A new report describes a way of mimicking the event horizon of a black hole—that infamous point of no return, the boundary beyond which light and matter are forever lost—using nothing more than light pulses transmitted along an optical fiber. With luck, researchers will be able to use the system to study faint particles emerging from the artificial horizon that are analogous to Hawking radiation, theorized by physicist Stephen Hawking to stream from real black holes.
The black holes that astronomers can see, such as the supermassive behemoths in the cores of galaxies such as our own Milky Way, are surrounded by clouds of hot gas that make it hard to study their event horizons in any detail. Although researchers have theorized that microscopic (quantum) black holes might spring from collisions inside the highest energy particle accelerators or when cosmic rays strike the atmosphere, these effects are still speculative.
"We're trying to create an object that is similar to the event horizon of a black hole, and then to study the quantum physics of the black hole in a real laboratory setting," says physicist Ulf Leonhardt of the University of St. Andrews in Scotland.
That may be easier said than done, notes theoretical physicist Ted Jacobson of the University of Maryland, College Park. "It does seem more feasible to set it up and make useful measurements in the near term than anything else I've seen," he says, but he notes that if the pulses can be tweaked to liberate Hawking radiation at all, the particles may still be drowned out by other effects at the horizon.
Black holes are born when matter and energy become so tightly packed that they implode under their own gravity, puncturing a hole of sorts in the fabric of spacetime, known as a singularity. Think of the singularity as a waterfall at the end of a (gravitational) river full of fish. The river speeds up as it approaches the cascade, and passing light waves are like fish swimming upstream. If they venture too close to the waterfall (the river's event horizon), they will take a plunge no matter how fast they swim.
The analogy is close enough that researchers have proposed creating black hole analogues in scores of different ways to try to study the still-mysterious interplay between gravity (the river) and quantum particles (the fish). You can make your own event horizon quite easily by pouring liquid onto a flat surface [see slideshow].
The key to producing a black hole analogue, Leonhardt and his colleagues explain, is to force a fluidlike medium to slosh faster than waves can ripple through it. For their demonstration, reported in Science, the researchers took advantage of the fact that as a light pulse zips through an optical fiber at light speed, it distorts the fiber's refractive index (a measure of how easily light waves pass through a material). In effect, the pulse acts as a speed bump for light.
The researchers sent a red pulse through a fiber, followed by a beam of longer wavelength, faster-moving infrared light to chase down the pulse. The wavelength of the infrared beam shortened, or blue-shifted, indicating that its wave fronts had piled up behind the trailing edge of the pulse, Leonhardt says. Technically, blue-shifting is a feature of a white hole event horizon, which is like an inside-out black hole in that it prevents any light from crossing it, but the researchers note that the leading edge of the pulse would mimic the horizon of a black hole.
Leonhardt says that if researchers can refine the technique to make the pulses sharper—like a shark fin instead of a hill—they may be able to study an effect imagined in the 1970s by physicist Stephen Hawking. He argued that quantum uncertainty should produce pairs of particles straddling a black hole's event horizon such that one member of each pair would flow downstream into the singularity whereas the other would escape. The outgoing Hawking radiation would leave the black hole glowing faintly like a warm coal and would gradually shrink it to nothing.
Hawking radiation as an idea has stood the test of time, but detecting it directly would be impossible amid the maelstrom surrounding a real black hole. Jacobson says he doesn't expect that demonstrating the analogous effect in optical fibers would confirm Hawking's prediction in anyone's mind. But it might stimulate new ideas, he says, adding that "it may even lead people to useful insights or techniques unrelated to the original problem of understanding Hawking radiation."