In general relativity, a black hole has long been regarded as a perfect mirror: no matter what fields fall onto it, it does not deform. The tidal Love numbers, which measure how a body deforms under an external tidal field, turned out to be zero for scalar, electromagnetic, and gravitational perturbations. That seemed like an iron rule, until fermions came along.

Fermions are particles with half-integer spin, such as neutrinos. Unlike bosons with integer spin (photons, gravitons), they obey the Pauli exclusion principle and behave differently. The authors of the new work decided to test how a black hole responds to fermionic perturbations and found that the mirror does reflect after all. For a Kerr black hole (rotating and uncharged), the tidal Love numbers turned out to be nonzero and were computed analytically.

This result overturns the previous understanding. It was thought that black holes have no tidal Love numbers at all, but now it is clear that fermionic fields (neutrinos, gravitinos) induce a nonzero conservative response. The dissipative numbers for static fermionic perturbations are identically zero, unlike the bosonic case, where they do not vanish due to frame dragging. This fundamental difference indicates a breaking of the ladder symmetries that explained the zero bosonic Love numbers, and suggests the possible existence of fermionic "hair" on black holes.

Looking ahead, the analysis can be extended to charged black holes, where the coupling of fermionic and electromagnetic perturbations would give rise to mixed Love numbers. Such effects could show up in gravitational-wave signals from binary systems with rapidly spinning black holes, opening a new channel to test general relativity.