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  • Open Access

Observing spacetime

Vijay Balasubramanian1,2,3,* and Tom Yildirim4,†

  • *Contact author: vijay@physics.upenn.edu
  • †Contact author: tom.yildirim@physics.ox.ac.uk
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Phys. Rev. D 113, 106034 – Published 29 May, 2026

DOI: https://doi.org/10.1103/p7wt-47rj

Abstract

Complex states of quantum gravity in flat and Anti de Sitter gravity can have features that are inaccessible to classical asymptotic observers. The missing information appears to such observers to be hidden behind a horizon or in a baby universe. Here we use the gravitational path integral to ask whether quantum observables can access the hidden data. We show that generic probes give a universal result and contain no information about the state. However, a probe appropriately fine-tuned to the state can give a large signal because of novel wormhole saddles in the path integral. Thus, in these settings, asymptotic observers cannot easily determine the state of the universe, but can check a proposal for it. Using these fine-tuned probes we show that an asymptotic observer can detect information hidden in a disconnected baby universe. Furthermore we show that the state of a two-boundary black hole can be detected using Lorentzian operators localized on just one of the boundaries.

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Outline

Information

Phys. Rev. D 113, 106034– Published 29 May, 2026
  • Received 7 April 2026
  • Accepted 27 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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