On 14 January, 2025, two colliding black holes sent the clearest gravitational wave signal ever recorded rippling across the universe to Earth’s detectors. This remarkably crisp signal, designated GW250114, has allowed physicists to conduct the most stringent test yet of Einstein’s general relativity by measuring multiple “tones” from the collision. The wave passed the test with flying colours, but researchers remain optimistic that future detections might finally reveal where Einstein’s century old theory breaks down, potentially offering the first glimpses of quantum gravity.