Fire in the Hole!
The trouble began, like so many confounding concepts in modern physics, in the brain of Stephen Hawking. Four decades ago, he realized that a black hole’s event horizon is inherently leaky; quantum processes allow a slow but steady flow of particles away from the black hole, a process now known as Hawking radiation. Given enough time, a black hole can evaporate completely. The idea of matter escaping the alleged point-of-no-return was surprising (it’s a central plot point in that other recent movie about black holes, the biographical The Theory of Everything), but the fate of information that falls into the black hole was what really troubled Hawking’s colleagues.
In the current view, which Hawking helped formulate, every event in the universe contains quantum information. When objects fall into a black hole, they take their information with them. That’s fine so long as the information stays in there, but if the black hole evaporates, things go all wonky. Coherent information goes in, but what comes out is just noise — Hawking radiation is 100 percent content-free.
Falling into a black hole seems to destroy a slice of reality, which just makes no sense. Hawking and other theorists began searching desperately for ways to prevent information from getting into the black hole in the first place, for ways to let it back out, or even for ways to make peace with the possibility that some information really can be lost forever.
In 2012, Joseph Polchinski of the University of California, Santa Barbara, offered a novel solution: As soon as an object gets pulled across the event horizon, it hits a firewall — an inferno so intense that it erases all the quantum information that object contained. No information gets lost because no information actually makes it through to the black hole’s interior. Polchinski pictures the event horizon as a kind of quantum eraser that neatly makes the information paradox go away.
If you think that sounds like a dodgy answer, rest assured that many of Polchinski’s colleagues (and even Polchinski himself) thought so, too — though perhaps not for the reasons you’d expect. The problem is that the rules of general relativity mandate physical continuity everywhere in the universe, even around a black hole. There should be no gap in the experience of, say, a doomed astronaut falling across the event horizon.
Or think of it this way: In relativity, the laws of physics look the same from all perspectives. The astronaut may get squished and stretched on the way in, but should still observe physics operating normally. The firewall, on the other hand, is about as abnormal and discontinuous as it gets. Now we’ve got a new paradox to deal with.
At this point, the story circles back to Hawking, who decided there must be a better way out. In a short paper presented last year, he suggests that the event horizon is not a defined boundary at all, but rather a zone of chaos where space and time are completely scrambled. No specific physical event occurs there — no Polchinski firewall — but any information passing through is rendered completely meaningless. Although Hawking dispenses with the traditional idea of the event horizon, he doesn’t dispute that black holes exist (as some breathless news stories claimed). Rather, he proposes that black holes are more like gray holes, defined by fuzzy edges that shed energy and garbled information.
My brain was starting to hurt, so I called on Juan Maldacena of the Institute for Advanced Study, a leading black hole theorist and a neutral third party. He doesn’t think much of the firewall concept because it fails miserably at the key task of reconciling quantum mechanics and relativity. “It’s telling us how not to do it,” he says. Does he like the Hawking approach, then? “No. He did not propose anything concrete,” Maldacena replies tersely.
When you are trying to solve the riddle of the black hole, nobody gets special treatment, not even Stephen Hawking.
Event Horizon Denialists
Here is where the deniers come in: If black holes keep sprouting paradoxes, the thinking goes, maybe the problem lies with our understanding of the black hole itself. I first encountered black hole denialism during a cosmology conversation with Laura Mersini-Houghton of the University of North Carolina, who mentioned, as a casual aside, “Oh, I also proved black holes don’t exist.” Over the past year, she has written two papers (one published, one in press) laying down the gauntlet.