Stephen Hawking's new idea says that even if matter falls toward a black hole, beyond scientists calculate the event horizon to be, it may reemerge in a highly jumbled form later. Here, an artist imagines what black hole Cygnus X-1 looks like as it pulls material from a nearby star. // NASA/CXC/M.Weiss
If a group of astronauts falls into a black hole, they may be able to get out, according to a paper Stephen Hawking posted online January 22. You would not recognize the unfortunate space travelers, though, and you would not be able to piece their particles back together in a recognizable form. But information about who they were would not technically be destroyed. Hawking’s new theory, not yet through the peer-review process, proposes that black holes do not have event horizons and may not even have singularities at their centers — tossing out the only anatomy most of us know.
A black hole — at least traditionally — curves space-time infinitely, hosting an infinitely dense “object” known as a singularity at its heart. At some distance around this singularity lies the event horizon, an invisible boundary beyond which even light cannot escape the black hole’s gravitational grasp. Hawking’s proposal turns all of that upside down (whatever that means in space).
His paper, entitled “Information Preservation and Weather Forecasting for Black Holes,” is an attempt to solve the firewall paradox, which was an attempt to solve the information paradox, which was a problem Hawking himself created. In the 1970s, he proposed that black holes leak energy, causing them slowly to lose mass. Eventually, this slow drip, known as “Hawking radiation,” will drain the black hole entirely away. But like a cosmic witness protection program, Hawking radiation has been stripped of any identifying information about its parent material. When the black hole is gone, so too is every bit of information about what fell in and made up the singularity.
But that’s not technically allowed, says quantum mechanics. Quantum mechanical laws, which rule the tiny parts of the universe, claim that information cannot be destroyed. By 2004, though, physicists had determined that Hawking radiation — somehow, in some way that seems like magic but is just as-yet-unexplained science — does carry information about the gas, stars, puppies, and apples that once fell into black holes. In 2012, a team led by Joe Polchinski of the Kavli Institute for Theoretical Physics in Santa Barbara, California, was working toward explaining the “how” when they gave themselves another problem: the firewall paradox.
General relativity says the aforementioned ill-fated astronauts will pass over the black hole’s event horizon without noticing and then will be stretched out — spaghettified, the technical term and something they would notice — as they approach the singularity. But if you look at a black hole through the lens of quantum mechanics, Polchinski found, the event horizon morphs into an inferno of energetic particles — a “firewall” that would zap and char the astronauts. That result, though, conflicts with relativity, which says the event horizon is more of a philosophical construct, an “unremarkable place,” as Nature News says. According to the paradox, either the firewalls are right and general relativity is wrong or black holes lose information and quantum mechanics is wrong.
Black holes have to change
In Hawking’s new paper, he attempts to reassure us that our most basic theories of nature can stick around as they are. It’s ideas about black holes that need to change. In his view, crazy quantum effects around these objects make space-time change wildly and rapidly. Those fluctuations make a staid, stable event horizon impossible. Instead, Hawking says nature’s most extreme objects have “apparent horizons,” which act like event horizons in that they imprison matter and energy but differ in that they dissolve, releasing their prisoners.
Hawking does not fully explain the mechanism behind this dissolution. But Don Page, a physicist at the University of Alberta in Edmonton, told Nature News that Hawking radiation eventually will shrink the black hole enough that the physics of the small — quantum mechanics — and the physics of the large — general relativity — work in tandem, making the apparent horizon disappear. According to Nature News, “matter would be only temporarily held behind the apparent horizon.” This horizon would shrink toward the black hole’s center but would never quite get there — and neither would infalling matter, which is why Hawking’s idea implies that singularities may be fiction.
Hawking is used to challenging our ideas, making our brains hurt, and spurring other physicists to make bold refutations. Let the flurry of activity to explain or disprove his new idea begin.