BLACK LIGHT: Black holes are believed to emit a faint glow of radiation due to quantum-mechanical effects. Although such radiation has never been observed, astronomers can identify black holes by the much more luminous radiation given off by matter falling into them. This image from the Chandra X-ray Observatory shows the region surrounding the Milky Way galaxy's central supermassive black hole. Image: NASA/CXC/MIT/F.K. Baganoff et al.
Stephen Hawking is famous for many things: provocative best-selling books, Simpsons guest stints and his long and productive life with amyotrophic lateral sclerosis among them. In the field of astrophysics, the University of Cambridge physicist is also known for his work on gravity and black holes, including his 1974 postulation of the eponymous Hawking radiation, a phenomenon by which a black hole should give off a stream of particles from its outer boundary.
Hawking radiation is predicted to arise from quantum fluctuations at the event horizon of a black hole, the point of no return beyond which even light is too slow to escape. Alongside light waves and regular matter falling into a black hole, Hawking realized, ought to be particles that pop into and out of existence. Quantum mechanics dictates that such short-lived particle pairs arise from even empty space, infusing the vacuum with its own ripples of activity. In most corners of the cosmos, those pairs quickly disappear together back into the vacuum, but at the edge of an event horizon one particle may be captured by the black hole, leaving the other free to escape as radiation.
The relatively faint emission has never been detected from a real black hole, so researchers have sought a number of laboratory proxies to demonstrate the general principles of the phenomenon. Now a group of Italian researchers reports what may be the first demonstration of a quantum-mechanical Hawking radiation analogue. In a paper set to be published in Physical Review Letters, the team reports observing photons trickling out from transient event horizons in a piece of glass.
"We've given what we think are initial indications that Hawking radiation can be measured in the lab," says Daniele Faccio, who led the research at the University of Insubria in Italy but is moving to the Heriot-Watt University in Scotland. Faccio and his colleagues created the event horizons in a two-centimeter-long section of fused silica glass, a medium in which intense laser pulses can locally perturb the refractive index, or speed of light passing through the material.
As that perturbation travels through the glass, it forms a moving blockade for light trying to pass. "If you have a light pulse that's approaching the perturbation from behind, that is trying to catch up to it, it will feel an increase in refractive index that will slow it down," Faccio says. "Imagine yourself sitting on top of this perturbation, and you will see this light wave approaching you until it stops." In other words, the laser-induced perturbation acts as a boundary beyond which light cannot pass—a sort of moving event horizon. If a pair of photons is produced close enough to the event horizon, they will become separated and will be unable to return to the vacuum from whence they came. In a true black hole, the separation would be more pronounced; one of the particles would be lost for good to the black hole.
The researchers recorded photons streaking outward from the event horizon, about one photon per 100 laser pulses, with the traits predicted of Hawking radiation. The photon emission was unpolarized, for instance, and appeared in the right wavelengths. After taking steps to rule out possible contamination from more mundane mechanisms, such as fluorescence, the group concluded that the photons appeared to be spontaneously produced from the same physics underlying Hawking radiation.
Physicists in the field disagree about exactly what the observation means. Ulf Leonhardt of the University of St. Andrews in Scotland, whose group in 2008 proposed the optical method of producing event horizons that Faccio and his colleagues used, says that the new research indeed represents the first observation of Hawking radiation.
But others are not as certain. "I still need to be convinced that what they are seeing is the analogue of what Hawking found for black holes," says William Unruh, a physicist at the University of British Columbia who has demonstrated a classical, or nonquantum, analogue of Hawking radiation in the lab by studying the propagation of waves on moving water surfaces. One possible issue is that the Faccio group's photons emerge from the glass at a 90-degree angle from the direction of the laser pulse. "That is the wrong direction, and it is really hard to see how that could happen," Unruh says.
The direction of emission is "a major point of ongoing discussion," Faccio says, noting that the position of the photon detector was chosen to minimize contamination from the laser. "I sort of prefer to think of it the other way around—we have a spectrum of photons that agrees exactly with theoretical predictions for the event horizon. Now we need to properly understand in detail why they're generated."
Ted Jacobson of the University of Maryland remains on the fence as well, noting that although the emitted photons have some attributes that would be expected from an analogue of Hawking radiation, other predicted features have yet to be confirmed. For instance, the experiment performed by Faccio's group does not allow the researchers to verify that the photons appear in quantum-mechanically correlated pairs at the event horizon.
"In our big piece of glass we have no way of saying where the other photon will end up," Faccio notes. But Leonhardt's group, which is investigating the same phenomenon in optical fibers rather than blocks of glass, might be able to detect both photons from a separated pair and show their common origin. "Once he does that, I think it will close all the discussions," Faccio says. "That will be an undeniable proof that this idea is correct."