In January 2025, the loudest gravitational wave ever recorded hid a second signal nobody knew how to read — and a decade after the first detection, it has just given physicists the spin rate of a black hole's event horizon - Space Daily

The signal arrived at LIGO on January 14, 2025. Two laser beams running down 4-kilometer vacuum tubes in Hanford, Washington and Livingston, Louisiana flickered by a distance roughly ten-thousandths the width of a proton, lasted a fraction of a second, and we…