Traveling to a black hole to see how it behaves is, to say the least, risky. Apart from being technically impossible to reach it with current means – the closest to Earth is some 3,000 light years away – if a seasoned astronaut wanted to get closer to browse, he could never go out to tell about it . Therefore, physicists have to settle for what telescopes pick up and analyze their trace, such as, for example, the gravitational waves that are believed to arise when two of them merge.
A third option is to study them in the laboratory, and for this scientists have developed systems that try to imitate their characteristics. According to cosmologist Stephen Hawking, one of the peculiarities of these bodies is that they spontaneously emit radiation . “It is due to the creation of pairs of particles, where one of them falls into the black hole and the other escapes; that is, one leaves and another enters”, explains Gonzalo Olmo, researcher at the Institute of Corpuscular Physics and the Department of Theoretical Physics from the University of Valencia.
A model that uses sound waves
A hypothetical external observer could appreciate this radiation, associated with the particle that comes out of the hole. Hawking formulated this theory in 1974, but it has not been proven, directly or indirectly. Now, physicist Jeff Steinhauer claims to have succeeded . In a study published in the journal Nature Physics , this scientist from the Israel Institute of Technology (Technion) describes how he has used a model analogous to the cosmic body recreated in a laboratory: an acoustic black hole . To determine its properties, instead of light it has used sound waves – called phonons – and an extremely cold gas, close to absolute zero.
This gas is the Bose-Einstein condensate, the fifth state of matter, postulated in the 1920s by Albert Einstein and the Indian physicist Satyendra Nath Bose. What the Technion researcher has done has been to measure the small variations that sound waves generate in the density of the condensate. These oscillations are located on both sides of the horizon through which the waves propagate in the acoustic hole. “Steinhauer has studied the correlations that exist between these fluctuations on both sides of the horizon and has found indications compatible with what would be expected if the acoustic horizon generated Hawking radiation”, summarizes Olmo.
Despite the promise of the finding, most researchers note that it cannot be extrapolated to a real black hole . What scientists measure in laboratories are the quantum properties of the system, characteristics that are practically undetectable in astrophysical holes due to their large size. “The effects of quantum mechanics are important when it comes to microscopic objects. Although, in principle, this theory can describe the entire universe, if it is about big things, classical mechanics is used”, argues Roberto Emparan, from the Physics Department Quantum and Astrophysics of the University of Barcelona.
The study has caused a great controversy
Scientists have been trying for decades to prove the existence of this radiation. “Since I was a doctoral student, more than 25 years ago, it has been announced that it is about to be achieved,” recalls Emparan. For this, throughout this time different strategies have been tested. These use superfluids, optical fibers, polaritons –hybrid particles of light and matter–, ion rings … Water flows have also been used, although, in this case, what we are trying to recreate is not a black hole, but a white, whose main characteristic is that nothing can enter it. The physicist Germain Rousseaux claims to have demonstrated the existence of Hawking radiation precisely in an experiment of this type.
In the opinion of this expert from the French National Center for Scientific Research, Steinhauer’s announcement is good news. ” That there are several physical systems where the effect is observed shows that it is something universal , although the first time it was predicted in an astrophysical context,” he says. But Ulf Leonhardt does not share this view. This physicist from the Weizmann Institute of Sciences in Israel, who has been searching for the elusive radiation for years, like his colleague from the Technion, questions the finding in an article published on the arXiv portal. “The experimental data of the study are valid; the conclusions and statements are not,” he says. Steinhauer’s response was swift, and through another article on the same website, he refutes up to fifteen of Leonhardt’s comments.
Other scientists, like the aforementioned Emparan, also have doubts. “The way the results are analyzed raises too much uncertainty to conclude that the effect that Hawking proposed has been unequivocally observed,” he says. In fact, before being published in Nature Physics , the research had been on arXiv for over a year and hadn’t caused much of a stir among physicists.
Faced with the most skeptical, there are researchers who are clear: we are facing a milestone in science . Juan Ramón Muñoz de Nova works with Steinhauer at the Technion. Although the discovery was made by this physicist alone, the Spaniard claims to have received congratulations from other colleagues who know that he is collaborating with him. ” There is even speculation that it could mean the Nobel for Hawking , although it is already known that these valuations are always highly inflated by the impression of the moment. You have to be cautious ”, recommends Muñoz de Nova, who is also part of the Department of Materials Physics at the Complutense University of Madrid. In his view, it has been shown that analogs can be used in terrestrial laboratories to study astrophysical phenomena. To this he adds that, according to the study, the mechanism proposed by Hawking works, at least, in a similar scenario.
Beyond theoretical models
For Christoph Adami, professor of Physics and Astronomy at Michigan State University, in the US, the work shows for the first time the quantum effects that had been predicted in black holes and that cannot be measured directly. “For now, we have no other experimental evidence of them, beyond the models with which we try to imitate them. Perhaps one day they can be studied from the gravitational waves emitted by black holes,” predicts Adami.
Similarly, Daniele Pranzetti, an astroparticle physicist at the International School for Advanced Studies in Trieste in Italy, believes the study’s results are remarkable. From his point of view, they shed some light on a topic much debated for more than four decades, although he agrees with other scientists that the data cannot be extrapolated to real black holes. “It could be seen as a leap of faith, but it is an interesting avenue to explore, especially in the absence of a better one,” Pranzetti argues.
In recent years, we have come across great scientific findings in different branches of physics. Thus, the confirmation of the existence of the Higgs boson earned Peter Higgs and François Englert the Nobel, who had raised it. Gravitational wave capture may win the accolade in future editions. If the Hawking radiation is shown to be there, would we be facing a finding as important as these two? It is not clear, although, in principle, it does not seem so.
“The Higgs boson supports the mechanism that gives mass to particles; it is a fundamental advance that makes the standard model of physics square, perhaps the most accurate theory in the history of mankind,” says Muñoz de Nova. As for gravitational waves, they confirm the existence of a prediction made by Einstein directly. “It is not an analog, as in the case of Hawking radiation” , compares the physicist. In his opinion, the most important thing is that they greatly expand our view of the universe. Until now we could only track the sky with electromagnetic radiation, but the discovery of these waves allows us to use a different tool. “It’s like having discovered second eyes”, describes Muñoz de Nova. Emparan matches. That ability to open new doors is what gives them their great value. “It has been a fundamental discovery. From now on it will be possible to do numerous studies in astrophysics, astronomy and gravitational physics that could not be carried out before,” he says.
Towards a quantum theory of gravity
On the contrary, this same scientist does not believe that the possible finding of Hawking radiation in acoustic black holes has a great impact. Another thing would be to detect this phenomenon in real black holes. However, some researchers point out that it is one more step towards the achievement of one of the great dreams of physics: the birth of a unified theory that is capable of harmonizing general relativity with quantum mechanics. Einstein himself spent thirty years of his life trying to find her, but he was unsuccessful. This theory of everything would make it possible to explain all the forces of nature and the characteristics of energy and matter, both at the cosmological and atomic levels .
“The radiation proposed by Hawking suggests that black holes might not be as black as was originally thought, as they would emit pairs of particles. This poses a curious paradox when it comes to unifying Einstein’s theory of general relativity with that of quantum mechanics “, declares Pranzetti. In this sense, “Steinhauer’s finding, if proven, could be very revealing,” adds the scientist.
For Olmo, from the most optimistic point of view, this line of study would help shape a quantum theory of gravity . “This would allow us to understand the essence of quantum field theory and Einstein’s general relativity and how they combine,” he says. “If it happened, it would be a much more important advance than the detection of two specific elements of these theories, such as, respectively, the Higgs boson and gravitational waves,” he adds.
But, according to this expert, even showing us more conservative, we find that the experiment, regardless of any other consideration, verifies that there is an acoustic horizon through which the waves propagate and that it has quantum properties. “This alone is an important result, as we have never directly observed these horizons in astrophysical systems,” says Olmo.
Black holes will remain a mystery
What real black holes have in common with laboratory ones is precisely the presence of this horizon, although their properties and behaviors are different. “Acoustics occur in a spatial dimension and their horizon is a point, not a sphere as in the real ones”, points out Emparan, who reiterates that studies with analogous systems do not allow to analyze the characteristics of the original objects.
This theoretical position clashes with that of many other scientists, who develop this type of experimental work in different research centers. “The study shows that laboratory tests can be carried out to study quantum phenomena in gravitational and cosmological contexts , which can greatly help their understanding and modeling,” argues Muñoz de Nova from Israel.
In any case, whether or not the existence of Hawking radiation is confirmed from Steinhauer’s results, black holes will continue to be among the most enigmatic objects in the universe. In fact, only fifty years ago it was doubted that these regions of the cosmos really existed from which nothing material, not even light, can escape . However, thanks to the discovery of gravitational waves, announced in February 2016, it is now possible to even observe how their echo reaches us through space and time.
Images: Alain r via Wikimedia / CC – Nitzan Zohar / Technion – LIGO / T. Pyle
