On September 14, 2015, a faint but extraordinary signal reached Earth after traveling 1.3 billion years across the cosmos. It was not light, but a subtle ripple in the very fabric of space-time: gravitational waves, first predicted by Albert Einstein a century earlier. That day, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured the first direct evidence of gravitational waves, a discovery announced to the world in February 2016.
Since then, LIGO’s detectors in Hanford (Washington) and Livingston (Louisiana), together with the Virgo detector in Italy and KAGRA in Japan, have formed an international network—known as LVK (LIGO–Virgo–KAGRA).
IMPAN is the leader of the Polish Consortium of the Virgo Project, consisting of the Institutes from Poland participating in the Virgo project.
This collaboration now detects about one black hole merger every three days. To date, the network has observed more than 300 black hole collisions, with many confirmed and others under analysis. During its ongoing fourth science run alone, LVK has identified about 230 merger candidates, more than double the total from its first three runs combined.
The dramatic improvement in detector sensitivity is highlighted by the recent discovery of GW250114 (recorded on January 14, 2025). Like the first detection, GW150914, this event involved two black holes, roughly 30–40 times the mass of the Sun, merging approximately 1.3 billion light-years away. But thanks to a decade of technological advances that reduced instrumental noise, the new signal is significantly clearer, allowing for more precise measurements.
In a paper published on September 10, 2025 (Phys. Rev. Lett. 135, 111405), the LVK team reports that this event provides the strongest observational evidence yet for Stephen Hawking’s Black Hole Area Theorem. First proposed in 1971 (Phys. Rev. Lett. 26, 1344), Hawking’s theorem states that the total surface area of black holes cannot decrease. Merging black holes radiate energy away as gravitational waves and may spin up in the process, both of which could reduce area. Yet the theorem predicts that, despite these effects, the combined black hole must always end up with a greater surface area than the sum of its parts. GW250114 confirms this prediction with unprecedented precision. The theorem has also been generalized to cosmological scenarios by Andrzej Królak, in two papers published in General Relativity and Gravitation (Vol. 16, Nos. 2 and 4, 1984).
Prof. Królak is the leader of the Polgraw-Virgo group, consisting of around 30 scientists from 10 Polish Institutions, which is a member of the Virgo Project. 16 members of the Polgraw group are on the author list of the discovery paper - GW250114: Testing Hawking’s Area Law and the Kerr Nature of Black Holes (Phys. Rev. Lett. 135, 111405).
prof. Andrzej Królak