Physicists who collaborated with the late cosmologist on his career-long endeavour to understand what happens to information when things fall into black holes have published Stephen Hawking’s last scientific publication.
The study, which addresses what theoretical physicists refer to as “the information paradox,” was finished in the days leading up to Hawking’s death in March. It has now been written up and released online by his colleagues at Cambridge and Harvard universities.
The information paradox has been “at the centre of Hawking’s existence” for more than 40 years, according to Malcolm Perry, a professor of theoretical physics at Cambridge and a co-author on the study, Black Hole Entropy and Soft Hair.
The puzzle’s roots may be traced back to Albert Einstein. Einstein presented his general theory of relativity in 1915, a tour de force that explained how gravity comes from the spacetime-bending actions of matter, and therefore why the planets orbit the sun. However, Einstein’s theory makes crucial predictions regarding black holes as well, most notably that a black hole can be entirely characterised by just three characteristics: its mass, charge, and spin.
Hawking added to the image about 60 years later. He contended that black holes had a temperature as well. Because hot things lose heat into space, a black hole’s ultimate destiny is to evaporate out of existence. However, this raises an issue. The laws of the quantum realm require that no information be lost. So, what happens to all the information contained in an object — for example, the nature of a moon’s atoms – when it falls into a black hole?
“The problem is that if you put anything into a black hole, it seems to evaporate,” Perry said. “How might the data in that item ever be retrieved if the black hole then vanishes?”
Hawking and his colleagues illustrate in their most recent study how some information, at the very least, may be retained. Toss a thing into a black hole, and the temperature of the black hole should vary. So will entropy, a measure of an object’s inherent disorder that grows as the temperature rises.
The scientists, led by Sasha Haco of Cambridge and Andrew Strominger of Harvard, demonstrate that the entropy of a black hole may be recorded by photons that surround the event horizon, the point beyond which light cannot escape the extreme gravitational attraction. This shine of photons is referred to as “soft hair.”
“What this article accomplishes is illustrate that’soft hair’ can account for entropy,” Perry said. “It’s informing you that your soft hair is doing the correct thing.”
However, this is not the end of the information dilemma. “We don’t know whether Hawking entropy accounts for everything you could potentially throw at a black hole, so this is only a first start,” Perry said. “We believe it’s a solid first step, but there’s still a lot more work to be done.”
Perry was at Harvard working on the manuscript with Strominger only days before Hawking died. He was unaware of Hawking’s condition and phoned to provide an update to the scientist. It may have been Hawking’s last scientific conversation. “It was difficult for Stephen to communicate, so I was placed on a megaphone to tell where we had arrived.” When I explained it to him, all he did was grin. I informed him that we had arrived at our destination. He was aware of the end consequence.”
Among the unknowns that Perry and his colleagues must now investigate are how entropy-related information is physically kept in soft hair and how that information emerges from a black hole as it evaporates.
“Is all of the information about what I toss in saved on the black hole’s horizon if I throw anything in?” Perry asked. “That’s what it takes to overcome the information dilemma.” If it’s just half of it, or 99 percent, that’s not enough to address the information paradox issue. It’s a start in the right direction, but it’s far from complete. We have a few less riddles than previously, but there are still some baffling situations.”
“Understanding the microscopic basis of this entropy – what are the underlying quantum states that the entropy counts? – has been one of the major difficulties of the past 40 years,” Marika Taylor, professor of theoretical physics at Southampton University and a former student of Hawking’s, said.
“This study presents a method for understanding entropy for astrophysical black holes based on event horizon symmetries.” The authors must make various non-trivial assumptions, and the next step will be to demonstrate that these assumptions are correct.”
“Hawking discovered that black holes had a temperature,” said Juan Maldacena, a theoretical physicist at Einstein’s alma school, the Institute for Advanced Studies in Princeton. Temperature is understood in ordinary items to be caused by the mobility of the system’s tiny elements. The temperature of air, for example, is determined by the speed of the molecules: the quicker they travel, the hotter it is.
“It is uncertain what such elements are for black holes, and if they may be related with a black hole’s horizon.” Thermal characteristics of certain physical systems with particular symmetries may be computed in terms of these symmetries. This article demonstrates that one of these particular symmetries exists at the black hole boundary.”
