Hunting for Hair on Coalescing Black Holes

A black hole resulting from the merger of two smaller black holes exists for the first few milliseconds in a distorted state. As its geometry stabilizes, it radiates a gravitational waveform whose shape carries information about its mass, spin, and in principle, electrical charge. General relativity predicts that these quantities alone are sufficient to describe any black hole—there are no other distinguishing features, or “hair.” Taking a new look at the first gravitational waves ever detected (known as GW150914), Maximiliano Isi, of the Massachusetts Institute of Technology, Cambridge, and colleagues from the California Institute of Technology, Pasadena, and the Flatiron Institute, New York, have now tested this no-hair theorem through an approach dubbed black-hole spectroscopy. The approach entails an in-depth analysis of the frequency spectrum of the gravitational-wave signal.