We thought we’d seen the final paper from the late, great Stephen Hawking, but there’s now another – published in partnership with colleagues from Cambridge and Harvard, the paper tackles black holes, one of the topics Hawking was so passionately interested in.
In particular, it examines the long-standing mystery of what happens to the information held by objects once they disappear into a black hole. It hasn’t been clear exactly how any of an object’s properties could survive as it gets devoured, but this latest paper offers an idea.
If an object falls into a black hole, it should increase the hole’s temperature, the researchers suggest, and its entropy (its disorder or randomness) should change as well in response.
Photons surrounding the black holes might be able to log that entropy shift – photons that Hawking and his colleagues are calling “soft hair”.
“What this paper does is show that soft hair can account for the entropy,” one of the team, theoretical physicist Malcolm Perry from the University of Cambridge in the UK, told Ian Sample at The Guardian. “It’s telling you that soft hair really is doing the right stuff.”
“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.”
Perry finished the work on the paper after Hawking’s death in March 2018. He says it’s a sign of progress in our understanding, but that there’s still a long way to go.
Some of Hawking’s most significant research has been around black holes. The formula for the temperature of a black hole – the Hawking temperature – is inscribed on the memorial to the great scientist at Westminster Abbey.
Having provided the theoretical framework for this temperature in 1974, Hawking and other physicists have been working to align this idea with the fundamental laws of classical and quantum physics – laws which state you’re not going to get anything back out of a black hole once it’s gone in.
It’s formally known as the black hole information paradox: how black holes might simultaneously wipe all information from an object and yet also retain it. Confusing, huh?
And that’s where this “soft hair” comes in, first proposed by Hawking in 2016: a way of explaining temperature and entropy shifts so that it fits with what we already know about how the Universe works.
While you need to be a scientist of Hawking’s calibre to properly understand the finer points of the new research, it seems a fitting tribute that perhaps the final paper to have his name attached is linked to one of his favourite topics.
Perry also recounts the last time he spoke to Stephen Hawking about the research they were both working on – it was only a few days before Hawking’s death.
“It was very difficult for Stephen to communicate and I was put on a loudspeaker to explain where we had got to,” Perry told the Guardian.
“When I explained it, he simply produced an enormous smile. I told him we’d got somewhere. He knew the final result.”
Hawking’s final paper has yet to be peer reviewed, but is available on the pre-print server at arXiv.org.