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DOE OSTI · 3001380

Absolute entropy and the observer’s no-boundary state

Blommaert, Andreas [Institute for Advanced Study, Princeton, NJ (United States)] (ORCID:0000000225990616)·Kudler-Flam, Jonah [Institute for Advanced Study, Princeton, NJ (United States); Princeton University, NJ (United States)] (ORCID:000000034352910X)·Urbach, Erez Y. [Institute for Advanced Study, Princeton, NJ (United States)] (ORCID:0000000199003911)
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Abstract

We investigate the no-boundary proposal for closed universes with an observer. We argue that the observer’s no-boundary state is the identity operator on the physical Hilbert space, i.e., the maximum entropy state and show this explicitly in Jackiw-Teitelboim gravity. Geometrically, the no-boundary state is a bra-ket wormhole. Expectation values in the no-boundary state provide a trace for the observer’s algebra, which allows one to define von Neumann entropy for observers in different universes as the relative entropy with respect to the no-boundary state. This result is consistent with all previously discussed cases of traces for invariantly defined regions.

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BibTeXRIS

Blommaert, Andreas [Institute for Advanced Study, Princeton, NJ (United States)] (ORCID:0000000225990616), Kudler-Flam, Jonah [Institute for Advanced Study, Princeton, NJ (United States); Princeton University, NJ (United States)] (ORCID:000000034352910X), Urbach, Erez Y. [Institute for Advanced Study, Princeton, NJ (United States)] (ORCID:0000000199003911). 2025-11-19. Absolute entropy and the observer’s no-boundary state. https://doi.org/10.1007/jhep11(2025)113

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