Saswato R. Das in IEEE Spectrum:
To make their event horizon, Leonhardt and colleagues used a titanium sapphire laser and a microstructured optical fiber—one containing a hexagonal arrangement of air-filled holes that ran its length. They first transmitted an ultrashort, intense laser pulse down the optical fiber. The optical fiber is susceptible to nonlinear effects, such that when an intense pulse of light hits the fiber, it changes the physical properties of the fiber. In this case, the first pulse created a distortion that amounted to a change in the fiber’s index of refraction, which moves along with the pulse. The pulse itself was slowed by the distortion. Leonhardt and colleagues then sent a “faster” stream of infrared laser light in pursuit of the first pulse. When the faster-moving second pulse encountered the distortion, it got trapped at its edge and couldn’t break past it. This edge became the fiber’s “event horizon.”
“Light propagating in a moving medium is similar to the light propagating in curved space” such as you would find near a black hole, explains Volovik. So “it is possible to create artificial horizons.”
Following Einstein’s theory of relativity, as light approaches the event horizon, it would slow down immensely and be stretched out; time would also proceed very slowly. Scientists have worked out what this deceleration would look like, and Leonhardt and colleagues say they observed the predicted effects in their optical-fiber event horizon.
Leonhardt and his colleagues hope their artificial event horizon will let experimentalists see whether anything can escape from a black hole. This highly counterintuitive idea was proposed by Stephen Hawking in the 1970s. Hawking applied tenets of quantum mechanics to existing black-hole theory and surmised that black holes are not black at all. Instead, they emit light—which has since come to be known as Hawking radiation.