One of the common descriptions of black holes is that their gravitational pull is so strong, not even light can escape it. Stephen Hawking is famous for (among other things) showing that this isn't actually true. The Hawking radiation that bears his name allows matter to escape from the grip of a black hole. In fact, Hawking's work suggests that an isolated black hole would slowly evaporate away and cease to exist.

But his work remains entirely theoretical. Hawking radiation is expected to be so diffuse that we could only detect it if we could somehow find or create a black hole isolated from all other matter. But Jeff Steinhauer of Israel's Technion has been on a sometimes single-handed quest to develop a system that can accurately model a black hole's behavior. And, in a recent paper in Nature Physics, Dr. Steinhauer describes how his model system generates what appears to be Hawking radiation.

Searching for the horizon

A feature called the event horizon plays a central role in both Hawking radiation and the new model system. At a real black hole, the space-time outside the event horizon may be distorted by the intense gravity, but the distortion is relatively limited. Inside the event horizon, however, space-time is stretched at a rate that's faster than the speed of light. Photons can't escape because the space-time they occupy is getting stretched away from the event horizon faster than the photon can move.

Quantum mechanics, however, does funny things at the event horizon. More accurately, it does funny things everywhere, but an event horizon isn't excluded. Among those funny things is the creation of virtual particle pairs. These pairs, one matter, one antimatter, spontaneously pop into existence and then quickly annihilate each other, thus having no net effect on the Universe.

Near a black hole, however, there's the possibility that the two particles will appear on opposite sides of the event horizon. When this happens, one is lost forever inside the black hole, while the second can potentially escape its draw. The particles that escape form Hawking radiation. And, despite what is effectively an infinite distance between them, the two particles are entangled.

So, to study Hawking radiation, you have to figure out how to create an event horizon in the lab. Steinhauer has apparently managed to build its equivalent for sound using a gas of ultracold atoms and some lasers.

A black hole for sound

Steinhauer creates a Bose-Einstein condensate by chilling a cloud of rubidium atoms down to where the atoms occupy the same quantum state. Here, the individual atoms in the condensate ensure behavior equivalent to the quantum nature of space-time. Rather than light or particles, however, Steinhauer focuses on phonons, which are individual quanta of vibrational energy. You can think of a phonon as an individual packet of sound.

To create an event horizon, Steinhauer first stretches the condensate out so that it behaves like a one-dimensional system. Then, he uses lasers to control the flow of the condensate, creating a critical transition. On one side of the transition, the atoms flow at speeds that are below the speed of sound in the condensate. On the other side of the transition, however, atoms flow away at faster than the speed of sound. Any phonons that are generated on that side of the transition can never cross it, since the condensate travels away from it faster than they can propagate.

This is clearly analogous to an event horizon. But Steinhauer has spent several years and multiple papers (sometimes alone, sometimes with collaborators) laying out that it's not just analogous, but physically equivalent. His work shows that the phonons that spontaneously appear in the condensate are the equivalent of Hawking particles, and correlations in their density would be evidence of entanglement.

The behavior isn't exactly identical; for example, the equations describing the lab system include terms that have no equivalent at a black hole. "The neglected terms represent correlations between widely separated phonons on opposite sides of the horizon with different frequencies," he writes in his new paper. "Hawking radiation would not create such correlations, and it is not clear what would." But the differences are all in things that can be ignored without affecting the properties that are equivalent.

With the groundwork complete, Steinhauer turns to measuring the correlation between phonon densities on either side of his event horizon. This is harder than it might sound, as the measurements necessitate "an ensemble of 4,600 repetitions of the experiment, requiring six days of continuous measurement."

But the measurements work. Phonons on opposite sides of the event horizon show a strong correlation in density that is the equivalent of entanglement. And these entangled pairs of phonons cover a broad spectrum of energy, exactly as predicted by Hawking's work.

The implications are significant, and Steinhauer puts them succinctly: "The measurement reported here verifies Hawking’s calculation, which is viewed as a milestone in the quest for quantum gravity. The observation of Hawking radiation and its entanglement confirms important elements in the discussion of information loss in a real black hole."

Nature Physics, 2016. DOI: 10.1038/nphys3863 (About DOIs).