Black Hole of Light: Laser pulses create model of event horizon

If you’ve ever drifted so close to a waterfall that you could no longer swim fast enough to get away, then you pretty much know what it’s like to fall into a black hole. Researchers have now created a laboratory analog of such a point of no return.

“Space-time really behaves like a river,” says Ulf Leonhardt of the University of St. Andrews in Scotland. “Gravity can be represented as if space were a medium that is flowing.” A swimmer’s best efforts correspond to nature’s ultimate speed limit, which is the speed of light in empty space.

Black holes are regions where gravity curls space-time so much that nothing inside can escape—think of a waterfall that would trap all swimmers, no matter how fast. Both a spaceship approaching a black hole (or a swimmer edging toward a waterfall) will cross a point of no return called an event horizon. That’s where space-time flows into a black hole’s region so fast that even light cannot escape.

In their experiment, Leonhardt and his colleagues created the optical analog of an event horizon. To do so, they exploited the fact that inside matter, light travels slower than in empty space, and at varying speeds. They piped different laser beams simultaneously down an optical fiber. One—playing the role of the river—was a pulse lasting 70 trillionths of a second, and traveling relatively slowly. This pulse was also able to slow other light waves crossing its path.

A second beam—playing the role of the swimmer—initially traveled slightly faster than the first. The light waves of this beam would almost catch up with the pulse, but then get slowed down so much by it that the light waves would fall behind.

The experiment, reported in the March 7 Science, showed that the “swimmer” waves were unable to get into the event horizon, rather than unable to escape it. Thus, it produced not a black hole event horizon, but its opposite, called a white hole. However, Leonhardt says that the pulse behaves like a black hole if the swimmer waves start out traveling slightly slower than the pulse.

Either way, the researchers hope that the optical event horizon will allow them to detect a kind of radiation similar to what Stephen Hawking of the University of Cambridge in England predicted for black hole event horizons in 1974. Hawking suggested that pairs of photons can pop out of empty space as the result of quantum fluctuations outside the event horizon, and one photon out of each pair shoots away, essentially making the black hole glow. Future experiments might produce analogous radiation from the optical event horizon.

“This opens the possibility to make the final step and see … Hawking radiation,” says Grigori Volovik, a theoretical physicist at Helsinki University of Technology in Finland.

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