Physics Viewpoint | American Physics Society | 2016 Jun 06
A black hole may carry “soft hair,” low-energy quantum excitations that release information when the black hole evaporates.
[img3="Hawking, Perry, and Strominger suggest that black holes might have “soft hair,” low-energy quantum excitations that release information when the black hole evaporates. (Credit: APS/Alan Stonebraker)"]http://physics.aps.org/assets/53b89fc5- ... medium.png[/img3][hr][/hr]Four decades ago, Stephen Hawking proposed that black holes could destroy information—a conclusion that is incompatible with standard laws of quantum physics. This idea started a controversy known as the “black hole information problem” that even now has not been resolved. A new study by Hawking himself and Malcom Perry, both at the University of Cambridge, and by Andrew Strominger at Harvard University shows that some of the assumptions that led to the information problem might be wrong. Their results do not completely solve the problem, but point to a promising research direction that might lead to its long-awaited solution.
According to Einstein’s general theory of relativity, stationary black holes are completely determined by just three observable parameters: their mass, charge, and angular momentum. Almost none of the information about what fell into the black hole is visible from the outside. Physicist John Wheeler described this idea by saying that “black holes have no hair.” ...
If both gravity and electromagnetism are described classically, the contribution to the new charges coming from the black hole horizon must vanish. But Hawking, Perry, and Strominger argue that the situation is very different when electromagnetism is described quantum mechanically. To understand the difference, first consider the vacuum state and then add one photon. The result is a new quantum state with energy equal to the energy of the photon. As Strominger showed, if one takes the limit as the photon energy goes to zero (that is, the photon becomes “soft,” with vanishing energy), the result is a new state, which can be called a new vacuum because it has essentially the same energy as the original vacuum state. The first vacuum is turned into the second by acting with an operator that is just the quantum version of the new conserved charge.
The authors’ work now shows that acting with this same operator on a black hole horizon adds photons with essentially zero energy. These photons make up what they call the “soft hair” on a black hole. Since there are an infinite number of new charges, there are an infinite number of soft hairs that a black hole can support. Furthermore, the researchers demonstrate that when a charged particle falls into the black hole, it excites some of this soft hair. The exact conservation of the new charges implies that when a black hole evaporates, the information about the hair on the horizon must come out in the Hawking radiation ...
Soft Hair on Black Holes - Stephen W. Hawking, Malcolm J. Perry, Andrew Strominger
- Physical Review Letters 116(23):1301 (2016 Jun 10) DOI: 10.1103/PhysRevLett.116.231301
arXiv.org > hep-th > arXiv:1601.00921 > 05 Jan 2016
