In a laboratory in Scotland, a revolutionary kind of laser is taking shape ? the first one to be made out of an artificial black hole.
Once complete, the device could help confirm mounting evidence that real black holes, despite their name, emit light. A black-hole laser could also find practical uses in devices that probe a material's properties without damaging it.
At the heart of such a laser is a phenomenon that Stephen Hawking predicted in the 1970s, and that physicists have been hunting ever since. Although not even light can escape their gravity, Hawking calculated that black holes should nonetheless emit a faint glow, now called Hawking radiation.
This is a consequence of quantum theory, which says that a vacuum is not truly empty, but fizzes with fleeting pairs of particles and their antimatter counterparts. Normally, these pairs rapidly annihilate and disappear again, but if a pair of photons pops out too close to a black hole, one falls in ? and the other escapes.
Artificial event horizon
The resulting glow around naturally occurring black holes, such as the one at the centre of our galaxy, would be too dim to see. So to confirm Hawking's theory, physicists have taken to building artificial analogues in the lab by mimicking the physics of a black hole's event horizon, the surface beyond which light cannot escape.
In 2010, Daniele Faccio of Heriot-Watt University in Edinburgh, UK, and colleagues built one such system. As gravitational event horizons are tough to make, not to mention potentially dangerous, their "horizon" was a pulse of laser light moving through a piece of glass. The pulse temporarily increased the glass's refractive index as it travelled, creating a gradient in its wake that would slow a second pulse of light trailing it to a greater extent the closer the two got. Crucially, the second pulse would never be able to cross the first, turning the first into an event horizon from the point of view of the second.
The researchers detected excess photons emanating from their horizon, which they take to be evidence of Hawking radiation. Other physicists disagree about whether that observation counts, since the team was not working with a real black hole.
Soon though, the Heriot-Watt team may be able to make their model black hole do something that real ones cannot: act as lasers. That might produce more conclusive evidence of Hawking's theory.
Conventional lasers build their bright beams by bouncing light back and forth between two mirrors and through a block of material called the gain medium. As photons fly, they excite the atoms in the gain medium, which spit out new photons of exactly the same frequency, eventually creating a focused beam of laser light.
Cosmological nonsense
Back in 1999 Ted Jacobson of the University of Maryland, College Park, suggested replacing the two mirrors with a black hole and its "reverse" ? a white hole. Never detected in the wild, a white hole allows photons to approach but never lets them in.
Faccio's team have worked out how to put Jacobson's idea into practice. Their plan is to send two pulses of light through an artificial diamond in quick succession, the equivalent of an artificial white hole nested inside a black hole. "This really doesn't make much sense from a cosmological point of view," says Faccio. "It would be nonsense. But in these lab models, you can do this."
Light injected in between the horizons would bounce back and forth, as it can neither enter the white hole nor escape from the black hole. Crucially, if Hawking radiation exists, it would get amplified as it bounces, forming laser light that should be easier to detect than the excess radiation spotted in previous experiments.
Unlike in an ordinary laser, however, the light would repeatedly change frequency, or colour, on its journey, Faccio's team predicts. That's because, unlike two mirrors facing each other, the event horizons of a white hole and a black hole have the opposite effect on light reaching them.
Squished and stretched
As the bouncing light approaches the white hole's event horizon, where the refractive index suddenly increases, its wavelength would get squished like a cartoon car hitting a wall, "blue shifting" from its original colour to ultraviolet. When this bounces back towards the black hole event horizon, exactly the opposite would happen ? the light would get stretched out to infrared or terahertz frequencies. "It's a really weird laser, because the light is constantly shifting between two extremes," Faccio says.
These plans are due to be published in the journal Classical and Quantum Gravity. The team has already succeeded in building the white-hole half of the laser, however, by sending a pulse of light into a diamond 0.5 millimetres long that is transparent to all wavelengths used in the experiment. When a pulse was injected behind it, the white hole converted the infrared light to ultraviolet light, just as predicted ? though the researchers haven't yet submitted this result for publication.
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