Black holes get their name because their gravity is so strong that it even traps light. And since the light cannot leave the black hole, then the information comes out too. Oddly enough, physicists have shown theoretical sleight of hand and came up with a way to extract a speck of information that fell into a black hole. Their calculation touches on one of the biggest mysteries in physics: how all the information trapped in a black hole leaks away as the black hole "evaporates". It is believed that this should happen, but no one knows how.
However, the new scheme should rather emphasize the complexity of the black hole information problem, rather than solve it. “Maybe others will be able to go further with this, but I don't think it will help,” says Don Page, a theoretician at the University of Alberta in Edmonton, Canada who was not involved in the work.
You can cut an electricity bill, but you cannot destroy information by throwing it into a black hole. This is partly because although quantum mechanics deals with probabilities - like the probability of an electron being in one place or another - the quantum waves that give these probabilities must evolve in a predictable way, so if you know the waveform at one point, you can predict it. exactly at any time in the future. Without this "unitarity," quantum theory would produce meaningless results like probabilities that don't add up to 100%.
Let's say you are throwing some quantum particles into a black hole. At first glance, the particles and the information they contain are lost. And this is a problem, because the part of the quantum state that describes the combined system of particles and black holes has been destroyed, which makes it impossible to predict the exact evolution and violates unitarity.
Physicists think they have found a way out. In 1974, British theorist Stephen Hawking argued that black holes can emit particles and energy. Thanks to quantum uncertainty, empty space is not really empty - it is full of paired particles that periodically come into existence and disappear. Hawking realized that if a pair of particles emerging from the vacuum hit the edge of a black hole, one would fly off into space and the other would fall into the black hole. Carrying away the energy of the black hole, the escaping Hawking radiation causes the black hole to slowly evaporate. Some theorists think that information appears again, being encoded in the radiation of the black hole - however, this is a completely incomprehensible moment, since the radiation seems to be completely random.
And so Aidan Chatwin-Davis, Adam Jermyn and Sean Carroll of California Institute of Technology in Pasadena have found a good way to get information from a single 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, to transfer the delicate quantum state of one particle, like an electron, to another. In quantum theory, the spin of an electron can be directed up, down, or up and down at the same time. This state can be described by a dot on the globe, where the north pole means up and the south pole means down. Lines of latitude mean different mixtures of up and down, and lines of longitude mean "phase," or how the tops and bottoms cross. But if Alice tries to measure this state, it "collapses" in one scenario or another, up or down, destroying the phase information. Therefore, she cannot measure the state and send information to Bob, but must send it untouched.
To do this, Alice and Bob can exchange an additional pair of electrons connected by a special quantum bond - entanglement. The state of each particle in the entangled pair is not defined - it simultaneously points to any point on the globe - but their states are correlated, so if Alice measures her particle from the pair and discovers that it is spinning, say, upward, she will instantly know that Bob's electron turns from top to bottom. So, Alice has two electrons - one the one whose state she wants to teleport, and her half of the entangled pair. Bob only has one of a confusing pair.
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To perform teleportation, Alice uses another strange property of quantum mechanics: that a measurement not only reveals something about the system, but also changes its state. Therefore, Alice takes her two unentangled electrons and makes a measurement that "projects" the entangled state onto them. This measurement breaks the entanglement between the pair of electrons she and Bob have. But at the same time, it leads to the fact that Bob's electron is in the state in which Alice's electron was, which she had to teleport. Through correct measurement, Alice transfers quantum information from one side of the system to the other.
Chatwin-Davis and his colleagues realized that they could teleport information about the state of an electron from a black hole as well. Suppose Alice is floating next to a black hole with her electron. It captures one photon from the Hawking radiation pair. Like an electron, a photon can spin in both directions and will be entangled with a photon partner that falls into a black hole. Alice then measures the total angular momentum, or spin, of the black hole - its size and, roughly speaking, how squarely it is in relation to a particular axis. Having these two bits of information in her hands, she throws her electron, losing it forever.
But Alice can recover information about the state of this electron, according to scientists in the work on Physical Review Letters. All she has to do is measure the spin and orientation of the black hole again. These measurements then entangle the black hole and the incident photon. They also teleport the state of the electron to the photon captured by Alice. Thus, the information of the lost electron will be extracted into the observable Universe.
Chatwin-Davis emphasizes that this design is not a blueprint for a practical experiment. Ultimately, Alice will need to instantly measure the spin of a black hole, which has the same mass as the sun. “We joke that Alice is probably the most advanced scientist in the universe,” he says.
This scheme also has many limitations. In particular, as the authors note, it works with one quantum particle, but not with two or more. This is because the recipe uses the fact that the black hole retains angular momentum, so its final spin is equal to its initial spin plus the spin of an electron. This allows Alice to extract exactly 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 the black hole.
To truly solve the black hole information problem, theorists need to account for the complex states of the black hole's interior, says Stefan Leichenhower, a theorist at the University of California, Berkeley. “Unfortunately, the biggest questions about black holes are about the inner workings,” he says. "So this protocol, which is certainly interesting in itself, will probably tell us little about the information problem of a black hole."