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Tiny sphere bends light like a black hole does

Previously seen at the megascale, gravitational lensing goes micro

BEND IT LIKE A BLACK HOLE  A beam of light curves around a glass sphere embedded in plastic. Light making a close encounter with a black hole would take a similar path.

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Light rays bend around a microscopic sphere just as they would around a gargantuan black hole thanks to a new chip-sized device. The experiment, detailed September 29 in Nature Photonics, demonstrates physicists’ newfound ability to mimic and miniaturize cosmic-scale physical processes in the lab.

“It’s a nice little demonstration that can bend light around 360 degrees, just as gravity can around a black hole,” says William Unruh, a theoretical physicist at the University of British Columbia in Vancouver.

Einstein’s general theory of relativity defines gravity as the curvature of space and time around objects with mass: The more massive the object, the more drastically it warps space and time. As a result, the trajectory of light bends as it whizzes past the universe’s densest, most massive objects, including black holes and neutron stars, the remnant cores of dead stars. Astronomers have confirmed this phenomenon, called gravitational lensing, by observing galaxies that distort light coming from behind them.

Scientists can’t experiment with massive stars and galaxies in the lab, but they are getting good at manipulating light. “Advances in optics over the last five to 10 years allow us to do amazing things,” says Dentcho Genov, a physicist at Louisiana Tech University in Ruston.

Genov and colleagues from Nanjing University in China set out to build a miniature star on a chip that could bend light using optics rather than gravity. Their stars come in the form of tiny glass spheres, each 32 micrometers in diameter, embedded in molten plastic. The researchers poured that mixture onto a silver chip and let it harden. The result was a roughly micrometer-thick layer of plastic, with thicker areas sloped up around the spheres.

The researchers thought that light would bend more as it moves through thicker plastic. That’s exactly what happened when the researchers shined light horizontally through the plastic layer. The light moved in a straight line if it encountered no spheres. But light rays that came close to a sphere got bent. Any beam that came within about 10 micrometers of a sphere was whipped around it. Some light even seemed to get trapped and forced into orbit.

The light’s behavior is consistent with the paths that light takes around a massive star, Genov says. “It’s a perfect analog to astronomical observations.” Similar optical devices could help astrophysicists understand the exotic conditions around black holes and neutron stars, he adds.

Unruh is impressed with the optics but not the potential to clarify the cosmos. Just because the experiment mimics the influence of general relativity doesn’t mean it’s a perfect simulation, he says.

Physicist Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, agrees, saying the device provides a way for nonscientists to visualize the abstract concept of curved space and time. It could find a place in the tech industry too, he says, because data engineers and energy scientists want to build microchips capable of trapping light.

A light beam increasingly curves as it nears a microscopic sphere, and it eventually wraps around the sphere. Light bends because of the optical properties of the plastic it’s passing through, but it would behave similarly in the vicinity of massive stars and black holes. CREDIT: C. Sheng et al, Nature Photonics

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