. After the merger, the surface area of the final black hole was measured at 400,000 square kilometres, just shy of California’s footprint. In other words, Hawking was right: the total surface area increased, not shrank.
“This is the strongest observational evidence yet that black holes behave like thermodynamic objects,” said Maximiliano Isi, physicist at Columbia University and Flatiron Institute, who co-authored the study. “Even though it’s a simple idea—areas can’t shrink—it has deep implications for how we think about information, entropy and gravity.”
The results carry a confidence level of 99.999%, making this one of the most robust confirmations of Hawking’s theory to date. A previous attempt in 2021 using earlier data had reached only 95%.
Ringing the Bell of Einstein’s Equations
What made this event even more compelling was what happened after the merger. As the new black hole stabilised, it entered a phase known as ringdown—emitting gravitational waves like a bell settling after a strike. These post-merger oscillations, called quasi-normal modes, carry the black hole’s “voice”—a frequency signature dictated only by its mass and spin.
This clean signal allowed physicists to test another major theory: the Kerr metric, formulated in 1963, which describes how spinning black holes can be fully characterised by just two variables. The team successfully identified two distinct frequencies in the ringdown phase, a first for any real black hole detection.
According to Gregorio Carullo, one of the lead researchers, this marks a milestone: “We’ve finally measured the ringdown in a way that rules out any hidden structure inside the black hole. It behaves exactly as Einstein predicted.”
Precision Astronomy, Ten Years in the Making
The success of GW250114 didn’t happen by chance. It’s the result of a decade of engineering and algorithmic refinement, driven by global collaboration. Since 2015, the LVK network has logged nearly 300 black hole mergers, but none have approached the clarity of this one.
LIGO now detects a black hole merger roughly every three days, thanks to innovations like quantum squeezing, ultra-pure mirror coatings, and AI-enhanced data filtering. Its arms, each four kilometres long, can detect distortions smaller than 1/10,000th the width of a proton.
As Caltech astrophysicist Katerina Chatziioannou put it: “We’re no longer just catching signals—we’re starting to take black holes apart, mathematically speaking. And what we’re seeing is deeply consistent with general relativity.”
Future detectors such as LIGO-India, Cosmic Explorer, and Europe’s Einstein Telescope promise even greater sensitivity. Scientists hope they’ll be able to “hear” black hole mergers from the earliest eras of the universe, perhaps within just a few years.