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arXiv · 2505.14771

Absolute entropy and the observer's no-boundary state

Andreas Blommaert·Jonah Kudler-Flam·Erez Y. Urbach
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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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Andreas Blommaert, Jonah Kudler-Flam, Erez Y. Urbach. 2025-05-20. Absolute entropy and the observer's no-boundary state. https://arxiv.org/abs/2505.14771

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