Is there a way to retrieve information that falls into a black hole? (Image: Richard Kail/SPL)

The fate of information that falls into a black hole is a mystery. Can it ever get out or does it disappear forever? Now, new work shows how in theory you can retrieve something from a black hole.

Sean Carroll of the California Institute of Technology in Pasadena and his colleagues approached the problem using techniques gleaned from the art of quantum teleportation, which involves moving quantum information from one place to another.


Quantum teleportation is based on entangled particles. When particles are entangled, measuring the state of one particle instantly influences the state of the other, no matter how far apart they are. An oft-called-upon example involves a couple called Alice and Bob, each of whom has access to one of a pair of entangled particles. To begin the teleportation, Bob sends one bit of information – a bit being the basic unit of information – to Alice. She uses that bit and her share of the entangled pair to move one bit of quantum information, or a qubit, from Bob’s location to where she is.

“That is the trick of quantum teleportation,” Carroll. “What we realised is that the black hole naturally provides you with that kind of set up.”

That is because a black hole keeps producing pairs of entangled particles, which make up so-called Hawking radiation. It begins with the spontaneous production of a particle and its anti-particle from the energy of the vacuum of space-time. Normally, such particles are short-lived: they coalesce and disappear in a puff of energy. But near a black hole’s event horizon, one particle falls into the black hole while the other goes outwards. This outward-going particle makes up Hawking radiation. The particle that falls in and the one that escapes are entangled with each other. If Alice were sitting outside the black hole, she would have access to one of the pair, and Bob inside the black hole would have access to the other. The only thing needed to move information from within the black hole to the outside would be some way for Bob to send classical information to Alice.

Drop a qubit

Carroll and colleagues showed that if Alice took a qubit, whose spin she doesn’t know anything about, and dropped it into the black hole, this would change the black hole’s spin or angular momentum – something that is measurable from outside the black hole. Easy, right? Just measure the tiny change in the spin of a black hole.

So, if Alice measures the black hole’s spin before and after dropping the qubit, the change in the spin is analogous to a bit of classical information transmitted by Bob from within the black hole to Alice outside.

By taking the change in the black hole’s spin, and her half of the Hawking radiation that is emitted after she drops the qubit, Alice can use the rules of quantum teleportation to work out the spin of the qubit she dropped into the black hole – and hence retrieve information from beyond the black hole’s event horizon.

But there is rather a large catch. The technique cannot be used to retrieve more than one qubit.

John Preskill, also of the California Institute of Technology, but not associated with the study, says the work “tells us something about one qubit’s worth of quantum information, but hardly anything about the vast amount of information that seems to fall into the black hole and become inaccessible from outside.”

Carroll agrees, but hopes their work “starts us thinking in slightly different ways about what it would mean to get qubits out [of a black hole]”, and thus solve the puzzle about what happens to the information that falls into a black hole.

Journal reference: ArXiv, arxiv.org/abs/1507.03592