The work probes the black hole ‘information paradox,' the idea that physical information could permanently disappear into a black hole. The paper was completed just days before Hawking's death in March. The work was summarized by his colleagues at Cambridge and Harvard universities.
According to Malcolm Perry, a professor of theoretical physics at Cambridge and co-author of the paper "Black Hole Entropy and Soft Hair," the information paradox was "at the center of Hawking's life" for over 40 years.
The beginning of the puzzle originated in 1915 when Albert Einstein published his theory of general relativity, in which the famed physicist determined that the laws of physics hold for all non-accelerating observers and that the speed of light in a vacuum is independent of the motion of observers. In addition, Einstein's theory predicted that black holes are defined by mass, charge and spin.
More than 60 years later, Hawking added to Einstein's work, claiming that black holes also have a temperature, suggesting that, because hot objects lose heat space in space, black holes ultimately must dissolve due to entropy. However, according to quantum physics, information contained in an object is never lost. The question that thus remains becomes: what happens to all the information that spills into a black hole?
"The difficulty is that if you throw something into a black hole it looks like it disappears," Perry said, cited on Wednesday by the Guardian. "How could the information in that object ever be recovered if the black hole then disappears itself?"
In his last publication, Hawking and his colleagues explain how some information tossed into black holes could be preserved. When an object is sucked into a black hole, the black hole's temperature should change. In addition, entropy, a measure of any object's internal disorder, should also change as well. In fact, entropy increases as the black hole's temperature increases.
Physicists show that a black hole's entropy can be recorded by photons around the black hole's event horizon, the boundary at which the gravitational pull of the black hole is so great that even light cannot escape. This layer of photons at the surface of the event horizon is now known as "soft hair."
"What this paper does is show that ‘soft hair' can account for the entropy," Perry told the Guardian. "It's telling you that soft hair really is doing the right stuff."
"We don't know that Hawking entropy accounts for everything you could possibly throw at a black hole, so this is really a step along the way. We think it's a pretty good step, but there is a lot more work to be done," Perry added.
Just a few days before Hawking died, Perry called the physicist to update him on the research.
"It was very difficult for Stephen to communicate and I was put on a loudspeaker to explain where we had got to. When I explained it, he simply produced an enormous smile. I told him we'd got somewhere. He knew the final result," Perry noted.
Perry and his colleagues are now planning on discovering how information connected to entripy is stored in soft hair and how that information is released when a black hole dissipates.
"If I throw something in, is all of the information about what it is stored [available] on the black hole's horizon?" Perry said. "That is what is required to solve the information paradox. If it's only half of it, or 99 percent, that is not enough, you have not solved the information paradox problem.
"It's a step on the way, but it is definitely not the entire answer. We have slightly fewer puzzles than we had before, but there are definitely some perplexing issues left," he added.
According to Marika Taylor, professor of theoretical physics at Southampton University and Hawking's former student, "Understanding the microscopic origin of this entropy — what are the underlying quantum states that the entropy counts?— has been one of the great challenges of the last 40 years."
"This paper proposes a way to understand entropy for astrophysical black holes based on symmetries of the event horizon. The authors have to make several non-trivial assumptions so the next steps will be to show that these assumptions are valid," Taylor added.
Juan Maldacena at the Institute for Advanced Studies in Princeton added: "Hawking found that black holes have a temperature. For ordinary objects we understand temperature as due to the motion of the microscopic constituents of the system. For example, the temperature of air is due to the motion of the molecules: the faster they move, the hotter it is."
"For black holes, it is unclear what those constituents are, and whether they can be associated to the horizon of a black hole. In some physical systems that have special symmetries, the thermal properties can be calculated in terms of these symmetries. This paper shows that near the black hole horizon we have one of these special symmetries," Maldacena noted.
Hawking — a British theoretical physicist, cosmologist, author and director of research at the Center for Theoretical Cosmology within the University of Cambridge — is the first scientist to set out a theory of cosmology explained by a union of the general theory of relativity and quantum mechanics.
The famous physicist suffered from a form of amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease after the famous baseball player who had the disease. ALS gradually paralyzed him, although Hawking surprised doctors by living for 55 more years following his diagnosis at the age of 21. The disease ordinarily causes death within just a handful of years.
Despite his immobility, Hawking continued to speak before wide audiences in a wheelchair using a voice synthesizer that quickly became his trademark.
His body is buried in London's Westminster Abbey, alongside Sir Isaac Newton and near Charles Darwin.