Video: Artificial black hole mimics curved spacetime

Light curving through the proton sphere, just as it would around a black hole (Image: C. Sheng, H. Liu, Y. Wang, S. N. Zhu and D. A. Genov)

A plastic black hole traps light just like the real deal, and is the first such structure, natural or artificial, that you can actually watch in action. Unlike the real thing, it isn’t dangerous – but it is helping to demystify one of nature’s weirdest objects and might even have applications for energy-harvesting devices like solar cells.

Black holes are most famous for swallowing light, or anything else in their path. But this fate only awaits objects that get sucked past a point called the event horizon.


Less well known is a black hole’s photon sphere, a region of warped space-time outside the horizon that merely traps light in curved paths. Astronomers have never observed a photon sphere – even outside genuine black holes – because, by definition, trapped light can’t escape and reach your eyes so you can see it.

So to visualise this process, Hui Liu at Nanjing University in China and colleagues built an artificial black hole.

In nature, black holes swallow and 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 the effect of gravity by varying its refractive index, the property that determines how much a substance bends light.

Making light curve

The refractive index is different for different materials. That is why a straw poking out of a glass of water appears crooked: water bends light more than air, so has a higher refractive index. A material with a constantly varying refractive index would take this to the extreme, with lots of little bends creating a smooth curve – rather like a black hole’s photon sphere.

Liu’s team added quantum dots, tiny pieces of semiconducting material that fluoresce when illuminated, to molten acrylic glass, then poured the mixture onto a rotating quartz sheet, slowly spreading it out.

They placed a microscopic polystyrene sphere at the centre, which served as an anchor, with the material thickest nearest the sphere and thinning as it got further away. “This makes the effective refractive index vary in the same way the curvature of space varies around black holes,” says Liu. In fact, the same Einstein field equations used to model black holes can describe the behaviour of light in the acrylic.

Shining a laser through the material allows you to watch the artificial black hole in action – and to visualise other familiar gravitational effects.

Beams that are relatively far away from the microsphere are slightly bent towards it before continuing on their way. When gravity causes the same effect in space, it is known as gravitational lensing. This occurs whenever a light beam passes a massive object such as a star or galaxy, altering the beam’s path as it travels along curved space-time- and can be used to get a better view of distant objects, such as exoplanets.

In the case of the artificial black hole, though, the pull increases as the laser moves closer to the polystyrene sphere, and eventually there is a point where it curves the light completely around. Previously artificial black holes have been created that mimic the event horizon of a black hole, in an attempt to detect a mysterious process called Hawking radiation – but this is the first artificial object to recreate the photon sphere.

Visible sphere

What’s more, unlike a real black hole, the photon sphere can be imaged, thanks to the quantum dots. While the actual light that is trapped remains invisible, as in a real black hole, the quantum dots absorb some of it and emit red light at a different angle, allowing it to escape the black hole’s grasp. This provides an exact trace of the true photon sphere’s path and can be imaged by a camera.

“Our work reports a quite simple and ingenious method to mimic light trapping around a black hole,” says Liu.

Ulf Leonhardt of the Weizmann Institute in Rehovot, Israel, who has previously created an artificial event horizon, says Liu’s structure provides another way to study black holes. “It illustrates that there is no big mystery in the lensing effects in general relativity, you can do the same thing with ordinary materials.”

Liu says the model could be used to study the effects of general relativity around a real black hole, but the ability to trap light could also have more practical applications. “It could be quite useful for solar cells, photon detectors, microlasers and many other energy harvesting devices.”

Journal reference: Nature Photonics DOI: 10.1038/nphoton.2013.247