Nothing but the hole truth

Scientists have doffed their collective hats to Stephen Hawking. On January 14, 2025, ripples washed over the Earth, carrying echoes of two black holes that had collided about 1.3 billion years ago to form a single, larger one. The wave, a study reports, revealed that the new black hole's surface area was larger than that of its two parents — just as Hawking had predicted in 1971.

Hawking had made his striking prediction based on Einstein's general theory of relativity. When two black holes merge, the surface area of the new black hole's horizon must always be larger than the total area of the two that formed it, contrary to what is seen in other matter.

Since the first detection of gravitational waves in 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. and Virgo in Italy have improved the sensitivity of their detectors threefold. Using these ultra-sensitive detectors, scientists analysed the merger event (called GW250114), confirming Hawking's area law and reinforcing Einstein's theory of general relativity.

"Black holes are made entirely of gravity, and they follow different rules," explains Parameswaran Ajith, a physicist at the International Centre for Theoretical Sciences (ICTS), Bengaluru. Ajith was a part of the international team of scientists who studied the phenomenon, and whose paper was published in Physical Review Letters in September 2025 (bit.ly/Hawking-BH).

Some questions had earlier remained unresolved. Were LIGO's signals truly from black holes or from exotic lookalikes? The area law may not have held for exotic compact objects. If general relativity were not the final word on gravity, the area theorem could fail. "If it turns out that the area is not increasing, then it either means that what we are observing is not black holes or that the true theory of gravity is not general relativity," Ajith explains. The paper clears the confusion.

A newly formed black hole after a merger wobbles before settling down — in what is known as its ringdown phase. Researchers can determine its mass and spin from the frequencies of the ringdown signal. From these values, they calculate the final surface area. By comparing this with the total area of the original black holes, they can test whether the final area is indeed larger.

Ajith's team had developed one of the earliest methods for consistency tests, known as the inspiral-merger-ringdown consistency test, more than a decade ago. "The current area law test is an advanced version of that idea," Ajith adds.

"These are profound results," says Debatri Chattopadhyay, an astrophysicist and postdoctoral researcher at Northwestern in the U.S. "Black holes may hide their inner workings, but from the outside they behave remarkably simply compared to the messy complexity of astrophysics," she adds.