For decades, Prof. Stephen Hawking spun theories about the universe without moving from his wheelchair or, for years, unable even to speak or move his arms or head. He could only blink his eyes at sensors on his computer screen. The mind of British theoretical physicist, cosmologist, and author at the University of Cambridge was a prisoner of his paralyzed body, the result of early-onset amyotrophic lateral sclerosis (ALS or Lou Gehrig‘s disease). But Hawking, who died in March 2018 at the age of 75, was a genius, and his confinement didn’t prevent him from thinking great thoughts or theories.
During his distinguished career, Hawking predicted that black holes – invisible and massive phenomena in space with such strong gravity that nothing, even light, can escape its deep, dark grasp – emit radiation. This “Hawking Radiation,” he suggested, should have a thermal spectrum, similar to the radiation from any warm object. The temperature of the Hawking radiation should agree with the temperature predicted by the late astrophysicist Jacob Bekenstein of the Hebrew University of Jerusalem who died three years before Hawking.
Now, a team of researchers from the Physics Faculty at the Technion–Israel Institute of Technology in Haifa have modeled an artificial black hole and, by devising a way to measure the spectrum of Hawking Radiation. They found that black holes do emit radiation like an ordinary warm object, as Hawking’s predictions asserted.
A paper explaining their groundbreaking research appeared in the May, 30, 2019 issue of the prestigious journal Nature.
Almost four decades ago, a theory emerged suggesting black holes could be artificially constructed and used to measure the physics of Hawking Radiation, a type of thermal radiation, the existence of which Hawking predicted in the 1970s and which now bears his name. But it was the task of creating an artificial black hole and devising ways to measure the spectrum of Hawking radiation and its temperature that led to the recent Technion breakthrough.
“Our artificially-constructed black hole provides confirmation of the thermality of Hawking radiation,” explained lead researcher Prof. Jeff Steinhauer. “In addition, we found that the temperature is determined by the artificial gravity at the surface of the artificial black hole, also consistent with Hawking’s predictions.”
The success of their work is yet another profound insight into the nature of black holes, among the most mysterious and least understood secrets of the universe. The concept of an object in space with gravitation fields too strong for light to escape were considered in the 18th century. The first modern theory about the existence of black holes was developed in 1916, but not fully characterized and considered to be just mathematical curiosities until the late 1960s, when theories were sparked by knowledge about the collapse of massive stars. Although they could not be seen, the consensus was soon reached that black holes existed in most galaxies.
Having studied this phenomenon and problem for a decade, the Haifa research team made constant improvements to their experimental tools over the last three years. The goal was to not only create artificial black holes but also to develop methods to make measurements to check Hawking’s predictions. To reach their conclusions, the Hawking radiation experiment was repeated 7,400 times, providing a density profile for each “run” from which the researchers computed averages.
“Theoretical works, combined with our long-term study of this subject, allowed for the observation of spontaneous Hawking Radiation in a black hole,” reported Steinhauer. “The improvements in our experimental apparatus allowed us to measure the thermality of the Hawking spectrum and compare its temperature with Hawking’s prediction, given by the surface gravity.”
According to the researchers, that temperature – as predicted – provides an interesting link between the theories of Hawking and those of astrophysicist Bekensteinm who, in 1972, also presented a theory on black hole thermodynamics.
“Remarkably, although their calculations were based on very different ideas, both Hawking and Bekenstein came up with the same conclusion that the temperature was determined by the gravity at the surface of the black hole,” said Steinhauer. “We confirmed their predictions.”
The discovery made by the Technion physicists makes clearer the nature of black holes by measuring the spectrum emitted, very similar to the spectrum that would be emitted by an ordinary warm object. The low levels of radiation not only confirm Hawking’s theory but could also lead to further research.
According to Steinhauer and his team, their findings provide not only hints about the nature of real black holes but also about the “information paradox.” According to Hawking, the radiation and its thermal spectrum contain very little information. This idea is the basis of the information paradox, which poses questions such as: What is the fate of information that falls into a real black hole? Does it disappear from the universe? And, if not, where does it go?
The researchers found that the spectrum of the Hawking radiation is indeed thermal. So, the information paradox remains unresolved, with future researchers needing to look elsewhere to investigate the information paradox enigma.