In 1974 Stephen Hawking postulated that black holes should give off a trickle of particles, or radiation, from their outer boundaries. The finding established Hawk­­ing’s reputation as a brilliant scientist and set the stage for his highly visible public profile, which includes provocative best-selling books and guest appearances on The Simpsons. In the midst of all the celebrity, the original theory of Hawking radiation, as the black hole phenomenon is known, has almost been forgotten, at least by the general public. The faint emission has never been detected from a real black hole, and researchers have not been able to produce the effect in the lab.

A few years ago a group of scientists in Italy decided to try a new approach to test Hawking’s thesis. They used a piece of glass to re-create a black hole’s “event horizon”—the point of no return beyond which even light is too slow to escape, where Hawk­ing believes the radiation would arise. Alongside ordinary matter and light falling into a black hole, he reasoned, ought to be particles popping in and out of existence. Quantum mechanics dictates that such short-lived particle pairs arise even from empty space; in most corners of the cosmos, those pairs quickly disappear together back into the vacuum. But at an event horizon, one particle may be captured by the black hole, leaving the other free to escape as radiation.

Daniele Faccio of the University of Insubria and his colleagues created the event horizon in a section of fused silica glass, a medium in which intense laser pulses can locally perturb the speed at which light passes through the glass. That perturbation forms a moving event horizon, blocking photons from over­taking it. If a pair of photons is produced close enough to that event horizon, they will become separated and will be unable to return to the vacuum. The researchers recorded photons streaking outward from the glass, about one photon per 100 laser pulses, with all the traits they had predicted for Hawking radiation. They recently published their results in Physical Review Letters.

Physicists disagree about exactly what the observation means. Ulf Leonhardt of the University of St. Andrews in Scotland says the new research indeed represents the first observation of Hawking radiation. Others are not as sure. Theodore A. Jacobson of the University of Maryland says he is more convinced by another group’s recent paper on a nonquantum analogue of Hawking radiation in flowing water. He points out that Faccio’s group cannot verify that photons appear in 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, who proposed the artificial event horizon scheme and is investigating the phenomenon in optical fibers, could detect both photons and show their common origin. “Once he does that, I think it will close all the discussions,” Faccio says.

This article was originally published with the title Hawking Was Right (Probably).

3 Comments

1. 1. Hooshang 06:29 PM 1/27/11

   The infinite negative energy "sea" in Dirac's theory, can be used to tap into, through the vacuum fluctuations (utilizing Heisenberg uncertainty principle) , to produce new particles. The mechanism, which avoids violating Paula Exclusion Principle, could be correlation between Fermions (pairing), which leads to a Bose-Einstein condensation, which in turn releases energy from the "sea" into the "open" positive energy states. One of the resulting pair escapes the "sea", and the other gets absorbed by the "sea" and wondering in it. Here the "sea" acts as a black hole!

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2. 2. Hooshang 06:45 PM 1/27/11

   John, do you enjoy the following similarity?
   The infinite negative energy "sea" in Dirac's theory, can be used to tap into, through the vacuum fluctuations (utilizing Heisenberg uncertainty principle) , to produce new particles. The mechanism, which avoids violating Paula Exclusion Principle, could be correlation between Fermions (pairing), which leads to a Bose-Einstein condensation, which in turn releases energy from the "sea" into the "open" positive energy states. One of the resulting pair escapes the "sea", and the other gets absorbed by the "sea" and wondering in it. Here the "sea" acts as a black hole! The pair production can happen, favorably, at the upper edge of the "sea", right next to the void (gap) 2M. The negative energy "sea" is a perfect degenerate nuclear matter.

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3. 3. r2vettes 11:02 AM 1/15/12

   The article didn't mention Hawking's postulation that only negatively charged particles will pass the event horizon (positrons,etc.), while the positively charged particles go on their merry little way. The big bet with John Preskill and scorn of Susskind, Penrose et al ended with Hawking's surrender, but fellow believer Kip Thorne remained unfettered. Susskin's book "Black Hole Wars" is provacative.

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