The real deal is a bit too big to fit on a lab bench (Image: Jupe / Alamy)

A model black hole that traps sound instead of light has been caught emitting quantum particles, thought to be the analogue of the theoretical Hawking radiation. The effect may be the first time that a lab-based black hole has created Hawking particles in the same way expected from real black holes.

Black holes are ultra-dense concentrations of matter left behind when a star or other massive body collapses. Their gravity is so strong that nothing, not even light, can escape from their edge – a boundary called the event horizon.

Given that, physicists expected that black holes would be, well, black. But in 1974, Stephen Hawking of the University of Cambridge predicted they should emit a faint glow of particles now known as Hawking radiation.


An oddity of quantum theory that says that the vacuum of space is not truly empty, but fizzes with pairs of particles and their antimatter counterparts. Normally, these pairs annihilate each other and disappear. But if one gets caught inside a black hole’s event horizon, the other is free to escape and becomes observable as Hawking radiation.

The glow from real-life black holes would be too faint to see so, to confirm Hawking’s prediction, physicists have built artificial black holes that mimic the event horizon.

New horizons

In 2010, a team led by Francesco Belgiorno at the University of Milan made a model black hole, the horizon of which trapped photons using laser pulses in a fibre-optic cable. The team claimed this had produced Hawking radiation – but other researchers questioned whether it used the same physics as a real black hole horizon.

A quantum mechanical fluid should be able to mimic the exact physics of a black hole’s event horizon, albeit at a much smaller scale. In 2009 Jeff Steinhauer at the Technion-Israel Institute of Technology in Haifa and his colleagues made just such a model black hole using Bose-Einstein condensates (BECs), a quantum state of matter where a clump of super-cold atoms behaves like a single atom.

Now, the team claims that their black hole has produced just the kind of Hawking radiation expected of a real black hole. “This tells us that the idea of Hawking actually works,” Steinhauer says. “A black hole should really produce Hawking radiation.”

Black hole laser

The team used one laser to confine the BEC to a narrow tube, and another to accelerate some of it faster than the speed of sound. This fast flow created two horizons: an “outer” one at the point where the flow became supersonic, and an “inner” one further on where the flow slowed down again.

The Hawking effect comes from quantum noise at the horizon, says William Unruh at the University of British Columbia in Canada, one of the first to propose fluid-based black hole analogues. The horizons create pairs of particles of sound, or phonons. One phonon escapes the horizon, and the other is trapped inside it.

A single phonon is too weak to observe, but the phonons inside the black hole bounce back and forth between the inner and outer horizons, triggering the creation of more Hawking phonons each time, much like a laser amplifies light. Physicists call this effect a black hole laser.

“The Hawking radiation exponentially grows, it self-amplifies,” Steinhauer says. “That allows me to observe it, because the amplitude has grown.” In the future he hopes to improve his detectors to sense radiation from a single horizon, which could help determine whether the pairs of phonons are entangled – another predicted feature of real black holes that may have fiery consequences.

Game changer

“This work is really impressive,” says Daniele Faccio at Heriot-Watt University in Edinburgh in the UK who was on the team that made the fibre-optic based black hole. Although he stands by his work as the first to show that Hawking-like effects can be measured, he admits that it was more open to interpretation than Steinhauer’s. “This work has really raised the bar and we now see, I think this is fair to say, the first clear cut evidence of Hawking-related effects seeded from the quantum vacuum. I think this work is really a game changer.”

Unruh is a bit more reserved. “I would not say that the case is proven… but it is probably the closest anyone has come,” he says. “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.”

Journal reference: Nature Physics, DOI: 10.1038/nphys3104