When Stephen Hawking died in March at the age of 76, the world mourned a beloved and visionary scientist. But it is some consolation that Hawking’s final paper has now been published on the preprint journal ArXiv, 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 authored by Hawking along with physicists Sasha Haco, Malcolm Perry, and Andrew Strominger.
“We are deeply saddened to lose our much-loved friend and collaborator Stephen Hawking whose contributions to black hole physics remained vitally stimulating to the very end,” Haco, Perry, and Strominger said on the second page of the paper. “This paper summarizes the status of our long-term project on large diffeomorphisms, soft hair, and the quantum structure of black holes until the end of our time together.”
The work is the third in a series from the team and addresses Hawking’s famous brainchild—the black hole information paradox. Like many physics conundrums, 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 exists a different and seemingly contradictory set of rules to 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 holy grails of modern physics, and was a lifelong fascination for Hawking.
Black holes are notable flashpoints for this tension between quantum field theory and general relativity. According to the quantum rulebook, it should be impossible for information about a particle—its spin, configuration, mass, and other features—to be permanently deleted from the universe. But what about matter that falls into black holes, objects with a reputation of not letting anything escape once it passes the event horizon? Can information be scrubbed inside black holes?
Hawking suggested that information could indeed be deleted through Hawking radiation, which is a type of theoretical radiation that can escape from inside a black hole. This process has never been empirically observed, but the radiation would supposedly be stripped of all information about its original properties—and that would violate the rules of the universe as we know them.
In his last paper, Hawking and his colleagues speculated that a phenomenon called “soft hair” might resolve the black hole information paradox. The idea is that trails of light and gravity particles might encircle the event horizon, and could store, at the very least, entropic information about matter that fell into the black hole.
The paper doesn’t resolve the black hole information paradox, but it does take us one step closer to understanding some of the headiest concepts in the known universe. Hawking’s unceasing dedication to these questions is a testament to his insatiable curiosity, and the memoriam included at the top of the paper hints at how much he will be missed by his friends, fans, and colleagues.
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