Black holes could act as cosmic Rosetta Stones

A QUANTUM makeover means black holes can be described in the two disparate languages of physics - gravity and quantum mechanics. As well as paving the way for a much-sought-after theory of quantum gravity, the idea helps solve some mysteries surrounding these bizarre objects.

Black holes are full of puzzles. Theory says they should evaporate and give off heat at a constant temperature, though no one knows why. For some reason, they also get hotter as they shrink.

Einstein's theory of general relativity, the most popular theory of gravity, is the usual way of describing black holes, which can weigh as much as billions of suns. Georgi Dvali of CERN near Geneva, Switzerland, and Cesar Gomez of the Autonomous University of Madrid, Spain, decided to try the language of quantum mechanics, usually reserved for very small objects.

The first step in building this "quantum portrait", says Dvali, was to define black holes in terms of particles. "In quantum field theory, the building blocks are particles," he said at the Harvard-Smithsonian conference on theoretical astrophysics on 16 May, where he presented the idea.

The pair picked gravitons, the hypothetical massless particles that are thought to carry the force of gravity, just as photons carry the electromagnetic force. Dvali and Gomez reasoned that since a black hole is the densest object known, the gravitons must be packed in as tightly as possible.

Quantum mechanics already has a word for such a system: a Bose-Einstein condensate. In this state particles are so cold and densely packed that they behave as a single quantum object, making quantum effects visible on a macroscopic scale. Considering black holes as an overpacked bucket of gravitons allowed the pair to solve several mysteries, including why black holes radiate energy and get hotter as they evaporate (arxiv.org/abs/1112.3359).

Due to quantum fluctuations, every so often a graviton will get enough energy to leap out of the bucket. An observer outside the black hole will see a temperature rise corresponding to that graviton's energy. With fewer gravitons in the bucket, those remaining cling to each other more tightly, so the next graviton to escape will need more energy.

The quantum portrait could be combined with existing gravitational pictures of black holes, allowing physicists to translate between the two like a Rosetta stone. That might lead to a theory of quantum gravity.

"In this picture, we write down gravitational properties of gravitons in the quantum mechanical language," Dvali says. "We are building a quantum version of Einstein's theory."

Not everyone is convinced. "In my view, black holes are something more subtle than just condensates of gravitons," says Gerard 't Hooft of Utrecht University in the Netherlands. But Dvali thinks the idea is elegant enough to be taken seriously.