Scientists have confirmed two long-standing theories relating to black holes—thanks to the detection of the most clearly recorded gravitational wave signal to date.

Ten years after detecting the first gravitational wave, the LIGO-Virgo-KAGRA Collaboration has (10 Sep) announced the detection of GW250114—a ripple in spacetime which offers unprecedented insights into the nature of black holes and the fundamental laws of physics.

The study confirms Professor Stephen Hawking’s 1971 prediction that when black holes collide, the total event horizon area of the resulting black hole is bigger than the sum of individual black holes—it cannot shrink.

A team of researchers led by the Instituto Galego de Física de Altas Enerxías (IGFAE) from the University of Santiago de Compostela (Spain) has measured for the first time the speed and direction of the recoil of a newborn black hole formed through the merger of two others. The result, published today in the journal Nature Astronomy, offers new insights into some of the most extreme events in the universe.

Gravitational waves (GWs) are ripples in the fabric of spacetime that travel away from their sources at the speed of light, encoding information about them. They provide a completely novel information channel that allows us to observe astrophysical phenomena that do not emit light—such as black hole mergers—and obtain new information about processes that do—such as supernovae or neutron-star mergers.

While Einstein predicted the existence of GWs in 1916, they are so weak that detecting them requires incredibly sensitive detectors and extremely violent astrophysical events such as black-hole mergers, supernovae or the Big Bang itself.