Black Hole Interior and Time-like Entanglement Entropy

Abstract

We investigate the possibility of using holographic time-like entanglement entropy (TEE) to probe the black hole interior. We choose the complex-valued extremal surface mixing spatial and temporal segments as one concrete example from various holographic candidates. In Schwarzschild-AdS black holes, we demonstrate that the TEE of time-like strips in such a holographic proposal exhibits linear growth at large temporal widths, possesses a physically significant imaginary part, and obeys a time-like entanglement first-law-like relation for its real part. By analyzing charged, scalar-hairy black holes, we find this geometric TEE detects a hidden "causal phase transition" separating Type-I and Type-II interiors. We identify a critical temporal width ${\tau }_c$ serving as a bulk diagnostic, potentially acting as a boundary order parameter: for strips narrower than ${\tau }_c$, the system enters a distinct phase dominated purely by time-like contributions up to an emergent effective scale; conversely, for strips wider than ${\tau }_c$, space-like entanglement re-emerges. Notably, a Cauchy horizon drives ${\tau }_c$ to infinity, leading to a pure time-like entanglement phase. These results suggest that the TEE may supply a novel boundary quantum-information measure to detect structure hidden inside the black hole and suggests a deep connection between TEE and cosmic censorship.

arXiv comment52 pages, 20 figures; v2: a new section added, minor modifications and references updated to match the published version