A lab-created analogue of a black hole has provided new evidence that these mysterious space objects really do emit radiation. This evidence is indirect: physicists have shown that the analogue has a temperature, which is a necessary prerequisite for the eponymous radiation predicted by Stephen Hawking.
Under general relativity, a black hole is inescapable. Its gravitational power is so intense, not even light – the fastest thing in the Universe – can achieve escape velocity. Therefore, a black hole under general relativity emits no electromagnetic radiation.
But according to a theory Hawking put forward in 1974, a black hole does emit something when you add quantum mechanics to the mix: a theoretical type of electromagnetic radiation called, fittingly, Hawking radiation.
This theoretical emission resembles black-body radiation, produced by the temperature of the black hole, which is inversely proportional to its mass. Actually detecting it, however, is so much easier said than done. This radiation, if it exists, would be way too faint to be found with our current instruments.
And taking a black hole’s temperature is no easy task either. A black hole with the mass of the Sun would have a temperature of just 60 nanokelvins. The cosmic microwave background radiation it would absorb would be far higher than the Hawking radiation it emits, and the bigger the black hole, the smaller the temperature.
Cue the lab analogues. We saw one earlier this year made from optical fibre. This time, the system is made from a cluster of ultracold rubidium atoms cooled to just a few billionths of a degree above absolute zero. These are called Bose-Einstein condensates.
When this condensate starts flowing, it creates something called an acoustic black hole: this object traps sound (phonons) instead of light (photons). On the high-energy side of the experiment, the condensate flows slowly; on the low-energy side, it flows faster. In-between is a sonic ‘event horizon’.
As physicist Jeff Steinhauer of Technion-Israel Institute of Technology and colleagues demonstrated in 2016, when a pair of entangled phonons appears at this event horizon, one is propelled away by the low-speed condensate – that’s the Hawking radiation analogue.
Meanwhile, the high-speed condensate moves faster than the other phonon, so it gets swallowed by the analogue black hole – or so Steinhauer’s team thought. But Ulf Leonhardt, who led the optical fibre experiment mentioned earlier, found this to be a statistical anomaly, so the team went back and refined their experiment.
Their new result once again shows one phonon getting propelled out into hypothetical space, while the other is swallowed by the hypothetical black hole. This time, there is much less room for uncertainty – Leonhardt seemed pretty thrilled.
“I really congratulate Jeff on his work, which is an important step for the community,” he told Physics World. “It’s something he should be proud of and something we should all celebrate as an excellent paper.”
But the experiment produced another result, too.
“The main novelty of de Nova and colleagues’ work is a clever detection scheme that they use to extract the temperature of the emitted radiation,” wrote mathematician Silke Weinfurtner of the University of Nottingham in an editorial accompanying the paper.
“The authors’ findings provide the first evidence of the Hawking temperature from a quantum simulator.”
So, the evidence that Hawking was right is mounting, but this new method for detecting the temperature of the analogue black hole could help us gain a deeper understanding of the thermodynamics of a black hole.
The research has been published in Nature.