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unknownskywalker:

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/

unknownskywalker:

First Observation of Hawking Radiation

For some time now, astronomers have been scanning the heavens looking for signs of Hawking radiation, predicted it in 1974. So far, they’ve come up with zilch. Today, it looks as if they’ve been beaten to the punch by a group of physicists who say they’ve created Hawking radiation in their lab. These guys reckon they can produce Hawking radiation in a repeatable unambiguous way, finally confirming Hawking’s prediction. Here’s how they did it.

Physicists have long realised that on the smallest scale, space is filled with a bubbling melee of particles leaping in and out of existence. These particles form as particle-antiparticle pairs and rapidly annihilate, returning their energy to the vacuum.

Hawking’s prediction came from thinking about what might happen to particle pairs that form at the edge of a black hole. He realised that if one of the pair were to cross the event horizon, it could never return. But its partner on the other side would be free to go. To an observer it would look as if the black hole were producing a constant stream of quantum particles, which became known as Hawking radiation.

Since then, other physicists have pointed out that black holes aren’t the only place where event horizons can form. Any medium in which waves travel can support an event horizon and in theory, it should be possible to see Hawking radiation in these media too.

Now, physicists at the University of Milan say they’ve produced Hawking radiation by firing an intense laser pulse through a so-called nonlinear material, that is one in which the light itself changes the refractive index of the medium. As the pulse moves through the material, so too does the change in refractive index, creating a kind of bow wave in which the refractive index is much higher than the surrounding material.

This increase in refractive index causes any light heading into it to slow down. By choosing appropriate conditions, it is possible to bring the light waves to a standstill. This creates a horizon beyond which light cannot penetrate, what physicists call a white hole event horizon, the inverse of a black hole.

White holes aren’t so different to black holes. And it’s not hard to imagine what happens to particle pairs that form at this type of horizon. If one of the pair crosses the horizon, it can make no headway and so becomes trapped. The other is free to go. So the horizon ought to look as if it is generating quantum particles.

It is this radiation that physicists say they’ve seen by watching from the side as a high power infrared laser pulse ploughs through a lump of fused silica. That’s an astounding claim and one that many physicists will want to pour over before popping any champagne corks.

Why is it important? One reason is that Hawking radiation is the only known a way in which black holes can evaporate and so a proof of its existence will have profound effects for cosmology and the way the universe will end. And now that it’s been observed once, expect a rash of other announcemetns as researchers race to repeat the result.

Image: In the experimental set-up, a laser beam strikes a sample of fused silica glass (FS). An imaging lens (I) collects the photons emitted at 90 degrees and sends them to a spectrometer and CCD camera.

• Source: The Physics ArXiv Blog • The paper is available at arXiv.org

NewScientist:

A supermassive black hole is thought to sit at the centre of most large galaxies. In some so-called active galaxies, enormous quantities of gas are swirling into the black hole, forming a disc of hot matter around it that often outshines the billions of surrounding stars.

Our own galactic monster is less well fed, surviving on only a thin gruel of gas streaming out from nearby stars. As this gas falls towards the hole it also heats up and shines, though more faintly than the disc in an active galaxy. All kinds of electromagnetic radiation are emitted, ranging from radio to X-rays (see image).

Of course, the black hole itself does not shine since it actually swallows light. That is how we hope to be able to see it: light from gas swirling round the hole will be devoured, so the hole should show up as a shadow or silhouette against the background of hot, shining gas.

Another black hole video and some images taken of a supermassive blackhole’s “shadow”, which consists of all matter the gets captured by its huge gravitational field, including light and other waves.