Based on the paper The black hole information puzzle and the quantum de Finetti theorem

Black holes are some of the most fascinating objects in the universe. They are extreme deformations of space and time, formed from the collapse of massive stars, with a gravitational pull so strong that nothing, not even light, can escape it. Apart from the astrophysical aspects of black holes (which are bizarre enough to warrant their own study), they provide the ideal theoretical laboratory for exploring various aspects of quantum gravity, the theory that seeks to unify the principles of general relativity with those of quantum mechanics.

One definitive way of making progress in this endeavor is to resolve the infamous *black hole information paradox* [1], and through a series of recent exciting developments, it appears that we might be closer to achieving this than we have ever been before [5, 6]! Paradoxes in physics paradoxically tend to be quite useful, in that they clarify what we don’t know about what we know. Stephen Hawking’s semi-classical calculations of black hole radiation treat the matter in and around black holes as quantum fields but describe them within the framework of classical general relativity. The corresponding results turn out to be in disagreement with the results obtained from a purely quantum theoretical viewpoint. The information paradox encapsulates this particular discrepancy. According to the calculations of Hawking and Bekenstein in the 1970s [3], black hole evaporation via *Hawking radiation* is completely thermal. This simply means that the von Neumann entropy of radiation (a measure of its thermality or our ignorance of the system) keeps growing with the number of radiation quanta, reaching a maximum when the black hole has evaporated completely. This corresponds to a complete loss of information, whereby even a pure quantum state entering a black hole, would be transformed into a mixed state of Hawking radiation and all previous information about it would be destroyed. This conclusion is in stark contrast to what one would expect when regarding the black hole from the outside, as a quantum system that must obey the laws of quantum mechanics. The fundamental tenets of quantum mechanics are determinism and reversibility, the combination of which asserts that all information must be conserved. Thus if a black hole is formed by collapsing matter in a pure state, the state of the total system including the radiation R must remain pure. This can only happen if the entropy S(R) that first increases during the radiation process, ultimately decreases to zero when the black hole has disappeared completely, corresponding to a final pure state [4]. This quantum mechanical result is depicted in the famous *Page curve* (Fig.1)

Certain significant discoveries in the recent past showing that the Page curve is indeed the correct curve and can in fact be reproduced by semi-classical approximations of gravitational path integrals [7, 8] may finally hold the key towards the resolution of this paradox. These calculations rely on the *replica trick* [9, 10] and take into account contribution from space-time geometries comprising of wormholes that connect various replica black holes. This simple geometric amendment to the gravitational path integral is the only factor different from Hawking’s calculations and yet leads to diametrically different results! The replica trick is a neat method that enables the computation of the entropy of the radiation field by first considering n identical copies of a black hole, calculating their Renyi entropies and using the fact that this equals the desired von Neumann entropy in the limit of . These in turn are calculated using the semi-classical gravitational path integral under the assumption that the dominant contributions come from the geometries that are classical solutions to the gravity action, obeying Z_{n} symmetry. This leads to two distinct solutions:

- The
*Hawking saddle*consisting of disconnected geometries (corresponding to identical copies of a black hole). - The replica wormholes geometry consisting of connections between the different replicas.

Upon taking the limit, one finds that all that was missing from Hawking’s calculations and the quantum compatible Page curve was the inclusion of the replica wormhole solution in the former.

In this paper, the authors attempt to find the reason for this discrepancy, where this extra information is stored and the physical relevance of the replica wormholes using insights from quantum information theory. Invoking the quantum de Finetti theorem [11, 12], they find that there exists a particular *reference* information, W and the entropy one assigns to the black hole radiation field depends on whether or not one has access to this reference. While Hawking’s calculations correspond to measuring the unconditional von Neumann entropy S(R), ignoring W, the novel calculations using the replica trick calculate the conditional von Neumann entropy S(R|W), which takes W into account. The former yields the entropy of the ensemble average of all possible radiation states, while the latter yields the ensemble average of the entropy of the same states. They also show that the replica wormholes are a geometric representation of the correlation that appears between the n black holes, mediated by W.

The precise interpretation of W and what it might appear to be to an observer falling into a black hole remains an open question. Exploring what it could represent in holographic theories, string theories and loop quantum gravity could open up a dizzying array of insights into black hole physics and the nature of space-time itself. It appears that the different pieces of the quantum gravity puzzle are slowly but surely coming together to what will hopefully soon give us the entire picture.

**References**

- The Entropy of Hawking Radiation
- Black holes as mirrors: Quantum information in random subsystems
- Particle creation by black holes
- Average entropy of a subsystem
- The entropy of bulk quantum fields and the entanglement wedge of an evaporating black hole
- Entanglement Wedge Reconstruction and the Information Paradox
- Replica wormholes and the black hole interior
- Replica Wormholes and the Entropy of Hawking Radiation
- Entanglement entropy and quantum field theory
- Generalized gravitational entropy
- Locally normal symmetric states and an analogue of de Finetti’s theorem
- Unknown quantum states: The quantum de Finetti representation

#### Aruna Rajagopal

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