Stephen Hawking’s Final Paper Tackles A Crucial Black Hole Mystery

Stephen Hawking in February 2015. Martin Hoscik/Shutterstock

Professor Stephen Hawking was an extremely prolific scientist and the research he contributed to continues to be published even months after his death, which took place on March 14 of this year.

The new paper now released on arXiv is centered on one of the central topics of his career, the information paradox. Black holes are objects from which nothing can escape, not even light. Once something crosses the boundary of a black hole, the event horizon, it is lost forever to our universe. A black hole can be described simply by its mass, electric charge, and angular momentum. Everything else doesn’t matter because it’s gone. This is why black holes are described as having "no hair" – they are simple and have few properties.

This description comes from general relativity, but it’s the other cornerstone of physics, quantum mechanics, that has issues with black holes. In quantum mechanics, the information must always be preserved. So it is a paradox that black holes could be the exception to this rule. Is the no-hair theorem wrong or is more subtle physics at work here?

Finding a solution to the paradox is not an easy matter and Hawking and many other physicists spent decades on it. Even this latest work, while contributing to the field, is not a revolution but rather an extra bit of the puzzle. The study proposes that black holes might have “soft hair”.

The key might be in the entropy of a black hole. This thermodynamic quantity has been key to a lot of Hawking's work on black holes. The team suggests that when something falls into the black hole it alters its temperature and thus its entropy. Photons at the edge of the event horizon, the researchers suppose, are affected by this change and in a way register part of the information that fell in. That’s why they are described as “soft hair”.

“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,” co-author Professor Malcolm Perry, from the University of Cambridge, told The Guardian. “If I throw something in, is all of the information about what it is stored on the black hole’s horizon? 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.”

The paradox is one of the several issues that pit general relativity against quantum mechanics. The hope is for the development of a “theory of everything” that will supplant them both.

[H/T: The Guardian]

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