It’s one of Stephen Hawking‘s most famous hypotheses (though one often co-credited to other researchers): According to the rules of quantum mechanics, a black hole—from which nothing should be able to escape—actually can emit material in the form of Hawking radiation. In the thirty-plus years since the reknowned physicist made his prediction Hawking radiation has remained theoretical, but a research team now claims to have seen it right in the lab.
First, a quick refresher on Hawking radiation:
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. [Technology Review]
The lonesome, unpaired particles streaming away would make it appear that the black hole was emitting radiation, Hawking argued.
The bad news is, you can’t just hunt for a black hole in space to study. Black holes decay faster the smaller they are, meaning the big ones out in space would require practically an eternity to evaporate through Hawking radiation. And the tiny ones, like those feared by people who thought the Large Hadron Collider would mean the end of the world, evaporate almost instantly.
But Franco Belgiorno and his team, the researchers who claim to have seen Hawking radiation in their lab, say they don’t need a black hole. They write in their study: “The essential ingredient of Hawking radiation was not the astrophysical black hole itself but rather the space-time curvature associated to the event horizon.” So they tried to create that curvature a different way.
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. [Technology Review]
Eventually, this can create what’s called a white hole event horizon, a point beyond which light cannot penetrate. Yet, the team writes:
We report experimental evidence of photon emission that on the one hand bears the characteristics of Hawking radiation and on the other is distinguishable and thus separate from other known photon emission mechanisms.
Again, extraordinary claims require extraordinary evidence, so now we await the experiments to come that will try to repeat this find and see whether Belgiorno’s team has really done it.
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