Two black holes spiral around each other before merging. Like tops on a table, each spins about its own axis. If the axes of both tops point the same way, they likely grew up together. If they point every which way, they met randomly in a crowd. But how can we see that from Earth when black holes are so far away?
Previously, scientists looked at two numbers: one showed how much spin was aligned with the orbit, the other how much was 'wobbling' sideways. But these numbers were often fuzzy. The new approach is to measure the angle between the total spin of the two holes and their orbit. It's like swapping two warped mirrors for one straight one: the picture becomes clearer. On simulated data, the new angle provided more information than the old methods.
One pair is particularly interesting. The larger black hole has its axis tilted by more than 73°, and its mass is about 58 solar masses. Such heavy holes shouldn't form from ordinary stars. It seems this hole itself was born from the merger of two others. If so, we've caught a second-generation black hole by its tilt for the first time.
Now that the new angle works, we can check new mergers faster and better understand where black hole pairs come from. It's like learning to read a compass needle that used to tremble.
The event GW241127 061008 has a Savage–Dickey ratio of 191.3—meaning the data are 191 times more likely under isotropic spins than under aligned spins. If its primary black hole indeed falls into the mass gap, this would be the first such case confirmed by spin tilt.
In Liu Cixin's novel 'The Three-Body Problem', the dynamics of triple systems lead to chaotic orbits and unexpected mergers — similar unpredictability of spin axes is given by dynamical encounters in globular clusters, which is exactly what the θLS parameter captures.