A newly published study conducted at the Technion presents findings that support the “Hawking radiation” theory, which was proposed more than 40 years ago and centers on the claim that black holes are not all black and in fact emit electromagnetic radiation.
The study, conducted by Los Angeles-born physicist Prof. Jeff Steinhauer of the Technion - Israel Institute of Technology in Haifa and published in this month's Nature Physics journal, demonstrates how sound particles are emitted from black holes. “This is experimental confirmation of Hawking’s prediction about the thermodynamics of the black hole,” states Steinhauer. Experts say the experiment is a very sensitive and precise analogue for measuring the activity of black holes. Some even think that the study could pave the way for famed physicist Stephen Hawking to win the Nobel Prize.
Since 2009, Steinhauer has been developing models for measuring “Hawking radiation” — a longstanding dilemma of theoretical physics — while building upon the research of physicist and Israel Prize laureate Prof. Jacob Bekenstein, who died last year. Steinhauer is also the sole author of the just-published paper, which is quite unusual in the world of experimental physics.
The new research serves as a kind of bridge between two conflicting physical theories: the theory of relativity and quantum theory, which theoretical physics has never been able to reconcile into a single theory.
The emergence of black holes is one of the most intriguing phenomena in the study of the cosmos. Most often, they are formed when a solar mass implodes — when its store of energy (mainly hydrogen and helium) is depleted and it can no longer withstand its own gravitational pull. When a star collapses at the end of its lifetime, a portion of its mass collapses inward, into its core, which gives rise to a black hole in which a large mass is condensed into a compact mass. The black hole’s gravitational field is so strong that it swallows up everything in its path, including light. Any particle or matter up to a certain boundary, known as the “event horizon,” is sucked in.
In the 1970s, Bekenstein and Hawking, working separately, both demonstrated that black holes can emit radiation. Bekenstein’s work on the thermodynamics of black holes and other aspects of the connections between information and gravitation led him to conclude that black holes do not swallow everything. These claims were dismissed at the time by most scientists, including Hawking. But not long afterward, Hawking changed his mind and also came to believe that these holes are not entirely “black.” In 1975, Hawking proposed that tools from quantum field theory be applied to the event horizon. According to Hawking’s theory, particle pairs that are formed near the event horizon split in such a way that one particle is swallowed by the black hole while the other is left outside of the boundary and emits electromagnetic radiation in all directions. The idea gradually gained acceptance and came to be referred to as Bekenstein-Hawking entropy.
In 1981, Canadian physicist William Unruh developed the idea of the “acoustic black hole” — a new model of the black hole based on sound waves. In the years since, various experiments based on Unruh’s idea have been conducted to attempt validating that “Hawking radiation” exists. Steinhauer’s experiment is one such trial.
In his study, Steinhauer showed how pairs of sound particles, called phonons, spontaneously appear near the event horizon of a black hole analogue. Then, in keeping with Hawking’s theory, one of the particles escapes the black hole while the other is swallowed up inside. Steinhauer says that the two particles, one inside and one outside the black hole, are connected by a quantum relationship known as “entanglement,” meaning that a change in one particle also affects its partner.
Steinhauer, raised in Los Angeles, earned his doctorate from UCLA and completed two post-doctoral fellowships, one under Prof. Nir Davidson at the Weizmann Institute of Science, and the other in the lab of Nobel Prize laureate Wolfgang Ketterle at MIT. He joined the physics faculty at the Technion in 2003, and in 2009 began researching acoustic black holes in his lab there, which is filled with lasers, mirrors, lenses and magnetic coils. He published his initial findings about Hawking radiation in the Nature Physics journal in 2014. While his work did not exactly reflect Hawking radiation theory, it did arouse great interest in the scientific community and beyond. When the study was published, The Economist wrote that if the Royal Swedish Academy of Sciences was in the right mood, the discovery might well lead Professor Hawking to Stockholm to claim a Nobel Prize.
“For a hundred years, physicists have been supposed to be studying only what can be measured, but there’s nothing more frustrating for a physicist than to discover a new phenomenon that cannot be validated by an experiment in the foreseeable future,” says Prof. Eliezer Rabinovici of Hebrew University’s Racah Institute of Physics. “As far as black holes go, in the foreseeable future we don’t see any possibility of measuring the temperature of a black hole, which is why analogue systems that simulate some of the characteristics of the black hole are so important, and why this latest experiment is of such great significance.”
However, Rabinovici points out that analogues do not give a complete depiction of black holes. “Black holes have some surprising aspects that are not all understood. In terms of measuring the temperature and the quantum nature of the radiation that is analogous to Hawking radiation, a great effort has been made to ensure that these phenomena can be isolated from the other aspects of black holes, but not all scientists are convinced.”
Prof. Tsvi Piran, also of the Racah Institute, says, “It’s clear that in the present situation we have no chance of learning about Hawking radiation — because black holes are very distant and also because Hawking radiation is very low wattage. On the other hand, the idea of Hawking radiation is of vast importance because it serves to connect two basic theories in nature: the theory of relativity and quantum theory.”
The theory of Hawking radiation gave rise to the black hole information paradox that physicists have pondered for decades, and in the past 20 years in particular. “The paradox says that, according to the theory of relativity, information that is pulled into a black hole should disappear forever. But quantum theory allows for a situation in which information escapes the black hole,” Piran explains.
Steinhauer’s article shines a light on these questions. Last October he posted a version of the article on the arXiv website — a site where physicists publish their work unofficially before it appears in scientific journals. The article drew great interest and Steinhauer was subsequently invited to Paris to present his findings before a gathering of European physicists. “They asked a lot of questions and after the event I made some additional measurements before the official publication. And the results of the experiment supported Hawking’s calculations,” Steinhauer tells Haaretz.
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