## Abstract

The black hole information problem provides important clues for trying to piece together a quantum theory of gravity. Discussions on this topic have generally assumed that in a consistent theory of gravity and quantum mechanics, quantum theory is unmodified. In this review, we discuss the black hole information problem in the context of generalisations of quantum theory. In this preliminary exploration, we examine black holes in the setting of generalised probabilistic theories, in which quantum theory and classical probability theory are special cases. We are able to calculate the time it takes information to escape a black hole, assuming that information is preserved. In quantum mechanics, information should escape pure state black holes after half the Hawking photons have been emitted, but we find that this get’s modified in generalisations of quantum mechanics. Likewise the black-hole mirror result of Hayden and Preskill, that information from entangled black holes can escape quickly, also get’s modified. We find that although information exits the black hole as predicted by quantum theory, it is fairly generic that it fails to appear outside the black hole at this point—something impossible in quantum theory due to the no-hiding theorem. The information is neither inside the black hole, nor outside it, but is delocalised.

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## Notes

For background reading, we refer the reader to the review of John Preskill [3].

Modulo certain assumptions, the thermodynamical entropy of any theory is given in terms of the states N which can be distinguished by a physical device (based on an adaptation of the argument of von Neumann, used to justify the fact that the von Neumann entropy has a thermodynamical interpretation [34]). Since the Bekenstein bound is derived from thermodynamical considerations, logN is the entropy which should appear in the entropy bound for potential theories of quantum gravity which go beyond ordinary quantum theory.

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## Acknowledgments

We would like to thank the organisers of the Horizons of Quantum Theory conference for the opportunity to present our work, and their wonderful hospitality. We gratefully acknowledge feedback on a draft from Samuel Braunstein, David Jennings, Don Page and Arun Pati. J.O. and M.M. are grateful to the Aspen Center for Physics and NSF Grant 1066293, where some of this research was completed. J.O. would like to thank the Royal Society for their support. O.D. acknowledges support from the National Research Foundation (Singapore). O.D. was frequently visiting Imperial College whilst undertaking this research. Research at Perimeter Institute for Theoretical Physics is supported in part by the Government of Canada through NSERC and by the Province of Ontario through MRI.

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Müller, M.P., Oppenheim, J. & Dahlsten, O.C.O. Hiding Information in Theories Beyond Quantum Mechanics, and It’s Application to the Black Hole Information Problem.
*Found Phys* **44**, 829–842 (2014). https://doi.org/10.1007/s10701-014-9809-x

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DOI: https://doi.org/10.1007/s10701-014-9809-x

### Keywords

- Black holes
- Black hole information
- Post-quantum
- Generalised probabilistic theories