At its ultracold temperature, the BEC undergoes only weak quantum fluctuations that are similar to those in the vacuum of space. And these should produce packets of sound called phonons, just as the vacuum produces photons, Steinhauer says. The partners should separate from each other, with one partner on the supersonic side of the horizon and the other forming Hawking radiation.

On one side of his acoustical event horizon, where the atoms move at supersonic speeds, phonons became trapped. And when Steinhauer took pictures of the BEC, he found correlations between the densities of atoms that were an equal distance from the event horizon but on opposite sides. This demonstrates that pairs of phonons were entangled — a sign that they originated spontaneously from the same quantum fluctuation, he says, and that the BEC was producing Hawking radiation.

By contrast, radiation that he observed in an earlier version of the set-up had to be triggered rather than arising from the BEC itself, whereas a previous experiment in water waves led by Unruh and Weinfurtner did not attempt to show quantum effects.

Just as real black holes are not black, Steinhauer’s acoustical black holes are not completely quiet. Their sound, if it were audible, might resemble static noise.

“For sure, this is a pioneering paper,” says Ulf Leonhardt, a physicist at the Weizmann Institute of Science in Rehovot, Israel, who leads a different attempt to demonstrate the effect, using laser waves in an optical fibre. But he says that the evidence of entanglement seems incomplete, because Steinhauer demonstrated correlations only for phonons of relatively high energies, with lower-energy phonon pairs seemingly not correlated. He also says he’s not confident that the medium is a true BEC, which, he says, means that there could be other types of fluctuation that could mimic Hawking radiation.

Also unclear is what analogues can say about the mysteries surrounding true black holes. “I don’t believe it will illuminate the so-called information paradox,” says Leonard Susskind, a theoretical physicist at Stanford University in California. In contrast to the case of astrophysical black holes, there is no information loss in Steinhauer’s sonic black hole because the BEC does not evaporate.

Still, if Steinhauser’s results were confirmed, it would be “a triumph for Hawking, perhaps in the same sense that the expected detection of the Higgs boson was a triumph for Higgs and company”, says Susskind. Few doubted that the particle existed, but its discovery in 2012 still earned Peter Higgs and another theorist, François Englert, who predicted it, a Nobel prize.