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New horizons for Hawking radiation

In 1974, Steven Hawking predicted that black holes were not completely black, but were actually weak emitters of blackbody radiation generated close to the event horizon—the boundary where light is forever trapped by the black hole’s gravitational pull.

Hawking’s insight was to realize how the presence of the horizon could separate virtual photon pairs (constantly being created from the quantum vacuum) such that while one was sucked in, the other could escape, causing the black hole to lose energy.

Hawking’s idea was significant in suggesting a possible optical signature of a black hole’s existence. Yet, even though the prediction created an extensive theoretical literature in cosmology, calculations have since shown that Hawking radiation from black holes is so weak that it would be practically impossible to measure.

It turns out, however, that the physics of how waves interact with a horizon does not depend in a fundamental way on the presence of gravity at all.

In principle, an analogous Hawking radiation should occur in other systems. The key requirement is simply that the interaction between waves and the medium in which they propagate causes there to be a boundary between zones where the wave and the medium have different velocities.

In a paper in Physical Review Letters, Franco Belgiorno at the Università degli Studi di Milano, in collaboration with researchers at several other institutes, also in Italy, describe a series of experiments where high-intensity filaments of light in glass perturb the optical propagation environment in an analogous manner to the way a gravitational field affects light near a black hole horizon.

This perturbation creates the optical equivalent of an event horizon that allows the team to make convincing measurements of analog Hawking radiation at optical frequencies. These results are highly significant in suggesting a system in which Hawking’s prediction can be fully explored in a convenient laboratory environment.

Image: Creating an analog gravitational potential in an optical system. (Left) A suitable change in the index of refraction in a moving medium creates an effective event horizon for photons that propagate with it. As indicated by the pink arrow, photons are forbidden from entering beyond the optical event horizon. The equation below relates the analog black body temperature of the optical white hole to how the change in the index varies in time (τ). (Right) Researchers use a high-intensity light filament to perturb the index of refraction in glass and create an optical event horizon and measure the analog Hawking radiation emerging at right angles from the filament.

• Source: Full article at APS Physics • The paper is available at http://physics.aps.org/pdf/10.1103/