Black hole mergers can't tell electric from magnetic charge

Electromagnetic duality — the symmetry that swaps electric and magnetic fields — has now been tested for the first time in full numerical relativity simulations of merging black holes. Across an entire family of electrically, magnetically, and 'dyonically' charged binaries, the gravitational waves are identical; only the polarization of the emitted electromagnetic radiation rotates by the duality angle. This means gravitational-wave detectors alone could never distinguish an all-electric black hole binary from a magnetic one, a concrete degeneracy with implications for interpreting future multi-messenger observations.