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A four-page scientific paper about a theoretical physics question

has been making the media rounds this week. That should be no surprise, though, given the author—Stephen Hawking—and the claim he appears to make: There are no black holes. But that's far from the end of the story.

The study, published on the open-access research site arXiv, does, in fact, include the words "there are no black holes." But the sentence continues on: "—in the sense of regimes from which light can't escape to infinity. There are, however, apparent horizons which persist for a period of time."

The problem of the black hole's event horizon, the boundary beyond which nothing can escape, is one Hawking raised in the 1970s. That's when he discovered, contrary to popular belief of the time, that black holes radiate energy. This means there is no such thing as a black hole from which nothing can escape.

So what is actually new about Hawking's latest paper?

"That's a really good question," says Don Marolf, a theoretical physicist who studies black holes at the University of California, Santa Barbara. "Most people that I know that read the paper see this as an expression of his opinion on a current debate without necessarily adding new scientific ingredients."

The debate comes down to a deceivingly simple question that has all kinds of implications for understanding the nature of the universe: If you drop information into a black hole, can you ever get it back?

To avoid time-travel paradoxes and preserve Einstein's theory of general relativity, information—anything that could, in principle, encode some kind of message—cannot travel faster than the speed of light. Because of the way light behaves at a black hole's event horizon, relativity defines the horizon as the point at which that information is lost to the outside universe forever. But the 1974 discovery of what is now called Hawking radiation changed the game. The implications of Hawking's find was that a black hole, if left alone, will eventually radiate all its energy away and disappear. The question arises, then: Where does all that information go? The laws of quantum mechanics say that information cannot be destroyed, and string theory, Marolf says, seems to indicate that the information does—"through a process we don't yet understand in detail, or even halfway in detail"—re-emerge into the universe once the black hole is gone.

Physicists refer to this problem as the information paradox, which is another part of the ongoing tension between the laws of relativity and the laws of quantum mechanics. Marolf threw his hat into the ring when he coauthored a paper in 2012 in the Journal of High-Energy Physics (and also published on arXiv) proposing a concept called a firewall, which suggests that a piece of general relativity known as the equivalence principle breaks down at the border of a black hole, which would help to unite three prongs of physics that clash in paradoxical ways in black holes. The firewall preserves the notion that information must be conserved while sacrificing a piece of relativity at the black hole's edge.

The theory has proved to be provocative, highlighting inconsistencies in physicists' understanding of, well, the laws of the universe. (A more technical discussion of firewalls, written by another of the paper's authors, can be found here.) It is this concept that Hawking explicitly calls out in his paper, positing "several objections to the firewall proposal."

"I completely agree with the complaints and criticisms," Marolf says. "On the other hand, for reasons that have been much discussed in the literature, the basic statement is, something has to give and we're not quite sure what it is. So maybe one of the criticisms that was raised is based on incorrect physics. Who knows?"

The potentially new element of Hawking's paper, Marolf says, is the suggestion that little-understood chaotic effects—the finer details of how matter, space, and time behave at the quantum level, where particles constantly dart in and out of existence —inside black holes could provide an answer for the information paradox.

"I don't think we understand what goes on well enough to say for certain whether or not that's true," Marolf said. "What I do think is clear is that some cherished principle of physics must be given up, and I think Stephen has not a made a suggestion about what that should be. And so I don't think this is a complete scenario."

Chaos could certainly be one component of the solution, Marolf said, but it's unclear if Hawking is proposing that it's the entire solution, an idea Marolf thinks would be "just incorrect."

All of this discussion is happening inside the realm of theoretical physics, where answers lie beyond what we can test or observe. There is no microscope to zoom in and look for the string theory's eponymous strings. And watching a black hole for an answer is highly unlikely, given that, even if it stopped ingesting new material now, it would take about 10 to 60 times the current age of the universe before it radiated away and we'd know if it spat its information back out.

But physicists care about the question given its implications for understanding how the fabric of reality actually works, and for its potential to resolve tensions between two governing theories of the universe that seem to be at odds.

"That's why people get very excited about this," Marolf said. "They're trying to understand if black holes and black hole evaporation require some change in the way we think about the fundamental laws of the universe and in particular quantum mechanics. Or is quantum mechanics still the correct answer at the most fundamental level and we need to modify other ideas, like whether information comes out of black holes."

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