Black holes got their name because their gravity is so strong that it even keeps light. And if light cannot leave a black hole, then the information comes out, too — get introduced to the information paradox of a black hole. Oddly enough, physicists have shown a theoretical sleight of hand and figured out a way to extract the speck of information falling into a black hole. The calculation covers one of the biggest mysteries in physics: how all the information trapped in a black hole, draining away as “evaporation” of a black hole. Believe that it should happen, but nobody knows.
However, the new scheme should rather emphasize the complexity of the information problem in black holes than to solve it. “Maybe others will be able to go further in this, but I don’t think it will help,” says don Paige, a theorist from the University of Alberta in Edmonton, Canada, who did not participate in the work.
You can cut a receipt for electricity, but it is impossible to destroy the information by throwing it into a black hole. Partly this is because although quantum mechanics deals with probabilities — such as the probability of finding the electron in one or another place — quantum waves, which give these probabilities must develop predictable, so if you know the waveform at one point, you can predict it exactly at any time in the future. Without such “unitarity”, quantum theory would produce nonsensical results like probabilities that sum up to 100%.
Suppose you throw a few quantum particles in a black hole. At first glance, particles, and the information they contain, are lost. And this is a problem because part of the quantum state describing the combined system of particles and the black hole was destroyed, making impossible accurate prediction of the evolution and violates unitarity.
Physicists think they found the exit. In 1974, the British theorist Stephen Hawking claimed that black holes can emit particles and energy. Thanks to quantum uncertainty, empty space is not really empty — it is full of pairs of particles, periodically coming into existence and disappearing. Hawking realized that if a pair of particles emerging from the vacuum, you get on the edge of a black hole, one will fly into space, and the other falls into the black hole. Carrying energy of the black hole, the leaking Hawking radiation causes the black hole slowly evaporates. Some theorists think that the information appears again, being encoded in the radiation of a black hole — however, it is absolutely unclear, since the radiation seems to be completely random.
And here Chatvin Aidan-Davis, Adam Jermyn and Sean Carroll from the California Institute of technology in Pasadena have found a good way to get information from one quantum particle, lost in a black hole using Hawking radiation, and the strange concept of quantum teleportation.
Quantum teleportation allows two partners, Alice and Bob, transfer delicate quantum state of one particle like electron other. In quantum theory, the electron spin can be oriented up, down, or up and down at the same time. This state can be described by the point on the globe where the North pole means the top and the South pole indicates the bottom. Latitude lines mean different mixture of top and bottom, and lines of longitude mean “phase”, or cross each other as the upper and lower parts. But if Alice tries to measure this state, it “collapses” to one or another scenario, up or down, destroying the information about the phase. Therefore, it cannot measure the state and send the information to Bob, and should send it intact.
To do this, Alice and Bob can exchange additional pair of electrons, connected by a special bond quantum — entanglement. The state of each particle in a confusing pair is not defined — it indicates at the same time anywhere in the globe — but their States are correlated, so if Alice measures her particle from the pair, and discovers that she moves, say upward, she instantly finds out that Bob the electron is spinning from top to bottom. So, Alice has two electrons — the one whose state she wants to teleport, and half of confusing pairs. The bean has only one of the confusing pairs.
To perform the teleportation, Alice uses another strange property of quantum mechanics: the measurement not only reveals something about the system, but changes its state. So Alice takes two unconfused electron and produces a measurement that “projecting” them in a complicated state. This measurement destroys the entanglement between a pair of electrons available to her and Bob. But at the same time, it leads to the fact that Bob the electron is in a state in which the electron was Alice, which she was supposed to teleport. Through correct measurement, Alice takes quantum information from one system to another.
Chatvin-Davis and his colleagues realized that they could teleport information about the state of the electron and also from the black hole. Suppose Alice swims near a black hole with its electron. It captures one photon pair born in the process of Hawking radiation. Like the electron, the photon can rotate in both directions and will be confused with its partner photon, which fell into a black hole. Then Alice measures the total moment, or spin, the black hole — its size and, roughly speaking, how exactly is it in relation to a particular axis. Having these two bits of information in hand, she throws his electron, losing it forever.
But Alice can recover information about the state of this electron, according to scientists in the work to Physical Review Letters. All she needs to do is again measure the spin and orientation of the black hole. These measurements are then confusing a black hole and the incident photon. They also teleported the state of an electron by a photon captured by Alice. Thus, the information of the lost electron will be ejected in the observed Universe.
Chatvin-Davis stresses that this scheme is not a practical experiment. In the end, Alice will take an instant measurement of the spin of the black hole, whose mass is equal to the mass of the sun. “We joke that Alice is probably the most advanced scientist in the Universe,” he says.
In this scheme there are also weight limitations. In particular, as the authors note, it works with a single quantum particle, but not two or more. That’s because the recipe uses the fact that the black hole preserves angular momentum, so its final spin is the initial spin plus spin of the electron. This allows Alice to retrieve two bits of information — the total spin and its projection along one axis — and this is enough to determine the latitude and longitude of the quantum state of one particle. But this is not enough to recover all the information captured by a black hole.
To really solve the information problem of black holes, theorists need to consider the complex state of the interior of a black hole, says Stephen Lehenbauer, a theorist at the University of California at Berkeley. “Unfortunately, the largest issues on the topic of black holes related to domestic work, he says. Thus, this Protocol is interesting in itself, probably little will tell us about the information problem black hole”.
In addition, penetration into black holes require quantum-mechanical theory of gravity. The development of such a theory is, perhaps, the greatest purpose of all theoretical physics for decades it eludes physicists.