Black-Hole Mimic Triumphs Result Could Be Closest Thing Yet to Observation of Hawking Radiation

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Cristian Morales, head of the Cuba office ZIKA IN THE CARIBBEAN of the Pan American Health Organization The total number of suspected and con rmed (PAHO), says that it is probably unrealis- cases of locally transmitted Zika virus in countries FLORIDA (USA) and territories throughout the Caribbean. tic for other countries to simply copy Cuba’s mosquito-control programmes. The country’s

6 SOURCE: PAHO/WHO health-care network is one of the best in the Sargasso developing world, and the decades-long Gulf of Sea Mexico stability of its government has ensured policy continuity and enforcement of measures such BAHAMAS TURKS AND CAICOS ISLANDS as fines. He adds that the most important 2 aspects of a response, for any country, include CUBA 3 collaboration between government sectors and PUERTO RICO increased surveillance. MEXICO Caribbean HAITI 8,766 ,490 Sea EVERYONE’S CHALLENGE JAMAICA 2,30 “Cuba probably does a better job of control- ling mosquitoes than any other country in BELIZE HONDURAS 3,863 DOMINICAN 5 REPUBLIC the Americas, but it hasn’t been totally effec- 30,087 tive,” says Gubler. This is partly due to dips in NICARAGUA 5,307 funding. A resurgence of dengue in 1997 was ,088 probably exacerbated by the fall of the Soviet Union, Cuba’s major trading partner, which decimated the economy and weakened health with the exception of one locally acquired self-monitoring for mosquito bites, even among funding. case in March, Cuba mostly managed to keep children. One of the most effective measures Another disadvantage stems from the Zika out until this month. was a heavy fine for people found to have mos- 56-year-old US trade embargo, which quitoes breeding on their property, says Duane prevents Cuba from acquiring drugs and ON THE BALL Gubler, an infectious-disease researcher at medical supplies that include components That success was the result of its excellent Duke–NUS Medical School in Singapore. With made in the United States. It must instead health-care system and an extensive surveil- all these measures in place, Cuba eliminated the buy them from other countries, such as lance programme for vector-borne diseases dengue outbreak in four months. China, often at higher cost. that the government set up 35 years ago, says Now, when another outbreak threatens, “it’s Yet success has come despite these issues. Ileana Morales, director of science and tech- no problem for us to reinforce our system” and According to PAHO, health workers have nology at Cuba’s public-health ministry. intensify such efforts, says Morales. intensified efforts to spray pesticides and In 1981, Cuba saw the first outbreak of In February, before any Zika cases had been eradicate standing water — where mosquitoes haemorrhagic dengue fever in the Ameri- detected in Cuba, the government dispatched can breed — within 150 metres of the homes cas, with more than 344,000 infections. “We 9,000 soldiers to spray homes and other build- of each of the two most recent people to get turned that epidemiological event into an ings, while workers killed mosquito larvae in Zika, in the southeastern province of Holguin. opportunity,” says Morales. The country sent habitats such as waterways. Airport officials Workers are also searching houses for infected medical workers to affected areas and began screened visitors arriving from Zika-infected people and collecting mosquitoes for study. intensively spraying pesticides to eradicate the countries and medical workers went from Guzmán adds that Cuban researchers have Aedes aegypti mosquito that carries the disease. door to door looking for people with symp- begun to plan work on a Zika vaccine. It also created a national reporting sys- toms. The health-care system already con- She says that international cooperation will tem, as well as a framework for cooperation ducts extensive prenatal examinations, so it be important in helping Cuba and others to between government agencies and public-edu- is primed to detect Zika-caused birth defects address Zika. “It’s a problem of everybody. It’s cation campaigns to encourage spraying and such as microcephaly. a new challenge for the world.” ■

PHYSICS Black-hole mimic triumphs Result could be closest thing yet to observation of Hawking radiation.

BY DAVIDE CASTELVECCHI more than 40 years ago, creating problems for from its experimental set-up. theoretical physics that still convulse the field. It is nearly impossible to observe Hawking lack holes are not actually black. Instead, Now, after seven years of often solitary study, radiation in a real black hole, and previous these gravitational sinks are thought Jeff Steinhauer, an experimental physicist at artificial-black-hole experiments did not trace to emit radiation that causes them to the Technion-Israel Institute of Technology in their radiation to spontaneous fluctuations. Bshrink and eventually disappear. This phe- Haifa, has created an artificial black hole that So the result, published on 15 August1, could nomenon, one of the weirdest things about seems to emit such ‘Hawking radiation’ on its be the closest thing yet to an observation of black holes, was predicted by Stephen Hawking own, from quantum fluctuations that emerge Hawking radiation.

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Steinhauer says that black-hole analogues controversially, Hawking also suggested that a undergoes only weak quantum fluctuations might help to solve some of the dilemmas that black hole’s disappearance destroys all infor- that are similar to those in the vacuum of the phenomenon poses for other theories, mation about objects that have fallen into it, space. And these should produce packets of including one called the black-hole information contradicting the accepted wisdom that the sound called phonons, just as the vacuum paradox, and perhaps point the way to uniting total amount of information in the Universe produces photons, Steinhauer says. The part- quantum mechanics with a theory of gravity. stays constant. ners should separate from each other, with one Other physicists are impressed, but they cau- In the early 1980s, physicist Bill Unruh of the partner on the supersonic side of the horizon tion that the results are not clear-cut. And some University of British Columbia in Vancouver, and the other forming Hawking radiation. doubt whether laboratory analogues can reveal Canada, proposed a way to test some of Hawk- On one side of his acoustical event horizon, much about real black holes. “This experiment, ing’s predictions2. He imagined a medium that where the atoms move at supersonic speeds, if all statements hold, is really amazing,” says experienced acceler- phonons became trapped. And when Stein- Silke Weinfurtner, a theoretical and experimen- “For sure, this ated motion, such as hauer took pictures of the BEC, he found cor- tal physicist at the University of Nottingham, is a pioneering water approaching relations between the densities of atoms that UK. “It doesn’t prove that Hawking radiation paper.” a waterfall. Like a were an equal distance from the event horizon exists around astrophysical black holes.” swimmer reaching a but on opposite sides. This demonstrates that It was in the mid-1970s that Hawking, a point where he cannot swim away fast enough pairs of phonons were entangled — a sign that theoretical physicist at the University of Cam- to escape the waterfall, sound waves that are they originated spontaneously from the same bridge, UK, discovered that the event horizon past the point in the medium that surpasses the quantum fluctuation, he says, and that the BEC of a black hole — the surface from which noth- speed of sound would become unable to move was producing Hawking radiation. ing, including light, can escape — should have against the flow. Unruh predicted that this By contrast, radiation that he observed in peculiar consequences for physics. point is equivalent to an event horizon — and an earlier version of the set-up had to be trig- His starting point was that the randomness that it should display a sonic form of Hawking gered, rather than arising from the BEC itself 3, of quantum theory ruled out the existence of radiation. whereas a previous experiment in water waves true nothingness. Even the emptiest region of Steinhauer implemented Unruh’s idea in a led by Unruh and Weinfurtner did not attempt space teems with fluctuations in energy fields, cloud of rubidium atoms that he cooled to a frac- to show quantum effects4. causing photon pairs to appear continuously, tion of a degree above absolute zero. Contained Just as real black holes are not black, Stein- only to immediately destroy each other. But, in a cigar-shaped trap a few millimetres long, the hauer’s acoustical black holes are not com- just as Pinocchio turned from a puppet into a atoms entered a quantum state called a Bose– pletely quiet. Their sound, if it were audible, boy, these ‘virtual’ photons could become real Einstein condensate (BEC), in which the speed might resemble static noise. particles if the event horizon separated them of sound was just half a millimetre per second. “For sure, this is a pioneering paper,” says Ulf before they could annihilate each other. One Steinhauer created an event horizon by accel- Leonhardt, a physicist at the Weizmann Insti- photon would fall inside the event horizon and erating the atoms until some were travelling at tute of Science in Rehovot, Israel, who leads the other would escape into outer space. more than 1 mm s−1 — a a different attempt to demonstrate the effect, This, Hawking showed, causes black holes supersonic speed for the NATURE.COM using laser waves in an optical fibre. But he both to radiate — albeit extremely feebly — condensate (see ‘Building Read more about the says that the evidence of entanglement seems and to ultimately shrink and vanish, because a black hole’). physicist who models incomplete, because Steinhauer demonstrated the particle that falls inside always has a ‘nega- At its ultracold black holes in sound: correlations only for phonons of relatively high tive energy’ that depletes the black hole. Most temperature, the BEC go.nature.com/2atqxhy energies, with lower-energy phonon pairs seemingly not correlated. He also says he’s not confident that the medium is a true BEC, BUILDING A BLACK HOLE which, he says, means that there could be other A black hole’s event horizon — the point beyond which the gravitational pull is too strong even for light types of fluctuation that could mimic Hawking to escape — has been mimicked in the lab using a cloud of ultracold atoms. The arti cial black hole seems to emit a form of Hawking radiation. radiation. Also unclear is what analogues can say about REAL BLACK HOLE Gravity-warped Event horizon the mysteries surrounding true black holes. “I space-time Quantum uctuations don’t believe it will illuminate the so-called in the vacuum of space produce information paradox,” says Leonard Susskind, virtual photons. Sometimes, one Hawking of a pair gets trapped behind the radiation a theoretical physicist at Stanford University event horizon before the two in California. In contrast to the case of astro- destroy each other, forcing both physical black holes, there is no information to become real particles. The photon that escapes is emitted loss in Steinhauer’s sonic black hole because as Hawking radiation. the BEC does not evaporate. Virtual photon pair Still, if Steinhauer’s results were confirmed, splits and becomes real. 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 ARTIFICIAL BLACK HOLE Atoms accelerated by laser company”, says Susskind. Few doubted that the Ultracold atoms in a particle existed, but its discovery in 2012 still tube undergo quantum uctuations ‘Hawking radiation’ that produce pairs of virtual earned Peter Higgs and another theorist, Fran- particles — in this case, packets of çois Englert, who predicted it, a Nobel prize. ■ sound called phonons. If one phonon falls in the supersonic 1. Steinhauer, J. Nature Phys. http://dx.doi. org/10.1038/nphys3863 (2016). region, it is trapped, leading to a Virtual phonon Beyond here, atoms travel 2. Unruh, W. G. Phys. Rev. Lett. 46, 1351–1353 sonic form of Hawking radiation. pair splits and faster than the speed of (1981). becomes real. sound in the tube, creating 3. Steinhauer, J. Nature Phys. 10, 864–869 (2014). an event horizon. 4. Weinfurtner, S. et al. Phys. Rev. Lett. 106, 021302 (2011).