HOW do you see an object when its gravity is so strong that not even light can escape it?
Black holes are inherently mysterious for precisely this reason. Now an artificial black hole has been made that traps light like the real deal – and can be watched in action, safely. As well as demystifying one of nature's weirdest objects, this lookalike may find uses in devices that trap light, like solar cells.
Black holes are most famous for swallowing light, or anything else in their path, a fate that awaits something unlucky enough to pass a point called the event horizon. Less well known is a black hole's photon sphere, a region of warped space-time outside the event horizon that merely traps light in curved paths. No photon sphere has ever been seen because, by definition, trapped light can't reach your eyes.
So to visualise one, Hui Liu at Nanjing University in China and colleagues built an artificial black hole. Real black holes trap light via their immense gravity, something that would be difficult, not to mention incredibly dangerous, to recreate in the lab. Instead, Liu's team used a sheet of plastic, and mimicked gravity by varying the plastic's refractive index, the property that determines how much a substance bends light.
They reasoned that a material with a varying refractive index could bend light like a black hole's photon sphere, but without dangerous levels of gravity.
They created the material by pouring molten acrylic glass onto a spinning quartz sheet and planted a microscopic polystyrene sphere at the centre. The sphere acts as an anchor, causing the glass to solidify thickly at the centre then gradually become thinner further away. "This makes the refractive index vary in the same way the curvature of space varies around black holes," says Liu.
Shining a laser through the glass allows you to watch the artificial black hole in action.
Beams that are relatively far away from the microsphere at the centre of the glass are slightly bent towards it before continuing on their way – mimicking an effect known as gravitational lensing. This can be used to detect galaxies, stars and planets when it occurs in space.
In the case of the artificial black hole, though, the pull increases as the laser moves closer in. Eventually there is a point where the light curves completely around – the first artificial photon sphere (Nature Photonics, doi.org/n25). Crucially, unlike in a real black hole, the photon sphere can be seen because Liu's team sprinkled tiny light emitters called quantum dots into the glass. These absorb trapped light and re-emit it at a different angle, allowing it to escape and provide a visible trace (see images, left).
Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, says it is remarkable to see the mathematics of real black holes – Einstein's equations of general relativity (see "A hairy problem") – playing out in a model. "It illustrates that there is no big mystery," he says. "You can do the same with ordinary materials." His team has built mimics of a black hole's event horizon, but not its photon sphere.
Liu adds that the device could be used to study a black hole's tug in more complicated scenarios, while its ability to trap light could be useful in solar cells, photon detectors or microlasers.
This article appeared in print under the headline "Light kidnapped by black hole lookalike"
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