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Quanta Magazine

By Philip Ball

Posted in Interesting

As experimental proposals go, this one certainly doesn’t lack ambition. First, take a black hole. Now make a second black hole that is quantum entangled with it, which means that anything that happens to one of the black holes will seem to have an effect on the other, regardless of how far apart they are.

The rest sounds a bit easier, but a lot weirder. Feed some information into the first black hole, encoded in a quantum particle. As it falls beyond the event horizon — the point beyond which not even light can escape — the information is rapidly smeared throughout the black hole and is scrambled seemingly beyond recall.

But have patience — if you’ve linked the two black holes in the right way, after a short wait the quantum information will pop out of the second one, fully refocused into readable form. To get there, it will have traveled through a shortcut in space-time that links the two objects — a wormhole.

That, at least, is what physicists have predicted. Now a group led by Sepehr Nezami of the California Institute of Technology has suggested how to actually perform this extraordinary experiment — and they are beginning to work with collaborators to put the idea to the test.

If the predictions are borne out, the work may offer clues about where to look for the most elusive theory in physics: one that unites quantum mechanics with the theory of general relativity that describes gravity. And, for good measure, it would support the idea that space-time is not the fundamental backdrop against which the universe plays out but is itself woven from the interconnections between particles described by quantum entanglement.

The Death and Resurrection of Information

This experiment, as you might have guessed, doesn’t require black holes in the usual sense, meaning massive stars that have collapsed by their own gravity to an infinitesimally small volume. The researchers say that it could be done on a lab benchtop using just a few atoms or ions. All the same, the idea arises out of theoretical research on astrophysical black holes that has struggled to resolve a deep and unsettling question: Do these all-devouring monsters destroy information irreversibly?

It’s widely thought that information, like energy, should obey a conservation rule: The total amount of information in the universe will always stay the same. That’s what quantum mechanics seems to imply: The wave functions that describe quantum entities always evolve smoothly in an information-conserving way and can’t be suddenly snuffed out.

But black holes do seem to remove information from the universe. If, say, a quantum bit, or “qubit,” falls into a black hole, it can no longer be observed from outside the event horizon.

One possible resolution of this “black hole information paradox” can be found within the radiation that black holes emit from their event horizons. Hawking radiation, predicted by Stephen Hawking in the 1970s, will cause a black hole to lose gravitational energy — and thus mass. In effect, black holes are not eternal. They slowly evaporate.

Hawking initially believed that even if a black hole fully evaporated, the information it had consumed would remain lost forever. But an idea known as the AdS/CFT correspondence shows how the photons of Hawking radiation might be able to encode information about the interior of the black hole, thereby carrying that information back out into the universe at large.

The AdS/CFT correspondence was postulated by the theoretical physicist Juan Maldacena in 1997, and it’s widely regarded as one of the most promising directions in which to pursue theories of quantum gravity. It suggests that the physical structure of space-time in, say, four dimensions is equivalent to the operation of a quantum theory at a three-dimensional boundary.

This connection is strange, deep and surprising. It says that if you construct a space-time with a particular kind of curvature (and thus gravity) known as an anti-de Sitter space — that’s the AdS part — the mathematical description turns out to be equivalent to the description of a kind of quantum field theory called a conformal field theory — that’s the CFT part — in one fewer dimension. In other words, the correspondence works like a hologram — all the information in the higher-dimensional space-time projection is encoded within the lower-dimensional quantum interactions. This “holographic principle” was first proposed by the physics Nobel laureate Gerard ’t Hooft, and Maldacena’s AdS/CFT correspondence provided the first concrete picture of how it might work for a particular form of space-time.

In this view, what looks like continuous space in the AdS universe manifests in the CFT quantum view as entanglement — the interdependence of quantum bits. Here, said Maldacena, “the emergence of space-time is supposed to happen in systems with a large number of qubits that are highly entangled and highly interacting.” In other words, quantum entanglement can produce a space-time that seems to have gravity in it. Gravity, you might say, is spun from quantum effects.

Rapid Scramblers

What does all this have to do with black holes? The black hole information paradox asks what happens to the information that gets tossed into a black hole. The AdS/CFT correspondence is a key component of one proposed solution, since it supplies the means by which quantum entanglement could imprint the information on Hawking radiation and prevent it from being irrevocably lost.

In 2004, Hawking himself explained how, assuming the AdS/CFT conjecture is true, we could recover this information by capturing every single Hawking photon a black hole radiates over its entire lifetime before fully evaporating. As Norman Yao of the University of California, Berkeley described it, “If you were God and you collected all these Hawking photons, there is in principle some ungodly calculation you can do to re-extract the information in [each swallowed] qubit.”

Up until the halfway point of a black hole’s evaporation, the information inside it remains concealed. After that point, however, the black hole starts to reveal its information in its Hawking radiation. So you have a long wait before you can start to get at it. And according to an argument made in 1993 by the physicist Don Page of the University of Alberta, it will then seep out gradually, at a constant rate.

But in 2007, Patrick Hayden and John Preskill revised this picture by showing that, in fact, after the halfway point the information emerges more rapidly than that. Weirdly enough, once the black hole is half-evaporated, any further quantum bit of information tossed into it “literally bounces right back,” said Yao. This is because the black hole has by that stage become so quantum-entangled with the Hawking radiation it has already emitted that any more information it swallows is effectively registered at once in any further radiation it emits. The black hole, Hayden and Preskill said, then acts like an “information mirror.”