In a breakthrough, researchers have observed two black holes merging, providing strong evidence that black holes grow during cosmic events and confirming Stephen Hawking’s decades-old theory.
The observation supports Hawking’s 1971 prediction that the surface area of a black hole’s event horizon, the boundary beyond which nothing can escape, can never shrink.
The discovery was made on Jan. 14 by the Laser Interferometer Gravitational-wave Observatory, or LIGO, which recorded gravitational waves generated by the collision.
The findings, published Sept. 10 in Physical Review Letters, were led by Adrian G. Abac, a doctoral student at the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
Collision confirms Hawking’s surface area theory
The event, designated GW250114, involved two black holes merging into a single, more massive object. Before the collision, their combined surface area measured approximately 93,700 square miles (243,000 square kilometers), about the size of Oregon.
After the merger, the resulting black hole had a surface area of roughly 154,500 square miles (400,000 square kilometers), comparable to the size of California.
Gravitational waves finally prove Stephen Hawking’s black hole theorem
An exceptionally loud collision between two black holes has been detected by the LIGO gravitational wave observatory, enabling physicists to test a theorem postulated by Stephen Hawking in 1971… pic.twitter.com/oUXO2LsUdT
— David Ullrich (@DavidUllrich202) September 10, 2025
This increase aligns with Hawking’s second law of black hole mechanics and confirms that the merged black hole’s area was greater than the sum of its parts.
Columbia University physicist Maximiliano Isi, a co-author of the study, explained that although Hawking’s law is a simple statement, that surface areas don’t decrease, it carries deep implications.
He noted that the theory mirrors the second law of thermodynamics, where entropy, or disorder, always increases over time.
Wave analysis reveals black hole properties
The team analyzed the pitch and duration of the gravitational waves to determine the size and shape of the black holes. Similar to how a musical instrument’s tone changes with its size, black holes leave unique signatures in the waves they emit.
This allowed the scientists to measure the event’s “ringdown,” a fading vibration in space-time that follows the merger, much like a struck bell.
Katerina Chatziioannou, a physicist at Caltech, said this ringdown phase offered a clear signal that the final black hole’s surface area exceeded the total area of the original two. The results also support Roy Kerr’s theory from the 1960s, which describes how spinning black holes behave under Einstein’s equations.
LIGO, operated by Caltech and MIT, currently runs two detectors in Washington and Louisiana. Together with observatories in Europe and Japan, they form the LIGO-Virgo-KAGRA collaboration.
A new site in India is expected to join the network by 2030, improving the precision of future detections. Additional projects, including the US-based Cosmic Explorer and Europe’s Einstein Telescope, are also in development to detect the earliest black hole mergers in the universe.
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