In 1974, Stephen Hawking predicted that black holes shouldn't black. Instead they should emit a light fizz of radiation, causing the slightest, faintest glow. Unfortunately, this glow is too faint to pick up here on Earth and, thus, Hawking's prediction is mostly confined to theory. This state of affairs helps enable goofball theories like Laura Mersini-Houghton's sketchy pronouncement that black holes might not even exist.
Physicists will always try though. If a thing is too bizarre or too distant, we might still simulate or create it here on Earth. In 2010, a group of Italian researchers claimed to have mimicked a black hole event horizon—the boundary past which light can longer escape—by tweaking the refractive index of fused silica glass. This tweaking results in a temporary barrier or boundary, a moving event horizon. Within this setup, the group observed streaks of traveling photons sharing many of the properties expected from Hawking radiation.
Those results have since been widely questioned, as some of the other properties observed of these photons were quite the opposite of Hawking's predictions. The Italians' experimental setup is not the only possibility, however, for creating Hawking radiation in a lab setting. Researchers at Technion-Israel Institute of Technology in Haifa have been working on a model black hole based on Bose-Einstein condensate, a state of matter consisting of atoms supercooled to the point that they begin to behave collectively at macroscopic scales, but still according to the bizarre quantum rule-book.
So, it's possible to have a bunch of atoms together acting like a single atom. Using this material, the Haifa researchers were able to create a black hole model that steals sound instead of light. The basic idea is to create a situation where the condensate particles are moving at supersonic speeds, such that sound waves can't keep up and then they effectively disappear within the material as a black hole would gobble up some photons.
We report the observation of Hawking radiation emitted by this black-hole analogue.
"The point where the flow velocity equals the speed of sound is the sonic event horizon," the Haifa team, led by physicist Jeff Steinhauer, explained in a 2009 paper. "The effective gravity is determined from the profiles of the velocity and speed of sound."
This model has been in progress since first being described in the study above, but now, the team is finally declaring that it's detected something comparable to Hawking radiation. "We report the observation of Hawking radiation emitted by this black-hole analogue, which is the output of the black-hole laser formed between the horizons," Steinhauer and his team write in their most recent report.
A quick refresher. Hawking radiation arises at the boundary of a black hole as a feature of quantum mechanics that allows for the spontaneous generation of matter/antimatter particle pairs. This is just what happens in empty space, which is disallowed in the quantum world (empty space is too "definite," too certain). So, empty space produces particles, which usually destroy each other immediately. But at the edge of a black hole, the event horizon might steal one half of the pair, leaving some undestroyed particle. These orphaned particles are Hawking radiation.
Instead of light particles, photons, the acoustic black hole produces pairs of phonons, or sound particles. One is lost, while the other escapes. What's more, as these phonons bounce around at the black hole's edge, they self-amplify, making them measurable.
"I would not say that the case is proven… but it is probably the closest anyone has come," William Unruh, one of the researchers that first proposed the acoustic black hole experiment, told New Scientist. "It is of course clear that black holes differ from flowing BECs, and showing that the effect occurs in a BEC does not prove it would occur in black holes. However, it sure increases my confidence that it does. The mathematics and the results are too similar to just be a coincidence."