It’s another shot in the black hole wars. The edge of a black hole might be a brick wall, says Nobel laureate Gerard ’t Hooft, against which information about in-falling stuff bounces back like a tennis ball.

’t Hooft was responding to Stephen Hawking’s 25 August announcement of a new solution to the information paradox – a problem that has plagued scientists for 40 years.

The paradox is this: if any object, be it an iPhone or an elephant, ventures into a black hole, it stays there. From the outside, we will never be able to learn about any of its characteristics, information about it disappears behind the black curtain.

But in 1974, Hawking discovered that quantum weirdness at their edges causes black holes to leak radiation in the form of photons. This radiation, dubbed Hawking radiation, makes black holes slowly lose mass and evaporate.

Eventually, they pop completely out of existence, snuffing out all the details that lived inside. But quantum mechanics says information can neither be created nor destroyed, so the information has to go somewhere. Where?
Doomed pachyderm

“The answer is that matter going into the hole has an effect on outgoing Hawking particles,” says ’t Hooft. “Hawking did not believe that at first, but gradually he is revising that opinion.”

Hawking’s new claim is that if, say, an elephant passes over the edge of a black hole, the information about its elephant-ness stays on the edge as a holographic imprint. When the Hawking radiation seeps out, it carries that imprint with it. But questions remained: how does the in-falling matter make a mark? And how does that mark tattoo itself onto the outgoing radiation?

“Hawking’s paper generated a lot of discussion,” says ’t Hooft. This made him revisit an idea he had first proposed in 1987. “I realised I can do a better calculation,” he says.

’t Hooft’s idea says gravity answers both questions. If an elephant starts to slip over the edge, the animal’s gravitational field changes. When outgoing Hawking radiation passes through that gravitational field, its path is altered, and can convey information about the doomed pachyderm.

Information about it, like its mass, then bounces back into space, although the animal itself is not so lucky.
Information duplication

“Even though we describe modes of infalling matter that ‘bounce back against the horizon’, these bounces only refer to the information our particles are carrying, while the particles will continue their way falling inwards,” ’t Hooft writes in the paper.

’t Hooft’s and Hawking’s ideas have a similar problem, though: information overload. Their solutions might actually make a second copy of the information, creating information instead of destroying it.

If the poor elephant goes into the black hole, all of its characteristics go with it. But that information also hangs out on the horizon in Hawking’s case, or bounces back out in ’t Hooft’s.

“Quantum mechanics forbids such duplication,” says Steven Giddings of the University of California, Santa Barbara. It’s also unclear how a gravity-only information transfer meshes with quantum mechanics. “The devil is in those details,” says Giddings.

The battle over black holes is far from over. We will have to wait for more, in a state of information underload, to see what really happens on the edge of a black hole.