On 14 September 2015, a signal reached Earth carrying information about a pair of remote black holes that had spiralled towards each other and eventually merged. The signal had travelled for approximately 1.3 billion years at the speed of light — but it was not composed of light. Rather, it was a ripple in space-time known as a gravitational wave, first predicted by Albert Einstein a century earlier.
On that day ten years ago, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever direct detection of gravitational waves. The LIGO and Virgo collaborations announced the discovery to the world in February 2016, following six months of rigorous analysis and verification.
This historic breakthrough opened a new observational window onto the cosmos, enabling researchers to explore the universe in a completely novel way. Previously, scientists had studied celestial phenomena via electromagnetic waves (including X-rays, visible light and radio waves) and high-energy particles such as cosmic rays and neutrinos. This was the first time a cosmic event had been observed through the warping of space-time itself.
For this achievement — the result of a vision first conceived over four decades earlier — three LIGO founders were awarded the 2017 Nobel Prize in Physics: Rainer Weiss (MIT, Professor Emeritus, recently deceased at age 92), Barry Barish, and Kip Thorne (both from Caltech).
A decade later, the international LIGO–Virgo–KAGRA (LVK) collaboration has announced a significant new result: the experimental confirmation of Stephen Hawking’s Black Hole Area Theorem, first proposed in 1971. According to this theorem, the total surface area of black holes can never decrease — not even during a merger.
Thanks to a remarkably clear signal, GW250114, detected in January 2025, researchers were able to precisely measure the increase in surface area following the merger of two black holes, reaching an unprecedented confidence level of 99.999%.
This detection was made possible by major technological advances that now allow the instruments to measure distortions in space-time as small as one part in 10,000 of a proton’s width — or 700 trillion times smaller than the diameter of a human hair.
“This is an amazing time for gravitational wave research: thanks to instruments such as Virgo, LIGO and KAGRA, we can explore a dark universe that was previously completely inaccessible. - said Massimo Carpinelli, professor at University of Milano-Bicocca, INFN researcher and director of the European Gravitational Observatory in Cascina - The scientific achievements of these 10 years are triggering a real revolution in our view of the Universe. We are already preparing a new generation of detectors such as the Einstein Telescope in Europe and Cosmic Explorer in the US, as well as the LISA space interferometer, which will take us even further into space and back in time. In the coming years, we will certainly be able to tackle these extraordinary challenges thanks to increasingly broad and solid cooperation between scientists, different countries and institutions, both at European and global level.