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At the death of Stephen Hawking, in March, at the age of 76, the whole world mourned a beloved and visionary scientist. But it's comforting to see that Hawking's latest article was published in the ArXiv preprint newspaper, demonstrating that even during his last days, he was still pursuing the epic cosmic questions that defined his career.
Entitled "Black Hole Entropy and Soft Hair", the paper was written by Hawking in collaboration with physicists Sasha Haco, Malcolm Perry and Andrew Strominger.
"We are deeply saddened to lose our very dear friend and collaborator, Stephen Hawking, whose contributions to black hole physics have always been extremely stimulating," said Haco, Perry and Strominger on the second page of the document. "This article summarizes the state of progress of our long-term project on large diffeomorphisms, soft hair and the quantum structure of black holes until the end of our life together."
This work is the third in a series of the team and tackles Hawking's famous original idea: the paradox of information about black holes. Like many riddles of physics, the paradox emerges from the lack of coherence between quantum field theory and general relativity. On the smallest scales of matter, where atoms and quarks abound, there is a different and apparently contradictory set of rules on the largest scale of matter, involving stars and galaxies. The search for a "theory of everything" that reconciles these two models is one of the jewels of modern physics and has been a fascination for the whole life of Hawking.
Black holes are notable neuralgic points for this tension between quantum field theory and general relativity. According to the quantum rule book, it should be impossible for information regarding a particle – its spin, its configuration, its mass and other features – to be permanently removed from the universe. But what about the matter that falls in the black holes, objects reputed not to let anything slip once past the event's horizon? Can the information be cleaned inside black holes?
Read more: Will Stephen Hawking's "information paradox" be solved?
Hawking suggested that the information could actually be suppressed by Hawking radiation, a type of theoretical radiation that could escape from inside a black hole. This process has never been observed empirically, but the radiation would be supposed to be devoid of any information on its original properties – and this would violate the rules of the universe as we know them.
In his last article, Hawking and his colleagues hypothesized that a phenomenon called "soft hair" could solve the paradox of black hole information. The idea is that streaks of light and gravity particles could surround the event horizon and could at least store entropic information about matter falling into the black hole.
The document does not solve the paradox of information about black holes, but it brings us even closer to understanding the most heady concepts of the known universe. Hawking's incessant dedication to these issues is testament to his insatiable curiosity. The memorandum at the top of the newspaper indicates how much we miss our friends, fans and colleagues.
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