Professor Jeff Steinhauer in his lab.

Image Credit: Nitzan Zohar, Technion Spokesperson's Office

The exotic cosmic objects we call black holes aren’t truly holes, and it turns out that they may not be totally black either. In an article that appears today in the journal, Jeff Steinhauer from the Israel Institute of Technology (Technion) outlines the strongest experimental evidence yet that energy can escape from a black hole.Black holes are extremely dense areas of space defined by an event horizon, a boundary beyond which nothing that gets sucked in can escape—not even light (hence the “black” in “black hole”). Theory predicts that black holes can be the size of an atom or millions of times as massive as the sun, although smaller ones are less stable. As strange and unique as they seem, there are likely millions of black holes in the universe, including at least one at the center of each galaxy.Nearly 50 years ago, bold work by then-graduate student Jacob Bekenstein inspired black hole expert Stephen Hawking to take a closer look at the theoretical physics governing black holes. In the process, a surprised Hawking discovered that quantum mechanics enables some energy to escape from black holes. Hawking realized that over time this could cause black holes to shrink and evaporate.Experimentally verifying or ruling out this “ Hawking radiation ” might seem like just a scientific curiosity, but it is actually an important test of our understanding of the universe and its behavior. Its existence would answer some questions, but raise others.One of the biggest unsolved problems in physics is how general relativity merges with quantum mechanics. Gravitational effects and quantum effects meet head-on in black holes, so they are an ideal place to study this. However, the Hawking radiation escaping from a cosmic black hole is so small that we aren’t able to detect it directly, at least not yet.If you can’t study Hawking radiation from a cosmic black hole, why not build your own black hole? Okay, how about a system with similar properties? The experiment reported in thearticle involved an analogous system called an acoustic black hole. Acoustic black holes don’t occur in nature, but they can be built out of a fluid whose flow changes from subsonic to supersonic. The idea was proposed in 1981 by William Unruh.An acoustic black hole is similar in many ways to a cosmic black hole, but it traps sound instead of matter and light. It turns out the equations that describe how gravity affects light are the same equations that describe how a flowing fluid affects phonons , which you can think of as a kind of particle that makes up sound waves.Like the event horizon of a black hole, the event horizon of an acoustic black hole is the point of no return. Any sound that goes in will not come out—unless the hole emits the phonon equivalent of Hawking radiation. The systems are so similar that if you detect phonon radiation coming from an acoustical black hole, Hawking radiation most likely exists too.Confirming Hawking radiation goes beyond detecting radiation outside of a black hole. The particles emitted by Hawking radiation are quantum mechanically connected or “entangled” with partner particles that are pulled into the black hole. Finding evidence of this is key to verifying its existence.Steinhauer’s work is the first experimental evidence of such entangled particles. In 2009, he and his colleagues created the first acoustic black hole. They did so in a Bose Einstein condensate (BEC), a special, quantum state of matter in which many very cold atoms behave like a single atom. Using a laser, the researchers created an event horizon at which the flow of atoms in the BEC goes from subsonic to supersonic.Since 2009, the researchers have improved the system and developed high-resolution imaging techniques for studying phonons and their partner particles. Today’s article presents their exciting results, which include observations of entangled phonons emitted by the acoustic black hole. The characteristics of these phonons are consistent with what you’d expect from Hawking radiation.This isn’t the only experimental attempt to detect Hawking radiation. Through a variety of experiments based on several different analogous systems, scientists around the world are searching for signs of Hawking-like radiation even as you read this. Together with Steinhauer’s work, these experiments should help us build a more complete picture of just how “black” black holes are.