Extreme black holes probably have ‘hair’
Black holes may not be so simple after all. According to a leading idea known as the ‘no-hair’ or ‘black hole uniqueness’ theorem, black holes can be fully characterised using just three data points – their mass, spin and electric charge. There’s no other observable information to be had about these light-gobbling behemoths, which therefore seem to be sleekly and uniquely ‘bald’. But new research casts doubt on the no-hair idea, or at least its universal application: computer simulations suggest that ‘extreme’ black holes – the ones whose spin or electrical charge is fully maxed out – do sport a few wispy hairs here and there.
“This new result is surprising because the black hole uniqueness theorems are well established, [as is] their extension to extreme black holes,” said Lior Burko of Theiss Research in La Jolla, California. “There has to be an assumption of the theorems that are not satisfied to explain how the theorems do not apply in this case.”
The inferred black-hole hairs manifest as a quantity that can be constructed from the spacetime curvature at the black hole horizon that is conserved, and measurable by a distant observer. This quantity depends on details of the black hole’s formation, so it goes beyond the bare-bones trio of mass, spin and charge. Excitingly, such hairs may not remain purely theoretical forever. Scientists might be able to spot them using gravitationalwave detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO).