IN A laboratory in Edinburgh, UK, a revolutionary laser is taking shape - the first to be built from an artificial black hole. It is all part of an effort to confirm Stephen Hawking's startling prediction that black holes emit light.
So-called Hawking radiation, dreamed up in the 1970s, is a consequence of quantum theory, which says that a vacuum is not empty but fizzes with fleeting pairs of particles and their antimatter counterparts. These rapidly annihilate, except if a pair of photons pops out too close to a black hole. Then one may fall in while the other escapes. So although not even light can escape their gravity, black holes should emit a faint glow.
This glow is too dim for us to observe, so physicists are creating experiments that mimic the behaviour of a black hole's event horizon, the threshold beyond which light cannot escape. Daniele Faccio of Heriot-Watt University in Edinburgh and colleagues do this by sending a pulse of light through a block of glass. The pulse temporarily increases the glass's index of refraction, creating a moving zone of high refractive index that acts as an event horizon. Just as light cannot escape a black hole, a second pulse of light cannot cross this zone because the high refractive index would always slow it down.
In 2010, the team reported that the artificial event horizon produced Hawking radiation. Though not everyone was convinced, the team is pressing ahead. Now their focus is on turning their "black hole" into a laser.
Conventional lasers bounce light back and forth between two mirrors. As the photons fly, they excite atoms in the material in between, which spit out new photons, producing a focused, amplified beam.
Faccio's team will replace the two mirrors with light pulses. One is equivalent to a black hole, the other to its "reverse", a white hole - which photons can approach but never enter. The overall set-up is akin to putting a white hole inside a black hole, such that any light injected in between the horizons would bounce back and forth, neither able to enter the white hole nor escape from the black hole (see diagram). Assuming that Hawking radiation exists, the light will also be amplified as it bounces.
Unlike the output of a conventional laser, this bouncing light would change its frequency, Faccio's team predicts. As it approaches the white hole, its wavelength would get squished like a cartoon car hitting a wall, "blue shifting" from its original colour to ultraviolet. When it approaches the black hole, it would get stretched to infrared wavelengths (arxiv.org/abs/1209.4993). The mixed laser beam should emerge from Faccio's set-up when the moving black hole reaches the end of the glass.
The researchers say they have already built the white-hole part using a pulse of light in a diamond. When an infrared pulse was then injected, they told New Scientist, the white hole converted it to ultraviolet, just as predicted. Although the researchers have yet to build the second half, Faccio is encouraged: "It's the Hawking mechanism which is making all this work."
The idea of nesting a white hole inside a black hole to create a laser was originally suggested in 1999 by Ted Jacobson of the University of Maryland, College Park. He warns that the team's model may not resemble a real black hole closely enough to put Hawking to the test.
Even if it doesn't, Faccio says that its behaviour is interesting. "An optical black-hole laser is something very unique," he says. "You can't go to the centre of the galaxy and put a white hole inside the black hole there."
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