Reduced Half-sided Translations and Islands

Abstract: An AdS$_2$ black hole in equilibrium with a finite temperature non-gravitating bath comes with a Hawking-like information paradox. The resolution of this paradox requires introducing an island for the bath after the Page time. Since the island region contains the black hole interior, the consistency of the island paradigm demands the failure of any interior reconstruction proposal that uses only the operators in the AdS$_2$ region outside the black hole horizon after the Page time. In this paper, we investigate the consistency of the island paradigm using the black hole interior reconstruction proposal due to Leutheusser and Liu. They argued that the operators in the interior of a black hole can be reconstructed by the half-sided translations of the operators outside the black hole horizon. However, in order to illustrate the island paradigm we need to modify the Leutheusser-Liu proposal by introducing the notion of the reduced half-sided translations. The reduced half-sided translations, unlike the half-sided translations, have non-trivial action only on the operators in the AdS$_2$ black hole region. As a result, the reconstructed interior operators are expressed only using the operators in the AdS$_2$ region outside the black hole horizon. We demonstrate the consistency of the island paradigm by showing that the reduced half-sided translation, which successfully reconstruct the operators in the black hole interior before the Page time, fails to reconstruct the interior operators after the Page time.