This galaxy, which sits about 2.5 billion light-years away, hosts two supermassive black holes (inset), visible because of the heated gas, dust, and stars around them. The two black holes are on a collision course, but astronomers still aren’t sure whether they will – or can – merge. (Credit: A.D. Goulding et al./Astrophysical Journal Letters 2019)

By now, merging black holes and the gravitational waves they produce are a scientific surety. Astronomers have observed several black hole merger