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First Observation of Hawking Radiation

Hawking predicted it in 1974. Now physicists say they’ve seen it for the first time

For some time now, astronomers have been scanning the heavens looking for signs of Hawking radiation. 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.

Today, Franco Belgiorno at the University of Milan and a few buddies 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,” say Belgiono and co. 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 (in fact Hawking argues that they are formally equivalent). 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 Belgiorno and co say they’ve seen by watching from the side as a high power infrared laser pulse ploughs through a lump of fused silica. Their pulse has frequency of 1055nm but the light they see emitted at right angles has a wavelength of around 850nm.

Of course, the big question is whether the emitted light is generated by some other mechanism such Cerenkov radiation, scattering or, in particular, fluorescence which is the hardest to rule out.

However, Belgiorno and pals say they can rule out all these sources of light for the radiation they see. In particular, they that the fluorescent light is well characterised and that it differs in various significant ways from the emissions they see. Therefore, they must be seeing Hawking radiation, they conclude.

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.

Ref: arxiv.org/abs/1009.4634: Hawking Radiation From Ultrashort Laser Pulse Filaments

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