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Quantum Hair on Black Holes

A black hole (M87) as captured by Event Horizon Telescope
An image of a black hole (M87) Credit: Event Horizon Telescope Collaboration

The signature feature of black holes is that they are black – objects which fall into the hole can no longer be seen and can no longer influence the outside universe. In classical general relativity, No Hair theorems severely restrict the properties of a black hole which can affect other objects. These are limited to its total mass, spin, and charge.

In an upcoming paper in Physical Review Letters, Xavier Calmet (University of Sussex), Roberto Casadio (INFN, University of Pisa), Stephen Hsu (Michigan State University), and Folkert Kuipers (University of Sussex) have shown that these properties no longer hold when quantum gravitational effects are considered.

They show that the quantum state of the graviton field caused by the black hole contains information about the interior state of the hole. This quantum influence extends past the event horizon and to the
furthest reaches of spacetime. As a specific example of quantum effects, they show that exchanges of virtual gravitons result in an extremely weak gravitational force, which is absent in classical physics. The size of this force depends on the specific arrangement of matter which formed the hole, not just on its total mass.

Their results may resolve Hawking’s Black Hole Information Paradox, which has challenged physicists since 1976. The “quantum hair” discovered by Calmet et al. allows information from behind the horizon to emerge in Hawking radiation, so that an evaporating black hole obeys a property called quantum unitarity.  There is also a longer College of Natural Science article on this result.