Israeli physicists believe it confirmed one of the late Stephen Hawking's most famous predictions by creating the sonic equivalent of a black hole made of an exotic superfluid of ultra-cold atoms. Jeff Steinhauer and colleagues from the Israel Institute of Technology (Technion) have described these fascinating experimental results in a recent article in Nature (1
Make the subtleties of quantum mechanics responsible for this Hawking radiation. From the quantum perspective, the vacuum of space continuously creates pairs of virtual particles (matter and antimatter) that arise and dissolve just as quickly. Hawking suggested that a virtual pair of particles, when it appears at the event horizon of a black hole, could have different fates: one might fall in, but the other escape, giving the impression that the black hole is emitting radiation. The black hole would lose some weight. The larger the black hole, the longer the evaporation takes. (Mini-black holes the size of a subatomic particle would disappear almost instantaneously.)
Hawking's prediction has enormous implications for theoretical physics, particularly the information paradox of black holes, but has been proven to be extremely difficult to test experimentally. The primary challenge is summarized by Silke Weinfurtner, a physicist at the University of Nottingham, in a perspective that accompanies the latest results: "The temperature associated with Hawking radiation, the so-called Hawking temperature, is inversely proportional to the Mass from the black hole, "she writes. "And for the smallest observed black holes whose mass is similar to the Sun, this temperature is around 60 nanokelvin, so Hawking radiation produces a tiny signal, and it seems that the phenomenon can not be verified by observation." 19659004] Thus, physicists have turned to so-called analog black holes, which were first proposed in 1981 by the physicist William Unruh of the University of British Columbia. He proposed a sonorous equivalent, which he called a "stupid hole", because behind a kind of event horizon no sound but sound is trapped. It's a bit like a waterfall where the water flows faster and faster over an edge until it flows faster than the speed of sound through water. This creates the equivalent of a point of no return. The liquid moves faster than the speed of sound, so no sound can escape from the liquid and escape in the opposite direction.
Unruh's work was theoretical, but in 2009 Steinhauer briefly created a black hole analog from a Bose-Einstein condensate (BEC) from 100,000 cooled rubidium atoms. When densely packed atoms are cooled to extremely low temperatures (billionths of a degree above absolute zero), they collectively convert to their lowest energy state, behaving essentially like a single large "superatom". This is a BEC, a kind of abundance. Steinhauer was able to accomplish the physical realization of Unruh's stupid holes in a BEC – a portion of the fluid was flowing faster than the speed of sound, effectively creating a supersonic region behind an event horizon that would propagate sound waves in the opposite direction of the flow prevented.
Steinhauer argued that his analogous system should also emit the equivalent of Hawking radiation with entangled phonons. In the words, sound waves should exit from the horizon of the sound event just as Hawking radiation emerges from a real black hole. By 2014, he had observed tantalizing evidence of the phenomenon, and two years later announced the observation of swallowed phonons emitted from their black sound hole. Granted, it was not exactly Hawking radiation – it's an analog sound wave. However, it has been the strongest proof of Hawking's theoretical prediction so far.
Now Steinhauer is back with the results of his new, improved experimental setup, which is able to produce a much stronger signal. He and his co-authors have shown that sonic wave pairs – the sound equivalents of virtual particles – occur at the analogue event horizon. One is emitted from the supersonic range while the other is absorbed. And just as Hawking predicted, "The radiation of the system … has a heat spectrum with a temperature determined only by the system's analog equivalent to gravity, a ratio between sound velocity and flow," wrote Ryan Mandelbaum at Gizmodo. "That means it emits a continuous spectrum of wavelengths rather than preferred wavelengths."
"The main novelty of the work of de Nova and colleagues is a sophisticated detection scheme that allows them to extract the temperature of the emitted radiation." Weinfurtner writes. "The authors' findings provide the first evidence for the Hawking temperature with a quantum simulator." She also notes that this work could prove useful for measuring other unusual quantum phenomena that might occur near the event horizon.
The next step is to repeat the experiment in the hope of measuring how this analogous version of Hawking radiation could change over time. "I'm interested in learning everything about real black holes and real gravity," Steinhauer told Gizmodo. "The way I see it, we saw that Hawking's calculations were correct."
DOI: Nature 2019. 10.1038 / s41586-019-1241-0 (About DOIs).