Albert McKeon

Jan 10th 2020

Stephen Hawking Had One Last Thing To Say About Black Hole Theory


Stephen Hawking would have turned 77 this month. The theoretical physicist continued studying the possible connections between the preservation of matter and the destructive nature of black holes until only days before his death in March 2018.

Prior to Hawking’s landmark work on black hole theory and quantum physics, physicists and mathematicians mostly accepted — or failed to challenge — Albert Einstein’s law of general relativity as scientific gospel. Einstein famously described black holes as killers of energy: nothing escaped these gravitational fields, not even matter or light.

But throughout his life, Hawking just as famously flipped some of Einstein’s suppositions. He first drew universal acclaim — on Earth, that is — in 1974 by showing how radiation can, in fact, flee the forces of a black hole. Lou Gehrig’s disease did little to slow Hawking’s own energy for studying the subject. In October, he posthumously had one last word: anything that’s fatally absorbed by a black hole actually leaves a record of existence, a “soft hair” of information.

Refuting Popular Theory Until He Didn’t

Black Hole Entropy and Soft Hair” was the last in a trilogy of papers written by Hawking and colleagues from Cambridge and Harvard universities. They focused on what is known as the “information paradox.”

Their work circled back to Hawking’s landmark 1974 paper, which claimed black holes had a temperature that allowed some quantum particles to leak. However, this theory posed a dire consequence for the history of any object consumed by a black hole. Essentially, the leaking would cause a black hole to evaporate and look like any other black hole that met the same fate. Writ large, Hawking’s idea meant if a black hole consumed our planet, there would probably be no record anywhere of Earth. The theory ran up against quantum physics, which posits that all information can be recovered if lost, creating the information paradox.

Hawking was a revolutionary about black holes until he wasn’t. He recanted his 1974 research 30 years later. Information is indeed retrievable from a black hole, he claimed in 2004. His announcement meant order in modern physics was restored. Among the most recent work to support that, as Live Science reported, was a 2015 Cambridge University study that said quantum teleportation of subatomic particles could be used to retrieve information from a black hole — albeit a small amount and possibly enough that would hurt the chances of retrieving more information.

Building on Stephen Hawking’s Legacy

Hawking’s changed stance on black hole theory didn’t stop him from further studying how information could be recovered from the abyss. As Cambridge University theoretical physics professor Malcom Perry, a co-author of Hawking’s last paper, told The Guardian, “The difficulty is that if you throw something into a black hole it looks like it disappears. How could the information in that object ever be recovered if the black hole then disappears itself?”

Hawking’s last paper takes a step toward answering the question. Essentially, Hawking and his colleagues posit that a black hole’s entropy might be recorded by a sheen of photons that surround the point at which light can’t escape the gravitational pull of the hole. Hawking’s team calls the sheen of photons “soft hair.”

“What this paper does is show that ‘soft hair’ can account for the entropy,” Perry said. “It’s telling you that soft hair really is doing the right stuff.” What the paper doesn’t do is account for everything that enters a black hole, so the information paradox remains. Although Hawking shed his countercultural status by recanting his 1974 theory on information, he was still a guiding light in his field because, among many achievements, he introduced the idea that black holes have a temperature.

Stephen Hawking’s legacy means his predecessors will keep trying to solve the information paradox with a big head start. “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,” Perry told The Guardian. “We think it’s a pretty good step, but there is a lot more work to be done.”